DS1388Z-33+ - Maxim Integrated

DS1388

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

For pricing, delivery, and ordering information, please contact Maxim Direct at

1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.

General Description

The DS1388 I2C real-time clock (RTC), supervisor, and

EEPROM is a multifunction device that provides a

clock/calendar, programmable watchdog timer, power-

supply monitor with reset, and 512 bytes of EEPROM.

The clock provides hundredths of seconds, seconds,

minutes, and hours, and operates in 24-hour or 12-hour

format with an AM/PM indicator. The calendar provides

day, date, month, and year information. The date at the

end of the month is automatically adjusted for months

with fewer than 31 days, including corrections for leap

year. A watchdog timer provides a reset for an unre-

sponsive microprocessor. It is programmable in 10ms

intervals from 0.01 to 99.99 seconds. A temperature-

compensated voltage reference and comparator circuit

monitors the status of VCC. If a primary power failure is

detected, the device automatically switches to the

backup supply and drives the reset output to the active

state. The backup supply maintains time and date

operation in the absence of VCC. When VCC returns to

nominal levels, the reset is held low for a period to allow

the power supply and processor to stabilize. The

device also has a pushbutton reset controller, which

debounces a reset input signal. The device is

accessed through an I2C serial interface.

Applications

Portable Instruments

Point-of-Sale Equipment

Network Interface Cards

Wireless Equipment

Features

oFast (400kHz) I2C Interface

oRTC Counts Hundredths of Seconds, Seconds,

Minutes, Hours, Day, Date, Month, and Year with

Leap Year Compensation Valid Up to 2100

oProgrammable Watchdog Timer

oAutomatic Power-Fail Detect and Switch Circuitry

oReset Output with Pushbutton Reset Input

Capability

o512 x 8 Bits of EEPROM

oIntegrated Trickle-Charge Capability for Backup

Supply

oThree Operating Voltages: 5.0V, 3.3V, and 3.0V

oLow Timekeeping Voltage Down to 1.3V

o-40°C to +85°C Temperature Range

oUL Recognized

TOP VIEW

DS1388

SCL

SDAGND

VCC

RSTX2

VBACKUP

Pin Configuration

RST

VCC

SCL

CPU

VCC

SDA

GND

VCC

RPU

RPU = tR/CB

RPU

VCC CRYSTAL

VBACKUP

DS1388

Typical Operating Circuit

19-4984; Rev 4 4/13

Denotes a lead(Pb)-free/RoHS-compliant package.

Ordering Information

PART TEMP RANGE PIN-PACKAGE TOP

MARK

DS1388Z-5+ -40°C to +85°C 8 SO (150 mils) DS1388-5

DS1388Z-33+ -40°C to +85°C 8 SO (150 mils) DS138833

DS1388Z-3+ -40°C to +85°C 8 SO (150 mils) DS1388-3

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

2 Maxim Integrated

DS1388

ABSOLUTE MAXIMUM RATINGS

RECOMMENDED DC OPERATING CONDITIONS

(TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to

absolute maximum rating conditions for extended periods may affect device reliability.

Voltage Range on VCC or VBACKUP Pins

Relative to Ground.............................................-0.3V to +6.0V

Voltage Range on Inputs Relative

to Ground ...............................................-0.3V to (VCC + 0.3V)

Junction-to-Ambient Thermal Resistance (θJA) (Note 1)..170°C/W

Junction-to-Case Thermal Resistance (θJC) (Note 1) ......40°C/W

Operating Temperature Range

(noncondensing) .............................................-40°C to +85°C

Storage Temperature Range .............................-55°C to +125°C

Lead Temperature (soldering, 10s) .................................+300°C

Soldering Temperature (reflow) .......................................+260°C

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

DS1388Z-5 4.5 5 5.5

DS1388Z-33 2.97 3.3 3.63

Supply Voltage VCC (Note 3)

DS1388Z-3 2.7 3 3.3

Logic 1 VIH (Note 3) 0.7 x

VCC

VCC +

0.3 V

Logic 0 VIL (Note 3) -0.3 +0.3 x

VCC V

Pullup Voltage (SCL, SDA),

VCC = 0V VPU 5.5 V

VBACKUP Voltage VBACKUP (Note 3) 1.3 3.0 5.5 V

DS1388Z-5 4.15 4.33 4.50

DS1388Z-33 2.70 2.88 2.97

Power-Fail Voltage VPF (Note 3)

DS1388Z-3 2.45 2.60 2.70

DC ELECTRICAL CHARACTERISTICS

(VCC = VCC(MIN) to VCC(MAX), TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

R1 (Notes 4, 5) 250

R2 (Note 6) 2000

Trickle-Charger Current-Limiting

Resistors

R3 (Note 7) 4000



Input Leakage (SCL) ILI -1 +1 μA

I/O Leakage (SDA) ILO -1 +1 μA

I/O Leakage (RST) ILORST (Note 8) -200 +10 μA

SDA Logic 0 Output

(VOL = 0.15 x VCC)IOLDOUT 3 mA

Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer

board. For detailed information on package thermal considerations, refer to www.maximintegrated.com/thermal-tutorial.

