Re: dspic33ep 512MC806 : QEI in external count with external up/down Mode
(I haven't used QEI to date, so the following is from an engineer's theoretical point of view.)
Not sure whether the figure is wrong or you've set your prescaler to 1:2. Both might apply.
Basically, the QEI module might be able to increment on both edges of the input signal (it does exactly this when operated in QEI mode). Cons8dering the simplicity of scaling down the counter, I personally would prefer a counter incrementing on both edges. But this is personal preference.
PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)