How wide is this falling edge pulse?
Do you realize that a short low going pulse on the MCLR pin is the same as a reset and therefore can be used as a wake up signal?
Maybe use a 10K pullup on MCLR to Vdd, put your pulse into the MCLR pin itself thru a capacitor. As the pulse goes low, so does the cap, for a short period of time (depending on the size of the cap and the value of the pullup), then shortly later, the pullup resistor charges the cap back to Vdd...BAM...instant pulse generator...also, instant wake up...
Try it and see what happens. It's worked for me before.
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