PIC16F74 Checksum(s)

But IIRC, there's an indirect way to do it, which is why it's possible to read its program-stored lookup tables.

Probably they just use different methods to calculate the checksum.

IIRC, that part lacks the capability of reading its own program memory directly, so its rather a moot point.

Best regards, Spehro Pefhany

^^^ it's

Best regards, Spehro Pefhany

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No, IIRC, this one requires 8-bit table entries to be stored as retlw (return literal in w) 14-bit instructions and you have to essentially 'call' the correct 'table' location (ugh).

Sometimes developers want to (or are required to by safety standards) run a check of the code memory checksum at startup or periodically to provide a higher degree of safety. IIRC, this particular microcontroller (like the 'C' version it replaces) is quite incapable of performing such a calculation.

Best regards, Spehro Pefhany

Ah, so it does. Looks like there are at least three 16F possibilities-- read only (F74 etc with above registers), read/write (F676 etc. with EEADRH etc.) neither read nor write (F628 etc.)

Best regards, Spehro Pefhany

Er. No. The F74, can read it's memory. Unlike many other 16F chips, it does have the extra special function registers to allow you to address the internal ROM. You load the address required into PMADR, and PMADRH, institute an EEPROM read, and the contents are returned in PMDATA, and PMDATH. It takes a total of about 13 machine cycles to read each location, but it can be done.

Best Wishes

t'is an' all.

Spot on. It is a real 'pain' at times...

Best Wishes