DS1744/DS1744P Y2K-Compliant, Nonvolatile Timekeeping RAMs

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Table 1. Truth Table

CE

OE

WE

MODE

DQ

POWER

X

X

Deselect

High-Z

Standby

X

Write

Data In

Active

Read

Data Out

Active

Read

High-Z

Active

X

X

X

Deselect

High-Z

CMOS Standby

X

X

X

Deselect

High-Z

Data-Retention Mode

SETTING THE CLOCK

As shown in Table 2, bit 7 of the century register is the write bit. Setting the write bit to a 1, like the read

bit, halts updates to the DS1744 registers. The user can then load them with the correct day, date, and time

data in 24-hour BCD format. Resetting the write bit to a 0 then transfers those values to the actual clock

counters and allows normal operation to resume.

STOPPING AND STARTING THE CLOCK OSCILLATOR

The clock oscillator can be stopped at any time. To increase the shelf life, the oscillator can be turned off

to minimize current drain from the battery. The

OSC bit is the MSB (bit 7) of the seconds registers (Table

2). Setting it to a 1 stops the oscillator.

FREQUENCY TEST BIT

As shown in Table 2, bit 6 of the day byte is the frequency test bit. When the frequency test bit is set to

logic 1 and the oscillator is running, the LSB of the seconds register toggles at 512Hz. When the seconds

register is being read, the DQ0 line toggles at the 512Hz frequency as long as conditions for access remain

valid (i.e., CE low, OE low, WE high, and address for seconds register remain valid and stable).

CLOCK ACCURACY (DIP MODULE)

The DS1744 is guaranteed to keep time accuracy to within

±1 minute per month at +25°C. The RTC is

calibrated at the factory by Dallas Semiconductor using nonvolatile tuning elements, and does not require

additional calibration. For this reason, methods of field clock calibration are not available and not

necessary. Clock accuracy is also affected by the electrical environment; caution should be taken to place

the RTC in the lowest-level EMI section of the PC board layout. For additional information, refer to

Application Note 58: Crystal Considerations with Dallas Real-Time Clocks.

CLOCK ACCURACY (PowerCap MODULE)

The DS1744 and DS9034PCX are individually tested for accuracy. Once mounted together, the module

typically keeps time accuracy to within

±1.53 minutes per month (35ppm) at +25°C. Clock accuracy is

also affected by the electrical environment and caution should be taken to place the RTC in the lowest-

level EMI section of the PC board layout. For additional information, refer to Application Note 58: Crystal

Considerations with Dallas Real-Time Clocks.