Rev. 1.43
User's
Guide
53
The Watchdog can be activated by setting the RTC's Watchdog register (internal offset $7ff7) with the
desired time-out value.
When the Watchdog is active, it decrement its internal counter until 0. To reload the counter, you can
either write again the RTC's watchdog register, either toggling the RTCWDI pin. (connected to the
8260's I/O PD12)
If the Watchdog counter reachs zero, the M48T37V generate an interrupt to its IRQ pin.
This pin is connected on two devices: the 8260 I/O port C5 and the programmable logic. This permits
to use the Watchdog in two manners:
A) The Watchdog pin is directly connected to the 8260's port C5, this pin has for special feature to be
able to generate an IRQ to the core.
B) For a more securised application, it is possible to connect, using a register, the Watchdog pin to the
Board's Power-On Reset signal. This is made writing the value $1 in the SRESR register. Once
activated, this connection can not be disabled until next Power-On Reset. In this case, when the
Watchdog's counter reachs zero, the VSBC-6862 will be completely Resetted.
The connection of the IRQ signal to the 8260 permits also to use the Alarm feature contained in the
M48T37V.
The following table gives the connection to MPC-8260 I/O ports.
MPC-8260
Peripheral function
I/O
Function
I/O
Signal
Description
RTC
PC5
PC5
I
RTCIRQ
IRQ from RTC
PD12
PD12
O
RTCWDI
Watchdog to RTC
Warning
Before the 'connection' of the Watchdog signal to the Power-On Reset with the
SRESR register, please clear the internal RTC's Watchdog register to avoid
unintended Reset.
Warning
The SNAPHAT housing plugged on the M48T37V contains both battery and
crystal. Thus, for the minimum frequency error, the user has to calibrate the
RTC after each battery replacement.
For detailed software setup, please refer to the corresponding ST Microelectronics
M48T37V datasheet and application notes.
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