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

Maxim Integrated 3

DS1388

DC ELECTRICAL CHARACTERISTICS (continued)

(VCC = VCC(MIN) to VCC(MAX), TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

VCC > 2V; VOL = 0.4V 3.0

1.8V < VCC < 2V; VOL = 0.2 x VCC 3.0

RST Logic 0 Output IOLSIR

1.3V < VCC < 1.8V; VOL = 0.2 x VCC 250 μA

DS1388Z-5 600

DS1388Z-33 250

VCC Active Current, EEPROM

Read, I2C Read/Write Access ICCER (Note 9)

DS1388Z-3 225

μA

DS1388Z-5 1.0

DS1388Z-33 0.70

VCC Active Current, EEPROM

Write Cycle ICCEW (Note 9)

DS1388Z-3 0.65

DS1388Z-5 270

DS1388Z-33 100 150

VCC Standby Current ICCS (Note 10)

DS1388Z-3 140

μA

VBACKUP Leakage Current

(VBACKUP = 3.7V,

VCC = VCC(MAX))

IBACKUPLKG 15 100 nA

TA = +25°C (guaranteed by design) 200k

EEPROM Write/Erase Cycles tWR TA = -40°C to +85°C (guaranteed by

design) 50k

Cycles

DC ELECTRICAL CHARACTERISTICS

(VCC = 0V, VBACKUP = 3.7V, TA= +25°C, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

VBACKUP Current, OSC On

(EOSC = 0), SDA = SCL = 0V IBACKUP (Note 11) 410 550 nA

VBACKUP Current, OSC Off

(EOSC = 1), SDA = SCL = 0V

(Data Retention)

IBACKUPDR (Note 11) 10 100 nA

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

4 Maxim Integrated

DS1388

AC ELECTRICAL CHARACTERISTICS

(VCC = VCC(MIN) to VCC(MAX), TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS

Fast mode 100 400

SCL Clock Frequency fSCL Standard mode 0 100 kHz

Fast mode 1.3

Bus Free Time Between a STOP

and START Condition tBUF Standard mode 4.7 μs

Fast mode 0.6

Hold Time (Repeated) START

Condition (Note 12) tHD:STA Standard mode 4.0 μs

Fast mode 1.3

LOW Period of SCL Clock tLOW Standard mode 4.7 μs

Fast mode 0.6

HIGH Period of SCL Clock tHIGH Standard mode 4.0 μs

Fast mode 0.6

Setup Time for a Repeated

START Condition tSU:STA Standard mode 4.7 μs

Fast mode 0 0.9

Data Hold Time (Notes 13, 14) tHD:DAT Standard mode 0 μs

Fast mode 100

Data Setup Time (Note 15) tSU:DAT Standard mode 250 ns

Fast mode 300

Rise Time of Both SDA and SCL

Signals (Note 16) tRStandard mode

20 +

0.1CB 1000

Fast mode 300

Fall Time of Both SDA and SCL

Signals (Note 16) tFStandard mode

20 +

0.1CB 300

Fast mode 0.6

Setup Time for STOP Condition tSU:STO Standard mode 4.0 μs

Capacitive Load for Each Bus

Line CB (Note 16) 400 pF

I/O Capacitance (SDA, SCL, RST) CI/O +25°C 10 pF

Pushbutton Debounce PBDB 160 180 ms

Reset Active Time tRST 160 180 ms

EEPROM Write Cycle Time tWEE 8 10 ms

Oscillator Stop Flag (OSF) Delay tOSF (Note 17) 20 ms

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

Maxim Integrated 5

DS1388

POWER-UP/POWER-DOWN CHARACTERISTICS

(TA= -40°C to +85°C) (Note 2) (Figures 1, 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

VCC Detect to Recognize Inputs

(VCC Rising) tRST (Note 18) 160 180 ms

VCC Fall Time; VPF(MAX) to VPF(MIN) t

F 300 μs

VCC Rise Time; VPF(MIN) to VPF(MAX) t

R 0 μs

OUTPUTS

VCC

VPF(MAX)

INPUTS

HIGH IMPEDANCE

RST

DON'T CARE

VALID

RECOGNIZED RECOGNIZED

VALID

VPF(MIN)

tRST

tRPU

VPF VPF

Figure 1. Power-Up/Down Timing

tRST

PBDB

RST

Figure 2. Pushbutton Reset Timing

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

6 Maxim Integrated

DS1388

WARNING: Under no circumstances are negative undershoots, of any

amplitude, allowed when device is in write protection.

Note 2: Limits at -40°C are guaranteed by design and are not production tested.

Note 3: All voltages are referenced to ground.

Note 4: Measured at VCC = typ, VBACKUP = 0V, register 0Ah, block 0h = A5h.

Note 5: The use of the 250Ωtrickle-charge resistor is not allowed at VCC > 3.63V and should not be enabled.

Note 6: Measured at VCC = typ, VBACKUP = 0V, register 0Ah, block 0h = A6h.

Note 7: Measured at VCC = typ, VBACKUP = 0V, register 0Ah, block 0h = A7h.

Note 8: The RST pin has an internal 50kΩpullup resistor to VCC.

Note 9: ICCA—SCL clocking at max frequency = 400kHz.

Note 10: Specified with I2C bus inactive.

Note 11: Measured with a 32.768kHz crystal attached to X1 and X2.

Note 12: After this period, the first clock pulse is generated.

Note 13: A device must internally provide a hold time of at least 300ns for the SDA signal (referred to the VIHMIN of the SCL signal)

to bridge the undefined region of the falling edge of SCL.

Note 14: The maximum tHD:DAT need only be met if the device does not stretch the LOW period (tLOW) of the SCL signal.

Note 15: A fast-mode device can be used in a standard-mode system, but the requirement tSU:DAT ≥250ns must then be met. This

is automatically the case if the device does not stretch the LOW period of the SCL signal. If such a device does stretch the

LOW period of the SCL signal, it must output the next data bit to the SDA line tR(MAX) + tSU:DAT = 1000 + 250 = 1250ns

before the SCL line is released.

Note 16: CB—total capacitance of one bus line in pF.

Note 17: The parameter tOSF is the period of time that the oscillator must be stopped for the OSF flag to be set over the voltage

range of 0V ≤VCC ≤VCC(MAX) and 1.3V ≤VBACKUP ≤3.7V.

Note 18: If the oscillator is disabled or stopped, RST goes inactive after tRST plus the startup time of the oscillator.

