K60P100M100SF2RM Datasheet, PDF (513/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Chapter 22 External Watchdog Monitor (EWM)
The EWM compare registers are used to create a service window, which is used by the
CPU to service/refresh the EWM module.
• If the CPU services the EWM when the counter value lies between CMPL value and
CMPH value, the counter is reset to zero. This is a legal service operation.
• If the CPU executes a EWM service/refresh action outside the legal service window,
EWM_out is asserted.
It is illegal to program CMPL and CMPH with same value. In this case, as soon as
counter reaches (CMPL + 1), EWM_out is asserted.
22.4.5 EWM Refresh Mechanism
Other than the initial configuration of the EWM, the CPU can only access the EWM by
the EWM Service Register. The CPU must access the EWM service register with correct
write of unique data within the windowed time frame as determined by the CMPL and
CMPH registers. Therefore, three possible conditions can occur:
Table 22-7. EWM Refresh Mechanisms
Condition
A unique EWM service occurs when
CMPL < Counter < CMPH.
A unique EWM service occurs when
Counter < CMPL
Counter value reaches CMPH prior to a
unique EWM service
Mechanism
The software behaves as expected and the counter of the EWM is reset to zero,
and EWM_out pin remains in the deasserted state.
Note: EWM_in pin is also assumed to be in the deasserted state.
The software services the EWM and therefore resets the counter to zero and
asserts the EWM_out pin (irrespective of the EWM_in pin). The EWM_out pin is
expected to gate critical safety circuits.
The counter value reaches the CMPH value and no service of the EWM resets
the counter to zero and assert the EWM_out pin (irrespective of the EWM_in
pin). The EWM_out pin is expected to gate critical safety circuits.
Any illegal service on EWM has no effect on EWM_out.
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
513