Freescale Semiconductor, Inc.
I/O Ports with Key Wake-up
10.4 Key Wake-Up Input Filter
The KWU input signals are filtered by a digital filter which is active only
during STOP mode. The purpose of the filter is to prevent single pulses
shorter than a specified value from waking the part from STOP.
The filter is composed of an internal oscillator and a majority voting logic.
The filter oscillator starts oscillation by detecting a triggering edge on an
input if the corresponding interrupt enable bit is set. The majority voting
logic takes three samples of an asserted input pin at each filter oscillator
period and if two samples are taken at the triggering level, the filter
recognizes a valid triggering level and sets the corresponding interrupt
flag. In this way, the majority voting logic rejects the short non-triggering
state between two incoming triggering pulses. As the filter is shared with
all KWU inputs, the filter considers any pulse coming from any input pin
for which the corresponding interrupt enable bit is set.
The timing specification is given for a single pulse. The time interval
between the triggering edges of two following pulses should be greater
than the tKWSP in order to be considered as a single pulse by the filter. If
this time interval is shorter than tKWSP, the majority voting logic may treat
the two consecutive pulses as a single valid pulse.
The filter is shared by all the KWU pins. Hence any valid triggering level
on any KWU pin is seen by the filter. The timing specification applies to
the input of the filter.
Technical Data
MC68HC912DG128 — Rev 3.0
I/O Ports with Key Wake-up
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