DS1747/DS1747P 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 one, like the

read bit, halts updates to the DS1747 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 zero then transfers those values to the

actual clock counters and allows normal operation to resume.

STOPPING AND STARTING THE CLOCK OSCILLATOR

The clock oscillator may 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,

see Table 2. Setting it to a one 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 will toggle at 512 Hz. When the

seconds register is being read, the DQ0 line will toggle at the 512 Hz 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 DS1747 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. The electrical environment also affects the clock accuracy, and caution should be taken to

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

refer to Application Note 58.

CLOCK ACCURACY (PowerCap MODULE)

The DS1747 and DS9034PCX are each individually tested for accuracy. Once mounted together, the

module typically keeps time accuracy to within

±1.53 minutes per month (35 ppm) 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, please refer to

Application Note 58.