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WDOG_PRESC field descriptions
Field
Description
15–11
Reserved
This field is reserved.
This read-only field is reserved and always has the value 0.
10–8
PRESCVAL
3-bit prescaler for the watchdog clock source. A value of zero indicates no division of the input WDOG
clock. The watchdog clock is divided by (PR 1) to provide the prescaled WDOG_CLK.
Reserved
This field is reserved.
This read-only field is reserved and always has the value 0.
25.9 Watchdog operation with 8-bit access
25.9.1 General guideline
When performing 8-bit accesses to the watchdog's 16-bit registers where the intention is
to access both the bytes of a register, place the two 8-bit accesses one after the other in
your code.
25.9.2 Refresh and unlock operations with 8-bit access
One exception condition that generates a reset to the system is the write of any value
other than those required for a legal refresh/update sequence to the respective refresh and
unlock registers.
For an 8-bit access to these registers, writing a correct value requires at least two bus
clock cycles, resulting in an invalid value in the registers for one cycle. Therefore, the
system is reset even if the intention is to write a correct value to the refresh/unlock
register. Keeping this in mind, the exception condition for 8-bit accesses is slightly
modified.
Whereas the match for a correct value for a refresh/unlock sequence is as according to the
original definition, the match for an incorrect value is done byte-wise on the refresh/
unlock rather than for the whole 16-bit value. This means that if the high byte of the
refresh/unlock register contains any value other than high bytes of the two values that
make up the sequence, it is treated as an exception condition, leading to a reset or
interrupt-then-reset. The same holds true for the lower byte of the refresh or unlock
register. Take the refresh operation that expects a write of 0xA602 followed by 0xB480
to the refresh register, as an example.
Chapter 25 Watchdog Timer (WDOG)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
479
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