DS1553 Maxim, DS1553 Datasheet - Page 10

• Manufacturer Part Number: DS1553
• Description: The DS1553 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 "8k x 8 nonvolatile static RAM (NV SRAM)
• Manufacturer: Maxim

DS1553 64kB, Nonvolatile, Year-2000-Compliant Timekeeping RAM
USING THE WATCHDOG TIMER
The watchdog timer can be used to detect an out-of-control processor. The user programs the watchdog
timer by setting the desired amount of timeout into the 8-bit Watchdog register (Address 1FF7h). The five
Watchdog register bits BMB4–BMB0 store a binary multiplier and the two lower-order bits
RB1–RB0 select the resolution, where 00 = 1/16 second, 01 = 1/4 second, 10 = 1 second, and
11 = 4 seconds. The watchdog timeout value is then determined by the multiplication of the 5-bit
multiplier value with the 2-bit resolution value. (For example: writing 00001110 in the Watchdog register
= 3 x 1 second or 3 seconds.) If the processor does not reset the timer within the specified period, the
Watchdog Flag (WF) is set and a processor interrupt is generated and stays active until either the
Watchdog Flag (WF) is read or the Watchdog register (1FF7) is read or written.
The most significant bit of the Watchdog register is the Watchdog Steering Bit (WDS). When set to 0, the
watchdog activates the
/FT output when the watchdog times out.
IRQ
When WDS is set to 1, the watchdog outputs a negative pulse on the
output for 40ms to 200ms. The
RST
Watchdog register (1FF7) and the FT bit are reset to 0 at the end of a watchdog timeout when the WDS
bit is set to 1.
The watchdog timer resets when the processor performs a read or write of the Watchdog register. The
timeout period then starts over. Writing a value of 00h to the Watchdog register disables the watchdog
timer. The watchdog function is automatically disabled upon power-up and the Watchdog register is
cleared. If the watchdog function is set to output to the
/FT output and the frequency test function is
IRQ
activated, the watchdog function prevails and the frequency test function is denied.
POWER-ON DEFAULT STATES
Upon application of power to the device, the following register bits are set to 0:
WDS = 0, BMB0–BMB4 = 0, RB0–RB1 = 0, AE = 0, and ABE = 0.
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