DS1644/DS1644P

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SETTING THE CLOCK

The MSB Bit, (B7) of the control register is the write bit. Setting the write bit to a 1, like the read bit,

halts updates to the DS1644 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 may be stopped at any time. To increase the shelf life, the oscillator can be turned off

a 1 stops the oscillator.

FREQUENCY TEST BIT

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

CLOCK ACCURACY (DIP MODULE)

The DS1644 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 and caution should be taken to

place the RTC in the lowest level EMI section of the PCB layout. For additional information please see

application note 58.

CLOCK ACCURACY (POWERCAP MODULE)

The DS1644 and DS9034PCX are each individually tested for accuracy. Once mounted together, the

module will typically keep 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 PCB layout. For additional information please see application note

58.