Hi! You're right, thats what I meant, and I also was in the (now obviously wrong) belief that the compiler had to take care of that problem. At first, I thought there must be a simple solution, but smarter people than me haven't found one ;-) Perhaps one of th cheapest solutions in PICs is like that: var word changingvar bit change_happened procedure isrpragma interrupt changing=new_value change_happened=trueend procedure ... REPEAT change_happened=false value=changingUNTIL ! change_happened It costs only one bit of RAM, one (or at most three) code words in the interrupt and also the reading does not have to compare bytes. Greets,Kiste
Am Samstag, 28. November 2020, 00:24:23 MEZ hat zmafoox <[email protected]> Folgendes geschrieben: You seem to be confusing a volatile operation from an atomic one - volatile means just what it says, the results will not be cached, and a write will happen once. Since the PIC is byte oriented, any read of a value greater than a byte has to be done in multiple operations. There is no way to make these operations atomic, so you need to do this yourself. It's been a while, but if I recall the order is:read the LSBread the MSBread the LSB2 (read the LSB again) if LSB2 < LSB read MSB again, I might be wrong. It's been a long time... On Thursday, November 26, 2020 at 10:51:12 PM UTC-8 Kiste wrote: Sorry... This isn't in fact the proper list for compiler issues, is it? This one is probably more severe. I'll start with the example. This is the data I've received: ?2807 2808 1?2807 2809 2?2807 2810 2?2807 2811 4?2807 2812 4?2807 2813 6?2807 2814 7?2807 2815 8?2807 2816 264 It's generated from this part of code, running on an PIC18F27K42. extra_time is increased in an interrupt every 16 seconds. It is declared as var volatile word, so the compiler should make sure that the bytes of the variable are in a consistent state: diff=word(extra_time-last_pulse_time_uw) if print_debug then print_debug=false serial="?" print_word_dec(serial,last_pulse_time_uw) serial=" " print_word_dec(serial,extra_time) serial=" " print_dword_dec(serial,diff) print_crlf(serial) end if Let me repeat some of the results above in hex: AFE-AF7 =7AFF-AF7 =8B00-AF7 =108 So, what has happened? The last line is clearly wrong. The subtraction was done just as extra_time was in the process of increasing. The low byte was processed before, the high byte was processed after the interrupt fired. So the actual subtraction done was BFF-AF7, with the result of 108. I've had a look at the asm file. Both bytes of the variable are just handled one after the other, both in the main program and in the interrupt service routine. The "volatile" declaration does not seem to do anything. Greets,Kiste -- You received this message because you are subscribed to the Google Groups "jallib" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/jallib/d6a6d597-2dd3-4a8c-be00-83d18a45a7a7n%40googlegroups.com. -- You received this message because you are subscribed to the Google Groups "jallib" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/jallib/519487440.2271703.1606547339764%40mail.yahoo.com.
