6502 Question

[guddler]

Ok, maybe this is a very noob question but you'll have to excuse me - I'm super tired!!

Can a 6502 reset itself without influence of the reset line?? Yes that sounds kinda dumb but I've got a MC pcb here that I can clearly see is resetting approximately once per second yet the reset line only goes low every third time. The watchdog is not disabled but all the same, note the reset line only going low every third time.

Errr? I thought the 6502 would lock up if the code crashed and the reset line didn't do it's thing.

Oh, unless there's a code routine that's performing a jump to a vector that's sending it off to it's start vector? Or am I talking complete sh*te?

Have to admit, this board is doing my head in! Never did like Missile Command. Apart from maybe at school in the youth wing over me pasty at lunch.

 

[DanP]

Presumably you can jump/branch to the reset Vector address in code? I think you're on the right lines, I think the processor will just keep loading instuctions even if they're garbage (well from what I've seen using MAME debug mode) until it either reaches the Reset Vector Address or a reset is initiated on the hardware line (by the watchdog for example). What happens to the reset sequence if you disable the watchdog? Does it go into a regular pattern?

Dan

 

[guddler]

I'll have to check again tonight.

 

[trm]

Looks like the 6502 has a reset vector at 0xFFFC and 0xFFFD so if you've got a Program Counter increment loop going on somehow then it seems entirely possible that the CPU is doing an internal reset. I can't find any documentation that states whether the /RESET line would dip, but I'd expect it doesn't matter as the CPU knows internally what is going on and it'd have to possibly compete against pull-up resistors or at least whatever gate is driving the /RESET pin.

At this point I'd probably slap the logic analyser onto the address bus and whatever M1 (well, the 6502 equiv of the Z80 M1 line) & READ equivalents are present and see what the address bus is doing when the machine resets.

 

[guddler]

Ok, I think I'll start up a new thread for the Missile Command shortly...

[EDIT] Or maybe tomorrow (film time)!
guddler2010-09-09 21:38:43