VCC FALLING vs. RST DELAY

DS1388 toc04

VCC FALLING (V/ms)

RESET DELAY (μs)

1010.1

100

1000

10,000

0.01 100

VCC = VPF + 0.1V TO 0V

IBACKUP SUPPLY CURRENT VOLTAGE

vs. TEMPERATURE

DS1388 toc02

TEMPERATURE (°C)

SUPPLY CURRENT (nA)

503520

10-25

300

350

400

450

500

550

600

250

-40 80

VBACKUP = 3V

OSCILLATOR FREQUENCY

vs. SUPPLY VOLTAGE

DS1388 toc03

SUPPLY (V)

FREQUENCY (Hz)

5.34.8

3.8 4.3

2.3 2.8 3.3

1.8

32768.05

32768.10

32768.15

32768.20

32768.25

32768.30

32768.35

32768.40

32768.45

32768.00

1.3

IBACKUP SUPPLY CURRENT VOLTAGE

vs. VBACKUP

DS1388 toc01

VBACKUP (V)

SUPPLY CURRENT (nA)

5.3

4.9

4.5

4.1

3.73.32.92.52.11.7

300

350

400

450

500

250

1.3

VCC = 0V

Typical Operating Characteristics

(VCC = +3.3V, TA = +25°C, unless otherwise noted.)

Maxim Integrated 7

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

DS1388

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

8 Maxim Integrated

DS1388

Pin Description

Block Diagram

PIN NAME FUNCTION

1 X1

2 X2

Connections for a Standard 32.768kHz Quartz Crystal. The internal oscillator circuitry is designed for

operation with a crystal having a specified load capacitance (CL) of 6.0pF. Pin X1 is the input to the

oscillator and can optionally be connected to an external 32.768kHz oscillator. The output of the internal

oscillator, pin X2, is left unconnected if an external oscillator is connected to pin X1.

3 VBACKUP

Connection for a Secondary Power Supply. Supply voltage must be held between 1.3V and 5.5V for proper

operation. This pin can be connected to a primary cell, such as a lithium button cell. Additionally, this pin

can be connected to a rechargeable cell or a super cap when used with the trickle-charge feature. If not

used, this pin must be connected to ground. UL recognized to ensure against reverse charging current

when used with a lithium battery (www.maximintegrated.com/qa/info/ul/).

4 GND Ground

5 SDA

Serial Data Output. SDA is the input/output for the I2C serial interface. This pin is open drain and requires

an external pullup resistor.

6 SCL

Serial Clock Input. SCL is the clock input for the I2C interface and is used to synchronize data movement

on the serial interface.

7 RST

Active-Low, Open-Drain Reset Output. This pin indicates the status of VCC relative to the VPF

specification. As VCC falls below VPF, the RST pin is driven low. When VCC exceeds VPF, for tRST, the RST

pin is driven high impedance. The active-low, open-drain output is combined with a debounced

pushbutton input function. This pin can be activated by a pushbutton reset request. It has an internal

50k nominal value pullup resistor to VCC. No external pullup resistors should be connected. If the

crystal oscillator is disabled, the startup time of the oscillator is added to the tRST delay.

8 VCC DC Power Pin for Primary Power Supply

BLOCK 2BLOCK 1BLOCK 0

CLOCK AND CALENDAR

REGISTERS

POWER CONTROL

AND

TRICKLE CHARGER

I2C

INTERFACE EEPROM

INTERFACE

EEPROM EEPROM

WATCHDOG

TIMER

STATUS CONTROL/

TRICKLE

VCC

GND

VBACKUP

RST

SDA

SCL

DS1388

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

Maxim Integrated 9

DS1388

Detailed Description

The DS1388 I2C RTC, supervisor, and EEPROM is a

multifunction device that provides a clock/calendar,

programmable watchdog timer, power-supply monitor

with reset, and 512 bytes of EEPROM. The clock pro-

vides hundredths of seconds, seconds, minutes, and

hours, and operates in 24-hour or 12-hour format with

an AM/PM indicator. The calendar provides day, date,

month, and year information. The date at the end of the

month is automatically adjusted for months with fewer

than 31 days, including corrections for leap year. A

watchdog timer provides a reset for an unresponsive

microprocessor. It is programmable in 10ms intervals

from 0.01 to 99.99 seconds. A temperature-compensat-

ed voltage reference and comparator circuit monitors

the status of VCC. If a primary power failure is detected,

the device automatically switches to the backup supply

and drives the reset output to the active state. When

VCC returns to nominal levels, the reset is held low for a

period to allow the power supply and processor to sta-

bilize. The device also has a pushbutton reset con-

troller, which debounces a reset input signal. The

device is accessed through an I2C serial interface.

Operation

The DS1388 operates as a slave device on the I2C bus.

Access is obtained by implementing a START condition

and providing a device identification code followed by

data. Subsequent registers can be accessed sequen-

tially until a STOP condition is executed. See the

Block

Diagram

, which shows the main elements of the serial

real-time clock.

Power Control

The power-control function is provided by a precise,

temperature-compensated voltage reference and a

comparator circuit that monitors the VCC level. The

device is fully accessible and data can be written and

read when VCC is greater than VPF. However, when

VCC falls below VPF, the internal clock registers are

blocked from any access. If VPF is less than VBACKUP,

the device power is switched from VCC to VBACKUP

when VCC drops below VPF. If VPF is greater than

VBACKUP, the device power is switched from VCC to

VBACKUP when VCC drops below VBACKUP. The regis-

ters are maintained from the VBACKUP source until VCC

is returned to nominal levels (Table 1). After VCC

returns above VPF, read and write access is allowed

after RST goes high (Figure 1).

On first application of power to the device, the time and

date registers are set to 00/00/00 00 00:00:00

(DD/MM/YY DOW HH:MM:SS). The user should initial-

ize all RTC registers to valid date and time settings.

Oscillator Circuit

The DS1388 uses an external 32.768kHz crystal. The

oscillator circuit does not require any external resistors

or capacitors to operate. Table 2 specifies several

crystal parameters for the external crystal. Using a

crystal with the specified characteristics, the startup

time is usually less than one second.

