DS1557P-100 DALLAS [Dallas Semiconductor], DS1557P-100 Datasheet - Page 6

• Manufacturer Part Number: DS1557P-100
• Description: 4MEG NV Y2KC Timekeeping RAM
• Manufacturer: DALLAS [Dallas Semiconductor]

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
The DS1557 frequency test mode uses the open drain
the Watchdog Steering bit (WDS) is a 1 or the Watchdog Register is reset (Register 7FF7h = 00h). The
32.768 kHz RTC oscillator. 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
ABE bits must be set as described below for the
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.
IRQ
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
/FT output and the frequency test mode can be used as a measure of the actual frequency of the
IRQ
/FT pin is an open drain output which requires a pullup resistor for
6 of 19
IRQ
/FT output to be activated for a matched alarm
IRQ
/FT output. With the oscillator running, the
DS1557