Date   : Sat, 21 Mar 1998 20:01:17 GMT
From   : Theo Markettos <theomarkettos@...>
Subject: Re: Wierd Hardware Problem!

On Sat, 21 Mar 1998 08:15:16 +0000, jasper@... said:

> In article <DJnvfAA+auE1EwtT@...>, Stuart William
> McConnachie <stuart@...> writes
>
> >But haven't we established that, as the keyboard is okay, the next
> >logical step is that the slow data bus is *NOT* working?
> >
> >I think that the "burrrr" is caused by the sound chip before it receives
> >the "reset" signal which is generated by a "power-up detect" circuit.
>
> The slow data bus is port A of the system VIA.  I'd always assumed that
> while the machine was starting up, before the OS initialised the system
> 6522, there was rubbish on the slow data bus which the sound chip
> interpreted to produce the characteristic "burrrr".  I'm probably wrong
> (again) <grin>

Probably :-)  Think about it - if the chip was getting rubbish, why would
produce the same sound every time?  It would be equally likely to go berrrr,
beeeee, or even shhhhh (don't you just love email sound effects...) Looking
at my rapidly disintegrating circuit diagram, there is only the
keyboard, the 76489, the TMS5220 speech processor (if present), and the
system 6522 on the slow data bus.  Since the VIA powers up with ports as
input, it would seem likely that either inside the keyboard, 76489 or the
6522 there are some pull-ups, which force the data bus all high on a reset.

> There appears to be a secondary reset, -RSTA, generated by D1, R20 and C10
> This is fed to pin 34 of the system 6522 (RST).  I don't know why this has
> a separate reset:  ISTR the service manual explained why, but cannot
> remember the reason (any chance of having a look in the manual for us,
> Mark?)

This is a standard power-on reset system - you're right in that it only seems
to go to the 6522, although my diagram is a bit small so I might have missed
somewhere.  My first thought we be for some of the state machines that need
to stay working over a soft reset - DRAM refresh being the main one, but as
far as I can tell they don't use it.  Incidentally the Beeb is one of the
few machines that can retain it's memory on a Break because the DRAM refresh
isn't suspendend on reset, unlike most other machines - a particularly nice
feature.

-- 
Theo Markettos         Home:   theomarkettos@...          
Liphook                        Work:   marketto@...                     
Hampshire
UK                     Web site, including Acorn backup software
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