SUPPLY CONDITION READ/WRITE

ACCESS

POWERED

VCC < VPF, VCC < VBACKPUP

No VBACKUP

VCC < VPF, VCC > VBACKUP

No VCC

VCC > VPF, VCC < VBACKUP

Yes VCC

VCC > VPF, VCC > VBACKUP

Yes VCC

PARAMETER

SYMBOL MIN TYP MAX

UNITS

Nominal

Frequency fO

32.768

kHz

Series

Resistance ESR 50

kΩ

Load

Capacitance CL6pF

The crystal, traces, and crystal input pins should be isolated

from RF generating signals. Refer to Application Note 58:

Crystal Considerations for Dallas Real-Time Clocks for addi-

tional specifications.

Table 1. Power Control

Table 2. Crystal Specifications*

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

10 Maxim Integrated

DS1388

Clock Accuracy

The accuracy of the clock is dependent upon the accu-

racy of the crystal and the accuracy of the match

between the capacitive load of the oscillator circuit and

the capacitive load for which the crystal was trimmed.

Additional error is added by crystal frequency drift

caused by temperature shifts. External circuit noise

coupled into the oscillator circuit can result in the clock

running fast. Figure 3 shows a typical PC board layout

for isolation of the crystal and oscillator from noise.

Refer to Application Note 58:

Crystal Considerations

with Dallas Real-Time Clock

s for detailed information.

Address Map

Figure 4 shows the address map for the DS1388. The

memory map is divided into three blocks. The memory

block accessed is determined by the value of the block

address bits in the slave address byte. The timekeep-

ing registers reside in block 0h. During a multibyte

access of the timekeeping registers, when the internal

address pointer reaches 0Ch, it wraps around to loca-

tion 00h. On an I2C START or address pointer incre-

menting to location 00h, the current time is transferred

to a second set of registers. The time information is

read from these secondary registers, while the clock

may continue to run. This eliminates the need to reread

the registers in case the main registers update during a

read. The EEPROM is divided into two 256-byte blocks

located in blocks 1h and 2h. During a multibyte read of

the EEPROM registers, when the internal address point-

er reaches FFh, it wraps around to location 00h of the

block of EEPROM specified in the block address.

During a multibyte write of the EEPROM registers, when

the internal address pointer reaches the end of the cur-

rent 8-byte EEPROM page, it wraps around to the

beginning of the EEPROM page. See the

Write

Operation

section for details.

To avoid rollover issues when writing to the time and

date registers, all registers should be written before the

hundredths-of-seconds register reaches 99 (BCD).

Hundredths-of-Seconds

Generator

The hundredths-of-seconds generator circuit shown in

the

Block Diagram

is a state machine that divides the

incoming frequency (4096Hz) by 41 for 24 cycles and

40 for 1 cycle. This produces a 100Hz output that is

slightly off during the short term, and is exactly correct

every 250ms. The divide ratio is given by:

Ratio = [41 x 24 + 40 x 1] / 25 = 40.96

Thus, the long-term average frequency output is

exactly 100Hz.

LOCAL GROUND PLANE (LAYER 2)

CRYSTAL

GND

NOTE: AVOID ROUTING SIGNAL LINES

IN THE CROSSHATCHED AREA

(UPPER LEFT QUADRANT) OF

THE PACKAGE UNLESS THERE IS

A GROUND PLANE BETWEEN THE

SIGNAL LINE AND THE DEVICE PACKAGE.

Figure 3. Layout Example

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

Maxim Integrated 11

DS1388

ADDRESS

BLK WORD BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION RANGE

0h 00h Tenth Seconds Hundredths of Seconds Hundredths of

Seconds 00–99

0h 01h 0 10 Seconds Seconds Seconds 00–59

0h 02h 0 10 Minutes Minutes Minutes 00–59

AM/

PM 10 Hour

0h 03h 0 12/24

10 Hour

Hours Hours

1–12+

AM/PM

00–23

0h 04h 0 0 0 0 X Day Day 01–07

0h 05h 0 0 10 Date Date Date 01–31

0h 06h 0 0 X 10

Month Month Month 01–12

0h 07h 10 Year Year Year 00–99

0h 08h Watchdog Tenths of Seconds Watchdog Hundredths of Seconds

Watchdog

Hundredth

Seconds

00–99

0h 09h Watchdog Ten Seconds Watchdog Seconds Watchdog

Seconds 00–99

0h 0Ah TCS3 TCS2 TCS1 TCS0 DS1 DS0 ROUT1 ROUT0 Trickle Charger —

0h 0Bh OSF WF 0 0 0 0 0 0 Flag —

0h 0Ch EOSC 0 0 0 0 0 WDE WD/RST Control —

1h 00–FFh 256 x 8 EEPROM EEPROM 00–FFh

2h 00–FFh 256 x 8 EEPROM EEPROM 00–FFh

Figure 4. Address Map

Note: Unless otherwise specified, the state of the registers is not defined when power (VCC and VBACKUP) is first applied.

X = General-purpose read/write bit.

0 = Always reads as a zero.

Clock and Calendar

The time and calendar information is obtained by read-

ing the appropriate register bytes. Figure 4 illustrates

the RTC registers. The time and calendar are set or ini-

tialized by writing the appropriate register bytes. The

contents of the time and calendar registers are in the

binary-coded decimal (BCD) format. The end of the

month date is automatically adjusted for months with

fewer than 31 days, including corrections for leap years

through 2099. The day-of-week register increments at

midnight. Values that correspond to the day-of-week

are user-defined but must be sequential (i.e., if 1

equals Sunday, then 2 equals Monday, and so on).

