m48t129v STMicroelectronics, m48t129v Datasheet - Page 18

• Manufacturer Part Number: m48t129v
• Description: 5.0 Or 3.3v, 1 Mbit 128 Kbit X 8 Timekeeper Sram
• Manufacturer: STMicroelectronics

M48T129Y, M48T129V
Figure 13. Back-up Mode Alarm Waveforms
Watchdog Timer
The watchdog timer can be used to detect an out-
of-control microprocessor. The user programs the
watchdog timer by setting the desired amount of
time-out into the Watchdog Register, address
1FFF7h. 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 sec-
ond, 10 = 1 second, and 11 = 4 seconds. The
amount of time-out is then determined to be the
multiplication of the five-bit multiplier value with the
resolution. (For example: writing 00001110 in the
Watchdog Register = 3*1 or 3 seconds).
Note: Accuracy of timer is a function of the select-
ed resolution.
If the processor does not reset the timer within the
specified period, the M48T129Y/V sets the WDF
(Watchdog Flag) and generates a watchdog inter-
rupt or a microprocessor reset. WDF is reset by
reading the Flags Register (Address 1FFF0h). The
most significant bit of the Watchdog Register is the
Watchdog Steering Bit (WDS). When set to a '0,'
the watchdog will activate the IRQ/FT pin when
timed-out. When WDS is set to a '1,' the watchdog
will output a negative pulse on the RST pin for 40
to 200 ms. The Watchdog register and the FT Bit
will reset to a '0' at the end of a Watchdog time-out
when the WDS Bit is set to a '1.'
18/30
V CC
V PFD (max)
V PFD (min)
V SO
AFE bit in Interrupt Register
AF bit in Flags Register
IRQ/FT
HIGH-Z
The watchdog timer can be reset by two methods:
1. a transition (high-to-low or low-to-high) can be
2. the microprocessor can perform a WRITE of
The time-out period then starts over. The WDI pin
should be tied to V
will be reset on each transition (edge) seen by the
WDI pin. In the order to perform a software reset
of the watchdog timer, the original time-out period
can be written into the Watchdog Register, effec-
tively restarting the count-down cycle.
Should the watchdog timer time-out, and the WDS
Bit is programmed to output an interrupt, a value of
“00h” needs to be written to the Watchdog Regis-
ter in order to clear the IRQ/FT pin. This will also
disable the watchdog function until it is again pro-
grammed correctly. A READ of the Flags Register
will reset the Watchdog Flag (Bit D7; Register
1FFF0h).
The watchdog function is automatically disabled
upon power-down and the Watchdog Register is
cleared. If the watchdog function is set to output to
the IRQ/FT pin and the frequency test function is
activated, the watchdog or alarm function prevails
and the frequency test function is denied.
applied to the Watchdog Input pin (WDI); or
the Watchdog Register.
tREC
SS
if not used. The watchdog
HIGH-Z
AI01678C