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Clocking and System Control
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SPRUI07–March 2020
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System Control and Interrupts
1.3.4.1 Servicing The Watchdog Timer
The WDCNTR is reset when the proper sequence is written to the WDKEY register before the 8-bit
watchdog counter (WDCNTR) overflows. The WDCNTR is reset-enabled when a value of 0x55 is written
to the WDKEY. When the next value written to the WDKEY register is 0xAA then the WDCNTR is reset.
Any value written to the WDKEY other than 0x55 or 0xAA causes no action. Any sequence of 0x55 and
0xAA values can be written to the WDKEY without causing a system reset; only a write of 0x55 followed
by a write of 0xAA to the WDKEY resets the WDCNTR.
Table 1-26. Example Watchdog Key Sequences
Step Value Written to WDKEY Result
1 0xAA No action
2 0xAA No action
3 0x55 WDCNTR is enabled to be reset if next value is 0xAA.
4 0x55 WDCNTR is enabled to be reset if next value is 0xAA.
5 0x55 WDCNTR is enabled to be reset if next value is 0xAA.
6 0xAA WDCNTR is reset.
7 0xAA No action
8 0x55 WDCNTR is enabled to be reset if next value is 0xAA.
9 0xAA WDCNTR is reset.
10 0x55 WDCNTR is enabled to be reset if next value is 0xAA.
11 0x32 Improper value written to WDKEY.
No action, WDCNTR no longer enabled to be reset by next 0xAA.
12 0xAA No action due to previous invalid value.
13 0x55 WDCNTR is enabled to be reset if next value is 0xAA.
14 0xAA WDCNTR is reset.
Step 3 in Table 1-26 is the first action that enables the WDCNTR to be reset. The WDCNTR is not
actually reset until step 6. Step 8 again re-enables the WDCNTR to be reset and step 9 resets the
WDCNTR. Step 10 again re-enables the WDCNTR ro be reset. Writing the wrong key value to the
WDKEY in step 11 causes no action, however the WDCNTR is no longer enabled to be reset and the
0xAA in step 12 now has no effect.
If the watchdog is configured to reset the device, then a WDCR overflow or writing the incorrect value to
the WDCR[WDCHK] bits will reset the device and set the watchdog flag (WDFLAG) in the WDCR register.
After a reset, the program can read the state of this flag to determine the source of the reset. After reset,
the WDFLAG should be cleared by software to allow the source of subsequent resets to be determined.
Watchdog resets are not prevented when the flag is set.
1.3.4.2 Watchdog Reset or Watchdog Interrupt Mode
The watchdog can be configured in the SCSR register to either reset the device (WDRST) or assert an
interrupt (WDINT) if the watchdog counter reaches its maximum value. The behavior of each condition is
described below:
• Reset mode:
If the watchdog is configured to reset the device, then the WDRST signal will pull the device reset
(XRS) pin low for 512 OSCCLK cycles when the watchdog counter reaches its maximum value.
• Interrupt mode:
If the watchdog is configured to assert an interrupt, then the WDINT signal will be driven low for 512
OSCCLK cycles, causing the WAKEINT interrupt in the PIE to be taken if it is enabled in the PIE
module. The watchdog interrupt is edge triggered on the falling edge of WDINT. Thus, if the WAKEINT
interrupt is re-enabled before WDINT goes inactive, you will not immediately get another interrupt. The
next WAKEINT interrupt will occur at the next watchdog timeout. If the watchdog is disabled before
WDINT goes inactive, the 512-cycle count will halt and WDINT will remain active. The count will
resume when the watchdog is enabled again.
If the watchdog is re-configured from interrupt mode to reset mode while WDINT is still active low, then
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