Illogical time and date entries result in undefined oper-

ation. The DS1388 can be run in either 12-hour or 24-

hour mode. Bit 6 of the hours register is defined as the

12- or 24-hour mode-select bit. When high, the 12-hour

mode is selected. In the 12-hour mode, bit 5 is the

AM/PM bit with logic-high being PM. In the 24-hour

mode, bit 5 is the 20-hour bit (20–23 hours). Changing

the 12/24 bit requires that the hours data be re-entered

in the proper format.

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

12 Maxim Integrated

DS1388

Watchdog Alarm Counter

The contents of the watchdog alarm counter, which is a

separate two-byte BCD down counter, are accessed in

the address range 08h–09h in block 0h. It is programma-

ble in 10ms intervals from 0.01 to 99.99 seconds. When

this counter is written, both the counter and a seed regis-

ter are loaded with the desired value. When the counter is

to be reloaded, it uses the value in the seed register.

When the counter is read, the current counter value is

latched into a register, which is output on the serial data

line and the watchdog counter reloads the seed value.

If the counter is not needed, it can be disabled and

used as a 16-bit cache of battery-backed RAM by set-

ting the WDE bit in the control register to logic 0. If all

16 bits of the watchdog alarm counter are written to a

zero when WDE = 1, the counter is disabled and the

WF bit is not set.

When the WDE bit in the control register is set to a logic

1 and a non-zero value is written into the watchdog reg-

isters, the watchdog alarm counter decrements every

1/100 second, until it reaches zero. At this point, the WF

bit in the flag register is set. If WD/RST = 1, the RST pin

is pulsed low for tRST and access to the DS1388 is

inhibited. At the end of tRST, the RST pin becomes high

impedance, and read/write access to the DS1388 is

enabled. The WF flag remains set until cleared by writ-

ing WF to logic 0. The watchdog alarm counter can be

reloaded and restarted before the counter reaches zero

by reading or writing any of the watchdog alarm

counter registers.

The WF flag and WDE bit must be set to zero before writing

the watchdog registers. After writing the watchdog regis-

ters, WDE must be set to one to enable the watchdog.

Power-Up/Down, Reset, and

Pushbutton Reset Functions

A precision temperature-compensated reference and

comparator circuit monitors the status of VCC. When an

out-of-tolerance condition occurs, an internal power-fail

signal is generated that blocks read/write access to the

device and forces the RST pin low. When VCC returns

to an in-tolerance condition, the internal power-fail sig-

nal is held active for tRST to allow the power supply to

stabilize, and the RST pin is held low. If the EOSC bit is

set to a logic 1 (to disable the oscillator in battery-back-

up mode), the internal power-fail signal and the RST pin

are kept active for tRST plus the oscillator startup time.

Access is inhibited whenever RST is low.

The DS1388 provides for a pushbutton switch to be

connected to the RST output pin. When the DS1388 is

not in a reset cycle, it continuously monitors the RST

signal for a low-going edge. If an edge is detected, the

part debounces the switch by pulling the RST pin low

and inhibits read/write access. After the internal timer

has expired, the part continues to monitor the RST line.

If the line is still low, it continues to monitor the line look-

ing for a rising edge. Upon detecting release, the part

forces the RST pin low and holds it low for tRST.

Special-Purpose Registers

The DS1388 has three additional registers (control,

flag, and trickle charger) that control the real-time

clock, watchdog, and trickle charger.

Flag Register (00Bh)

Bit 7: Oscillator Stop Flag (OSF). A logic 1 in this bit

indicates that the oscillator has stopped or was

stopped for some period of time and may be used to

judge the validity of the clock and calendar data. This

bit is edge triggered and is set to logic 1 when the

internal circuitry senses the oscillator has transitioned

from a normal run state to a STOP condition. The follow-

ing are examples of conditions that can cause the OSF

bit to be set:

1) The first time power is applied.

2) The voltage present on both VCC and VBACKUP are

insufficient to support oscillation.

3) The EOSC bit is turned off.

4) External influences on the crystal (i.e., noise, leak-

age, etc.).

This bit remains at logic 1 until written to logic 0. This

bit can only be written to logic 0. Attempting to write

OSF to logic 1 leaves the value unchanged.

Bit 6: Watchdog Alarm Flag (WF). A logic 1 in this bit

indicates that the watchdog counter reached zero. If

WDE and WD/RST are set to 1, the RST pin pulses low

for tRST when the watchdog counter reaches zero and

sets WF = 1. At the completion of the pulse, the WF bit

remains set to logic 1. Writing this bit to logic 0 clears

the WF flag. This bit can only be written to logic 0.

Attempting to write logic 1 leaves the value unchanged.

Bits 5 to 0: These bits read as zero and cannot be

modified.

Flag Register (00Bh)

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

OSFWF000000

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

Maxim Integrated 13

DS1388

Control Register (00Ch)

Bit 7: Enable Oscillator (EOSC). When set to logic 0,

the oscillator is started. When set to logic 1, the oscillator

is stopped when the DS1388 switches to battery power.

This setting can be used to conserve battery power

when timekeeping operation is not required. This bit is

cleared (logic 0) when power is first applied. When the

DS1388 is powered by VCC, the oscillator is always on

regardless of the status of the EOSC bit. The clock can

be halted whenever the timekeeping functions are not

required, which minimizes VBAT current (IBACKUPDR).

Bits 6 to 2: These bits read as zero and cannot be

modified.

Bit 1: Watchdog Enable (WDE). When set to logic

one, the watchdog counter is enabled. When set to

logic 0, the watchdog counter is disabled, and the two

registers can be used as NV RAM. This bit is cleared

(logic 0) when power is first applied.

Bit 0: Watchdog Reset (WD/RST). This bit enables the

watchdog alarm output to drive the RST pin. When the

WD/RST bit is set to logic 1, RST pulses low for tRST if

WDE = 1 and the watchdog counter reaches zero.

When the WD/RST bit is set to logic 0, the RST pin is

not driven by the watchdog alarm; only the watchdog

flag bit (WF) in the flag register is set to logic 1. This bit

is logic 0 when power is first applied.

