Z8F640x/Z8F480x/Z8F320x/Z8F240x/Z8F160x
Z8 Encore!®
74
mode. Refer to the Reset and Stop Mode Recovery chapter for more information on
STOP Mode Recovery.
If interrupts are enabled, following completion of the Stop Mode Recovery the eZ8 CPU
responds to the interrupt request by fetching the Watch-Dog Timer interrupt vector and
executing code from the vector address.
WDT Reset in Normal Operation
If configured to generate a Reset when a time-out occurs, the Watch-Dog Timer forces the
Z8F640x family device into the Short Reset state. The WDT status bit in the Watch-Dog
Timer Control register is set to 1. Refer to the Reset and Stop Mode Recovery chapter for
more information on Short Reset.
WDT Reset in Stop Mode
If configured to generate a Reset when a time-out occurs and the Z8F640x family device is
in STOP mode, the Watch-Dog Timer initiates a Stop Mode Recovery. Both the WDTsta-
tus bit and the STOPbit in the Watch-Dog Timer Control register are set to 1 following
WDT time-out in STOP mode. Refer to the Reset and Stop Mode Recovery chapter for
more information.
Watch-Dog Timer Reload Unlock Sequence
Writing the unlock sequence to the Watch-Dog Timer Control register (WDTCTL)
unlocks the three Watch-Dog Timer Reload Byte registers (WDTU, WDTH, and WDTL)
to allow changes to the time-out period. These write operations to the WDTCTL register
address produce no effect on the bits in the WDTCTL register. The locking mechanism
prevents spurious writes to the Reload registers. The follow sequence is required to unlock
the Watch-Dog Timer Reload Byte registers (WDTU, WDTH, and WDTL) for write
access.
1. Write 55H to the Watch-Dog Timer Control register (WDTCTL)
2. Write AAH to the Watch-Dog Timer Control register (WDTCTL)
3. Write the Watch-Dog Timer Reload Upper Byte register (WDTU)
4. Write the Watch-Dog Timer Reload High Byte register (WDTH)
5. Write the Watch-Dog Timer Reload Low Byte register (WDTL)
All three Watch-Dog Timer Reload registers must be written in the order just listed. There
must be no other register writes between each of these operations. If a register write
occurs, the lock state machine resets and no further writes can occur, unless the sequence
is restarted. The value in the Watch-Dog Timer Reload registers is loaded into the counter
when the Watch-Dog Timer is first enabled and every time a WDT instruction is executed.
PS017610-0404
Watch-Dog Timer
ZILOG [ ZILOG, INC. ]
