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DS1388

I2C RTC/Supervisor with Trickle Charger and 512 Bytes EEPROM

Maxim Integrated 

I2C RTC/Supervisor with Trickle Charger

and 512 Bytes EEPROM

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 reset to 01/01/00 01 00:00:00

(MM/DD/YY DOW HH:MM:SS).

Table 1. Power Control

SUPPLY CONDITION

VCC < VPF, VCC < VBACKPUP

VCC < VPF, VCC > VBACKUP

VCC > VPF, VCC < VBACKUP

VCC > VPF, VCC > VBACKUP

READ/WRITE

ACCESS

No

No

Yes

Yes

POWERED

BY

VBACKUP

VCC

VCC

VCC

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.

Table 2. Crystal Specifications*

PARAMETER

Nominal

Frequency

SYMBOL MIN TYP MAX UNITS

fO

32.768

kHz

Series

ESR

Resistance

50

kΩ

Load

Capacitance

CL

6

pF

*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.

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