Trickle-Charge Register (00Ah)

The simplified schematic of Figure 5 shows the basic

components of the trickle charger. The trickle-charge

select (TCS) bits (bits 4–7) control the selection of the

trickle charger. To prevent accidental enabling, only a

pattern on 1010 enables the trickle charger. All other

patterns disable it. The trickle charger is disabled when

power is first applied. The diode-select (DS) bits (bits 2

and 3) select whether or not a diode is connected

between VCC and VBACKUP. If DS is 01, no diode is

selected, yet if DS is 10, a diode is selected. The ROUT

bits (bits 0 and 1) select the value of the resistor con-

nected between VCC and VBACKUP. Table 3 shows the

resistor selected by the resistor select (ROUT) bits and

the diode selected by the diode-select (DS) bits.

Warning: The ROUT value of 250Ωmust not be select-

ed whenever VCC is greater than 3.63V.

The user determines the diode and resistor selection

according to the maximum current desired for battery

or super cap charging. The maximum charging current

can be calculated as illustrated in the following exam-

ple. Assume that a system power supply of 3.3V is

applied to VCC and a super cap is connected to

VBACKUP. Also, assume that the trickle charger has

been enabled with a diode and resistor R2 between

VCC and VBACKUP. The maximum current IMAX would

be calculated as follows:

IMAX = (3.3V - diode drop) / R2 ≈(3.3V - 0.7V) / 2kΩ≈

1.3mA

As the super cap charges, the voltage drop between

VCC and VBACKUP decreases and therefore the charge

current decreases.

Control Register (00Ch)

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

EOSC 00000WDEWD/RST

Table 3. Trickle-Charge Register

TCS3

TCS2 TCS1 TCS0

DS1 DS0

ROUT1 ROUT0

FUNCTION

X X X X 0 0 X X Disabled

X X X X 1 1 X X Disabled

X X X X X X 0 0 Disabled

10100101No diode, 250Ω resistor

10101001One diode, 250Ω resistor

10100110No diode, 2kΩ resistor

10101010One diode, 2kΩ resistor

10100111No diode, 4kΩ resistor

10101011One diode, 4kΩ resistor

00000000Initial default value—disabled

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

14 Maxim Integrated

DS1388

EEPROM

The DS1388 provides 512 bytes of EEPROM organized

into two blocks of 256 bytes. Each 256-byte block is

divided into 32 pages consisting of 8 bytes per page.

The EEPROM can be written one page at a time. Page

write operations are limited to writing bytes within a sin-

gle physical page, regardless of the number of bytes

actually being written. Physical page boundaries start at

addresses that are integer multiples of the page size (8

bytes) and end at addresses that are integer multiples

of [page size -1]. For example, page 0 contains word

addresses 00h to 07h. Similarly, page 1 contains word

addresses 08h to 0Fh. If a page write command

attempts to write across a physical page boundary, the

result is that the data wraps around to the beginning of

the current page (overwriting data previously stored

there), instead of being written to the next page as

might be expected. Therefore, it is necessary for the

application software to prevent page write operations

that would attempt to cross a page boundary.

I2C Serial Data Bus

The DS1388 supports a bidirectional I2C bus and data

transmission protocol. A device that sends data onto

the bus is defined as a transmitter and a device receiv-

ing data is defined as a receiver. The device that con-

trols the message is called a master. The devices that

are controlled by the master are slaves. The bus must

be controlled by a master device that generates the

serial clock (SCL), controls the bus access, and gener-

ates the START and STOP conditions. The DS1388

operates as a slave on the I2C bus. Connections to the

bus are made through the open-drain I/O lines SDA

and SCL. Within the bus specifications, a standard

mode (100kHz maximum clock rate) and a fast mode

(400kHz maximum clock rate) are defined. The DS1388

works in both modes.

The following bus protocol has been defined (Figure 6):

• Data transfer can be initiated only when the bus is

not busy.

• During data transfer, the data line must remain stable

whenever the clock line is high. Changes in the data

line while the clock line is high will be interpreted as

control signals.

Accordingly, the following bus conditions have been

defined:

Bus not busy: Both data and clock lines remain

high.

Start data transfer: A change in the state of the data

line from high to low, while the clock line is high,

defines a START condition.

Stop data transfer: A change in the state of the data

line from low to high, while the clock line is high,

defines a STOP condition.

Data valid: The state of the data line represents valid

data when, after a START condition, the data line is

stable for the duration of the high period of the clock

signal. The data on the line must be changed during

the low period of the clock signal. There is one clock

pulse per bit of data.

250Ω

2kΩ

4kΩ

VCC VBACKUP

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

TCS3 TCS2 TCS1 TCS0 DS1 DS0 ROUT1 ROUT0

TRICKLE-CHARGE REGISTER (00Ah)

1 0F 16 SELECT

NOTE: ONLY 1010b ENABLES CHARGER

1 OF 2

SELECT

1 OF 3

SELECT

TCS0-3 = TRICKLE-CHARGE SELECT

DS0-1 = DIODE SELECT

ROUT0-1 = RESISTOR SELECT

Figure 5. Programmable Trickle Charger

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

Maxim Integrated 15

DS1388

Each data transfer is initiated with a START condition

and terminated with a STOP condition. The number

of data bytes transferred between the START and the

STOP conditions is not limited, and is determined by

the master device. The information is transferred

byte-wise and each receiver acknowledges with a

ninth bit.

Acknowledge: Each receiving device, when

addressed, is obliged to generate an acknowledge

(ACK) after the reception of each byte. The master

device must generate an extra clock pulse, which is

associated with this acknowledge bit. The DS1388

does not generate any acknowledge bits if access to

the EEPROM is attempted during an internal pro-

gramming cycle.

