Watchdog that should be taken out and shot

I don't remember seeing a password protected one but there are a few that only allow tickles in a specific interval. So if it's tickled at t then if you tickle outside of t + t1 +/- delta it resets. Now your accidental reset sequence has to match the correct frequency not just be faster than x.

Robert

Hello Robert,

I have seen those as well. However, that may cost you a timer or at least one of the CCRs.

Yes, true. But that doesn't necessary put all the CPU hardware in the same state as a hardware reset. You'd think so, wouldn't you, but it doesn't.

If you've done any hardware design, you'll know about excluded states (which are hardware states that normal sequencing won't exit from). Following an EMI glitch (not normal software behaviour, or even software runaway), the state of the CPU is effectively random. If you're really unlucky, you could wind up with an excluded state somewhere on the CPU. (Or more likely a microcontroller - more on-chip subsystems.)

I've seen this happen in practice, with code which did indeed reset everything as per the datasheet. The CPU partially worked, but not completely. This wasn't my design, and I was trying to convince the designer to use a hardware watchdog, when this came up. Result: he used a hardware watchdog.

Steve

Hello Steve,

Yes, it can happen. But it shouldn't. Unstable states are often a sign of a not too careful hardware or chip design. I mostly design around discretes and logic. There I pay meticulous attention to undefined states. IOW that there is always some way out. Mostly this is easy, for example when a register can set five different parameters I make sure that the unused states 5 through 7 map to something meaningful and don't just point to lalaland.

If in doubt always have the watchdog do a full HW reset.

That's the beauty of design reviews. Even if it's just two people in an informal review. All of us can make mistakes.

"Tom Lucas" wrote in message news: snipped-for-privacy@despina.uk.clara.net...

Oooh, we hate when that happens.

Appears to be wrong only on the one page. In the register descriptions it is correct.

Scott

Ah Ha! Tom, you didn't RTWFM! W->Whole

~Dave~

I'm sorry I didn't report the addresses that I poked when trying this out. Not that it would have helped much since it (correctly) says FFFE2000 in my Sharp 754xx manual. It might have prompted you to look again though. But why use different addresses on different chips?

Wrong information is worse than no information at all. At least in the latter case you know that you don't know.

Peter

If we wanted it to be easy we wouldn't have become engineers :-D

Actually I became an engineer because it was easier than understanding humans. It was also easier and better paid than what I was educated for - cosmology.

Peter

Well I think it is safe to assume that no-one got into it for the chicks ;-)