Date   : Sat, 21 Mar 1998 08:15:16 +0000
From   : Mike Tomlinson <jasper@...>
Subject: Re: Wierd Hardware Problem!

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>

>Can someone confirm that the beeb had one of these that ensures a clean
>boot up when power is applied?  And that it is connected to the 76489
>sound chip??  I lost my Beeb circuit diagram out of my AUG years ago
>(regretably).

According to my cct diagram, the reset line is not connected to the
76489.  The 76489 is enabled/disabled by the system 6522 (ic3) through a
74LS259 (ic32) though.

Yes, there is a power up reset.  It's generated by ic16 (555) and ic33
(74LS04) to provide RST and -RST.

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?)

>>The burrrrrrrrr-beeep of the BBC's power up sequence was IMHO one of
>>its most endearing features.
>I concur!  Must get round to making pcBBC do that too....

Sample the BBC power up beep and play it as a .wav file?

-- 
Mike Tomlinson
+++  Divide by cucumber error.
     Please reinstall universe and reboot  +++