A device that acknowledges must pull down the SDA

line during the acknowledge clock pulse in such a

way that the SDA line is stable low during the high

period of the acknowledge-related clock pulse. Of

course, setup and hold times must be taken into

account. A master must signal an end of data to the

slave by generating a not-acknowledge (NACK) bit on

the last byte that has been clocked out of the slave. In

this case, the slave must leave the data line high to

enable the master to generate the STOP condition.

Figures 7 and 8 detail how data transfer is accom-

plished on the I2C bus. Depending upon the state of

the R/Wbit, two types of data transfer are possible:

Data transfer from a master transmitter to a slave

receiver. The first byte transmitted by the master is

the slave address. Next follows a number of data

bytes. The slave returns an acknowledge bit after

each received byte. Data are transferred with the

most significant bit (MSB) first.

Data transfer from a slave transmitter to a master

receiver. The first byte (the slave address) is trans-

mitted by the master. The slave then returns an

acknowledge bit. Next follows a number of data

bytes transmitted by the slave to the master. The

master returns an acknowledge bit after all received

bytes other than the last byte. At the end of the last

received byte, a NACK is returned.

The master device generates all the serial clock

pulses and the START and STOP conditions. A trans-

fer is ended with a STOP condition or with a repeat-

ed START condition. Since a repeated START

condition is also the beginning of the next serial

transfer, the bus is not released. Data are transferred

with the most significant bit (MSB) first.

SDA

SCL

IDLE

1–7 8 9 1–7 8 9 1–7 8 9

START

CONDITION STOP CONDITION

REPEATED START

SLAVE

ADDRESS

R/W ACK ACKDATA ACK/

NACK

DATA

MSB FIRST MSB LSB MSB LSB

REPEATED IF MORE BYTES

ARE TRANSFERRED

Figure 6. I2C Data Transfer Overview

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

16 Maxim Integrated

DS1388

Device Addressing

The slave address byte is the first byte received follow-

ing the START condition from the master device. The

slave address byte consists of a 4-bit control code. For

the DS1388, this is set as 1101 binary for read and

write operations. The next three bits of the slave

address byte are the block select bits (B2, B1, B0). B2

is always logic 0 for the DS1388. These bits are used

by the master device to select which of the three blocks

in the memory map are to be accessed. These bits are

the three most significant bits of the word address. The

last bit of the slave address byte defines the operation

to be performed. When set to 1, a read operation is

selected; when set to 0, a write operation is selected.

Write Operation

Slave Receiver Mode (Write Mode)

Following the START condition from the master, the

device code (4 bits); the block address (3 bits); and the

R/Wbit, which is logic-low, is placed onto the bus by

the master transmitter. This indicates to the DS1388

that a byte with a word address follows after the

DS1388 has generated an acknowledge bit during the

ninth clock cycle. The next byte transmitted by the

master is the word address and will set the internal

address pointer of the DS1388, with the DS1388

acknowledging the transfer on the ninth clock cycle.

The master device can then transmit zero or more

bytes of data, with the DS1388 acknowledging the

transfer on the ninth clock cycle. The master generates

a STOP condition to terminate the data write.

Byte Write

The write-slave address byte and word address are

transmitted to the DS1388 as described in the

Slave

Receiver Mode

section. The master transmits one data

byte, with the DS1388 acknowledging the transfer on

the ninth clock cycle. The master then generates a

STOP condition to terminate the data write. This initiates

the internal write cycle, and, if the write was to the

EEPROM, the DS1388 does not generate acknowledge

signals during the internal EEPROM write cycle.

EEPROM Page Write

The write-slave address byte, word address, and the

first data byte are transmitted to the DS1388 in the

same way as in a byte write. But instead of generating

a STOP condition, the master transmits up to 8 data

bytes to the DS1388, which are temporarily stored in

the on-chip page buffer and are written into the memo-

ry after the master has transmitted a STOP condition.

Data bytes within the page that are not written remain

unchanged. The internal address pointer automatically

increments after each byte is written.

If the master should transmit more than 8 data bytes prior

to generating the STOP condition, the address pointer

rolls over and the previously received data is overwritten.

As with the byte write operation, once the STOP condi-

tion is received an internal write cycle begins.

RTC Multibyte Write

Writing multiple bytes to the RTC works much the same

way as the EEPROM page write, except that the entire

contents of block 0h can be written at once. The 8-byte

page size limitation does not apply to the block 0. If the

master should transmit more bytes than exists in block

0 prior to generating the STOP condition, the internal

address pointer rolls over and the previously received

data is overwritten. As with the byte write operation,

once the STOP condition is received an internal write

cycle begins.

Slave Address Byte

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

1 1 0 1 B2B1B0R/W

OPERATION CONTROL CODE BLOCK SELECT R/W

Read Clock 1101 000 1

Write Clock 1101 000 0

Read Lower Block of EEPROM 1101 001 1

Write Lower Block of EEPROM 1101 001 0

Read Upper Block of EEPROM 1101 010 1

Write Upper Block of EEPROM 1101 010 0

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

Maxim Integrated 17

DS1388

Acknowledge Polling

Since the DS1388 does not acknowledge during an

EEPROM write cycle, acknowledge polling can be used

to determine when the cycle is complete (this feature

can be used to maximize bus throughput). Once the

master issues the STOP condition for a write command,

the DS1388 initiates the internally timed write cycle.

ACK polling can be initiated immediately. This involves

the master sending a START condition, followed by the

slave address byte for a write command (R/W= 0) to

the EEPROM. If the device is still busy with the write

cycle, then a NACK is returned. If the cycle is com-

plete, then the device returns the ACK and the master

can then proceed with the next read or write command.

The RTC registers in block 0 are accessible during an

EEPROM write cycle.

Read Operation

Read operations are initiated in the same way as write

operations with the exception that the R/Wbit of the

slave address is set to 1. There are three basic types of

read operations: current address read, random read,

and sequential read.

