User guide

DS1254
6 of 17
AM/PM/12/24 MODE
Bit 7 of the hours register is defined as the 12-hour or 24-hour mode select bit. When high, the 12-hour
mode is selected. In the 12-hour mode, bit 5 is the AM/PM bit with logic high being PM. In the 24-hour
mode, bit 5 is the second 10-hour bit (20–23 hours).
OSCILLATOR BIT
Bit 5 of the day register controls the oscillator. When set to logic 1, the oscillator is off. When set to logic
0, the oscillator turns on and the watch becomes operational.
ZERO BITS
Registers 1, 2, 3, 4, 5, and 6 contain one or more bits that will always read logic 0. When writing these
locations, either a logic 1 or logic 0 is acceptable.
BATTERY MONITORING
The DS1254 automatically monitors the battery in an attached DS3800 battery cap on a 24-hour time
interval. Such monitoring begins within t
REC
after V
CC
rises above V
PF
and is suspended when power
failure occurs.
After each 24-hour period has elapsed, the battery is connected to an internal 1MΩ test resistor for one
second. During this one second, if the battery voltage falls below the battery voltage trip point (~2.6V),
the battery warning output BW is asserted. Once asserted, BW remains active until the attached DS3800
battery cap is replaced. However, the battery is still retested after each V
CC
power-up, even if was active
on power-down. If the battery voltage is found to be higher than ~2.6V during such testing, BW is de-
asserted and regular testing resumes. BW has an open-drain output driver.