Post by thread:Table Read Confusion

Hi ppl, I have a stupid (newbish) question about table read using TBLPTRU, TBLPTRH, and TBLPTRL. I understand what they do and how to use them. The question is: if I need to read the table in the interrupt, is it a good idea for me to store the original values of TBLPTRs or should I disable interrupts when I need to do program flash memory reads/write in the main loop (like a critical section in x86)? Also, shouldn't this problem also apply to FSRnH / FSRnL? How come the manual doesn't mention this at all? In the example in the PIC18 Reference, the manual only mention to save WREG/STATUS/BSR... ~Terry (Seitai) Chen

> The question is: if I need to read the table in the interrupt, is it a good > idea for me to store the original values of TBLPTRs or should I disable > interrupts when I need to do program flash memory reads/write in the main > loop (like a critical section in x86)? Six of one, half dozen of the other. ANY register which gets may be changed by an interrupt handler should be dealt with very carefully by non-interrupt code, since its current value can never be known during execution of non-interrupt code. If you need to do table reads and writes in an interrupt, you really should save and restore the table pointer values. Also note that fast interrupt context saving doesn't save the table pointer (or FSR) vaules, so you need to do it manually in your code. > Also, shouldn't this problem also apply to FSRnH / FSRnL? How come the > manual doesn't mention this at all? > In the example in the PIC18 Reference, the manual only mention to save > WREG/STATUS/BSR... See above- ANY register which gets changed in the interrupt handler is a good candidate for backing up, if it's important that its state be known elsewhere in the code. This includes all configuration registers, pointers, etc. Also, remember that the objective of the manual is NOT to teach you how to program a PIC. The objective of the manual is to provide explicit definition of the behaviour of the PIC under certain conditions. They assume the broad strokes, generally, but attempt to fill in the blanks. I think a better question is, why do they provide an example of context saving at all? Mike H.