Current Address Read

The DS1388 contains an address pointer that main-

tains the last address accessed, internally increment-

ed by 1. Therefore, if the previous access (either a

read or write operation) was to address n, the next cur-

rent address read operation would access data from

address n + 1. Upon receipt of the slave address with

the R/Wbit set to 1, the DS1388 issues an acknowl-

edge and transmits the 8-bit data byte. The master

issues a NACK followed by a STOP condition, and the

DS1388 discontinues transmission.

Random Read

Random read operations allow the master to access any

memory location in a random manner. To perform this

type of read operation, first the word address must be

set. This is done by sending the word address to the

DS1388 as part of a write operation. After the word

address is sent, the master generates a START condi-

tion following the acknowledge. This terminates the write

operation, but not before the internal address pointer is

set. Then the master issues the slave address byte

again but with the R/Wbit set to 1. The DS1388 then

issues an acknowledge and transmits the 8-bit data

byte. The master issues a NACK followed by a STOP

condition, and the DS1388 discontinues transmission.

Sequential Read

Sequential reads are initiated in the same way as a ran-

dom read except that after the DS1388 transmits the

first data byte, the master issues an acknowledge as

opposed to a STOP condition in a random read. This

directs the DS1388 to transmit the next sequentially

addressed 8-bit byte. To provide sequential reads, the

DS1388 contains an internal address pointer, which is

incremented by one at the completion of each opera-

tion. This allows the entire memory contents of the

block specified in the slave address to be serially read

during one operation. The master terminates the read

by generating a NACK followed by a STOP condition.

No page boundaries exist for read operations. When

the address pointer reaches the end of an EEPROM

block (FFh), the address pointer wraps to the beginning

(00h) of the same block.

The DS1388 can operate in the two modes illustrated in

Figures 7 and 8.

...

AXXXXXXXXA1101000S 0 XXXXXXXX A XXXXXXXX A XXXXXXXX A P

<R/W> <WORD ADDRESS (n)> <DATA (n)> <DATA (n + 1)> <DATA (n + X)

S - START

A - ACKNOWLEDGE (ACK)

P - STOP

R/W - READ/WRITE OR DIRECTION BIT ADDRESS

DATA TRANSFERRED

(X + 1 BYTES + ACKNOWLEDGE)

MASTER TO SLAVESLAVE TO MASTER

<SLAVE

ADDRESS>

Figure 7. Data Write—Slave Receiver Mode

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

18 Maxim Integrated

DS1388

...

AXXXXXXXXA1101BBBS 1 XXXXXXXX A XXXXXXXX A XXXXXXXX A P

B - BLOCK SELECT

S - START

A - ACKNOWLEDGE (ACK)

P - STOP

A - NOT ACKNOWLEDGE (NACK)

R/W - READ/WRITE OR DIRECTION BIT ADDRESS

DATA TRANSFERRED

(X + 1 BYTES + ACKNOWLEDGE)

NOTE: LAST DATA BYTE IS FOLLOWED BY A NACK.

MASTER TO SLAVE SLAVE TO MASTER

<SLAVE

ADDRESS>

Figure 8. Data Read—Slave Transmitter Mode

B - BLOCK SELECT

S - START

Sr - REPEATED START

A - ACKNOWLEDGE (ACK)

P - STOP

A - NOT ACKNOWLEDGE (NACK)

R/W - READ/WRITE OR DIRECTION BIT ADDRESS

<SLAVE

ADDRESS> <R/W>

AXXXXXXXXA1101BBB 1101BBBSSr0 A1

DATA TRANSFERRED

(X + 1 BYTES + ACKNOWLEDGE)

NOTE: LAST DATA BYTE IS FOLLOWED BY A NACK.

MASTER TO SLAVE SLAVE TO MASTER

AXXXXXXXX XXXXXXXX A XXXXXXXX A XXXXXXXX A P

...

Figure 9. Data Write/Read (Write Pointer, Then Read)—Slave Receive and Transmit

Chip Information

SUBSTRATE CONNECTED TO GROUND

PROCESS: CMOS

Package Information

For the latest package outline information and land patterns, go

to www.maximintegrated.com/packages. Note that a “+”, “#”,

or “-” in the package code indicates RoHS status only.

Package drawings may show a different suffix character, but

the drawing pertains to the package regardless of RoHS status.

PACKAGE

TYPE

PACKAGE

CODE

OUTLINE

NO.

LAND

PATTERN NO.

8 SO S8+4 21-0041 90-0096

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

DS1388

Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent

licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and

max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.

Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000

Revision History

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 4/05 Initial release. —

Removed the leaded parts from the Ordering Information. 1

Indicated the pullup voltage for SDA and SCL in the Pin Description table. 8

Added the oscillator bias circuit to the Block Diagram and removed the original Figure 8, 10

Added time and date POR values in the Power Control section. 9

Changed the last sentence of the Watchdog Alarm Counter section (first paragraph) to

“When the counter is read, the current counter value is latched into a register, which is

output on the serial data line and the watchdog counter reloads the seed value.”

Added “Access is inhibited whenever RST is low.” To the end of the Power-Up/Down,

Reset, and Pushbutton Reset Functions section (first paragraph). 12

In the Control Register (00Ch) section, bit 7 description, added a statement that EOSC is

used to reduce VBAT current when timekeeping is not required. 13

1 9/08

Replaced the I2C read and write figures. 17, 18

Corrected the RTC power-on default values in the Power Control section. 9

Corrected the date register range in Figure 4. 11

2 10/09

Added WF flag clear to the Watchdog Alarm Counter section. 12

In the Absolute Maximum Ratings section, added the thermal information, lead

information, and new Note 1, and updated the soldering information. 2–6

In the Pin Description, corrected X2 pin name and bias limit on SDA, SCL. 8

3 8/10

Added the package code and land pattern no. to the Package Information table. 18

44/13 Updated Absolute Maximum Ratings,Power Control and Clock and Calendar sections 2, 9, 11

Mouser Electronics

Authorized Distributor

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