You can get them running plenty hot by simply wiring a DIP switch to the
address bus with NOP encoded into it using the R/W line as the low to which
the bus is switched. Each time it reads the bus, that's what it will see. It
will require pullups on the order of 4.7K to provide the high level. That's
one way of determining how fast a particular chip can be run, though you have
to try it with a wide range of instructions. With the 6800, of course, you
have to run the clock generator with it as well, however, and that may not
work as fast as the CPU before it goes out of specified limits.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <cctalk_at_classiccmp.org>
Sent: Friday, June 07, 2002 12:03 PM
Subject: Re: [CCTECH] Interesting tidbit on 6502
> >
> > I don't see an HCF in the Mot listings of their instruction set.
> >
>
> Many processors (and other chips for that matter) have undocumented
> instructions used for factory testing.
>
> > I've heard about this fiction over several decades now, and, surely, it
must
> > be clear to you that there's no way the microprocessor can cause that
effect
> > without the aid of external hardware.
>
> It's not clear to me. What I am thinking of is the following :
>
> The test instruction puts the processor into a mode where one of the
> external lines toggles at the clock frequency, whereas normally it only
> toggles at (say) 1/4 of the clock frequency (at most). This signal is
> driving a bus line that has considerable capacitance to ground. At the
> higher freqeucny, the drive buffer passes a higher current (to
> charge/discharge that bus capacitance) and overheats.
>
> I doubt you could get the chip to catch fire, but I think it would be
> possible to get the chip to run hot.
>
> -tony
>
>
Received on Fri Jun 07 2002 - 14:01:03 BST