I wrote

> (like mistakes in the datasheets and errors in sample code)

Here's one - well, two actually, in just 8 lines. Datasheet Example
10-1 for the 4520, a fairly recent device

Initializing PortA to <RA3:0> as i/p, RA<5:4> as o/p

CLRF PORTA
CLRF LATA
MOVLW 07H
MOVWF ADCON1
MOVWF 07H
MOVWF CMCON
MOVLW 0CFH
MOVWF TRISA

What a bloody waste of a Sunday afternoon. The MOVWF 07H
is an easy one to spot. The killer was 07 into ADCON1. It should
be 0F. BTW, I didn't copy that routine verbatim, but did set ADCON1
to 07 (because that's what you do on many other PICs, so it didn't
seem out of the ordinary). Not until I'd pulled my code apart line by
line, including cross-checking the above with the A2D section did I
finally get some pushbuttons working on PortA of a 2520

grrrrrrrrrr

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