Information

EWM_CMPH field descriptions
Field Description
7–0
COMPAREH
To prevent runaway code from changing this field, software should write to this field after a CPU reset
even if the (default) maximum service time is required.
22.4 Functional Description
The following sections describe functional details of the EWM module.
22.4.1 The EWM_out Signal
The EWM_out is a digital output signal used to gate an external circuit (application
specific) that controls critical safety functions. For example, the EWM_out could be
connected to the high voltage transistors circuits that control an AC motor in a large
appliance.
The
EWM_out signal remains deasserted when the EWM is being regularly serviced by
the CPU within the programmable service window, indicating that the application code is
executed as expected.
The EWM_out signal is asserted in any of the following conditions:
• Servicing the EWM when the counter value is less than CMPL value.
• If the EWM counter value reaches the CMPH value, and no EWM service has
occurred.
• Servicing the EWM when the counter value is more than CMPL and less than CMPH
values and EWM_in signal is asserted.
• After any reset (by the virtue of the external pull-down mechanism on the
EWM_out
pin)
On a normal reset, the EWM_out is asserted. To deassert the EWM_out, set EWMEN bit
in the CTRL register to enable the EWM.
If the
EWM_out signal shares its pad with a digital I/O pin, on reset this actual pad defers
to being an input signal. It takes the EWM_out output condition only after you enable the
EWM by the EWMEN bit in the CTRL register.
When the EWM_out pin is asserted, it can only be deasserted by forcing a MCU reset.
Chapter 22 External Watchdog Monitor (EWM)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 403