Information

Note
EWM_out pad must be in pull down state when EWM
functionality is used and when EWM is under Reset.
22.4.2 The EWM_in Signal
The EWM_in is a digital input signal that allows an external circuit to control the
EWM_out signal. For example, in the application, an external circuit monitors a critical
safety function, and if there is fault with this circuit's behavior, it can then actively initiate
the EWM_out signal that controls the gating circuit.
The EWM_in signal is ignored if the EWM is disabled, or if INEN bit of CTRL register
is cleared, as after any reset.
On enabling the EWM (setting the CTRL[EWMEN] bit) and enabling EWM_in
functionality (setting the CTRL[INEN] bit), the EWM_in signal must be in the deasserted
state prior to the CPU servicing the EWM. This ensures that the
EWM_out stays in the
deasserted state; otherwise, the EWM_out pin is asserted.
Note
You must update the CMPH and CMPL registers prior to
enabling the EWM. After enabling the EWM, the counter resets
to zero, therefore providing a reasonable time after a power-on
reset for the external monitoring circuit to stabilize and ensure
that the EWM_in pin is deasserted.
22.4.3 EWM Counter
It is an 8-bit ripple counter fed from a clock source that is independent of the peripheral
bus clock source. As the preferred time-out is between 1 ms and 100 ms the actual clock
source should be in the kHz range.
The counter is reset to zero, after a CPU reset, or a EWM refresh cycle. The counter
value is not accessible to the CPU.
22.4.4 EWM Compare Registers
The compare registers CMPL and CMPH are write-once after a CPU reset and cannot be
modified until another CPU reset occurs.
Functional Description
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
404 Freescale Semiconductor, Inc.