DS1554 Maxim, DS1554 Datasheet - Page 8

• Manufacturer Part Number: DS1554
• Description: The DS1554 is a full-function, year 2000-compliant (Y2KC), real-time clock/calendar (RTC) with an RTC alarm, watchdog timer, power-on reset, battery monitor, and 32k x 8 nonvolatile static RAM
• Manufacturer: Maxim

DS1554 256k, Nonvolatile, Y2K-Compliant Timekeeping RAM
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 DS1554 (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 (DIP MODULE)
The DS1553 is guaranteed to keep time accuracy to within 1 minute per month at 25C. The RTC is
calibrated at the factory by Maxim 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 clock accuracy. 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.
CLOCK ACCURACY (PowerCap MODULE)
The DS1554 and DS9034PCX are each individually tested for accuracy. Once mounted together, the
module is will typically keep time accuracy to within 1.53 minutes per month (35 ppm) at 25°C. The
electrical environment affects clock accuracy. 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.
FREQUENCY TEST MODE
The DS1554 frequency test mode uses the open drain IRQ/FT output. With the oscillator running, the
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
32.768 kHz RTC oscillator. The IRQ/FT pin is an open-drain output that requires a pullup resistor for
proper operation. The FT bit is cleared to a 0 on power-up.
USING THE CLOCK ALARM
The alarm settings and control for the DS1554 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
/FT output to be activated for a matched alarm
IRQ
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 DS1554 is in the battery-backed state of
operation to serve as a system wakeup. Alarm mask bits AM1 to 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.
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