DS1557

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SETTING THE CLOCK

The 8th 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 DS1557 (7FF8h-7FFFh) registers. After setting the write bit to a 1, RTC Registers can be

loaded with the desired RTC count (day, date, and time) in 24-hour BCD format. Setting the write bit to a

0 then transfers the values written to the internal RTC Registers and allows normal operation to resume.

CLOCK ACCURACY

The DS1557 and DS9034PCX are each individually tested for accuracy. Once mounted together, the

module is guaranteed to keep time accuracy to within

±1.53 minutes per month (35 ppm) at 25°C. The

DS1557 does not require additional calibration and, in most applications, temperature deviations will

have a negligible effect on accuracy. For this reason, methods of field clock calibration are not available

and not necessary. Attempts to calibrate the RTC that may be used with similar device types (M48T5x

family) will not have any effect even though the DS1557 appears to accept calibration data.

FREQUENCY TEST MODE

IRQ

/FT output will toggle at 512 Hz when the FT bit is a 1, the Alarm Flag Enable bit (AE) is a 0, and

the Watchdog Steering bit (WDS) is a 1 or the Watchdog Register is reset (Register 7FF7h = 00h). The

IRQ

/FT output and the frequency test mode can be used as a measure of the actual frequency of the

proper operation. The FT bit is cleared to a 0 on power-up.

USING THE CLOCK ALARM

The alarm settings and control for the DS1557 reside within Registers 7FF2h-7FF5h. Register 7FF6h

contains two alarm enable bits: Alarm Enable (AE) and Alarm in Backup Enable (ABE). The AE and

condition.

The alarm can be programmed to activate on a specific day of the month or repeat every day, hour,

minute, or second. It can also be programmed to go off while the DS1557 is in the battery backed state of

operation to serve as a system wake-up. Alarm mask bits AM1-AM4 control the alarm mode. Table 3

shows the possible settings. Configurations not listed in the table default to the once per second mode to

notify the user of an incorrect alarm setting.