Date   : Tue, 26 Feb 2002 13:12:11 +0000 (GMT)
From   : Sprow <rsprowso@...>
Subject: Re: Circuits involving a SAA5050?

On Tue, 26 Feb 2002, Kris Adcock wrote:

> > -----Original Message-----
> > From: owner-bbc-micro@...
> > [mailto:owner-bbc-micro@...]On Behalf Of Sprow
> > Sent: 25 February 2002 15:16
> > To: bbc-micro@...
> > Subject: Re: [BBC-Micro] Circuits involving a SAA5050?
> >
> > > For several years now I've been maintaining a RiscPC system
> > > with a teletext decoder card in it.
> >
> > [snip]
> >
> > > I'm looking to build my own
> > > 'serverely simplified' version of the card. Using an
> > SAA5050 chip seems to be an obvious choice

[snip]
 
> I've had a proper look over the SAA5050 datasheets, and realised I'd
> misunderstood it the first time. I had in mind that I could just send the
> ASCII to the chip, and the chip would turn it into a video signal. Looking
> over it now (as someone else has pointed out) you send the /bitpattern/ to
> the chip.
> 
> What I had in mind is a circuit which just allows me to send the ASCII data
> once, which it will then display until I send the next set of data. This is
> how the existing podule works, which communicates with the RiscPC via the
> IIC pins.

Silly question,but what's the card called? Would getting a second card
help?
Alternatively,look for a Mode 7 board by Jafa Systems for the Electron -
it's a Mode 7 upgrade board for the Elk,which must be quite small.

The SAA5050 when used in the BBC micro is used in parallel loading mode,so
it is effectively taking the ASCII value from DRAM and outputting RGB
video.

To make this into a stand alone board you'll need to supply your own
RAM (see my earlier 2k bank switching suggestion) and some interface to
the podule bus - be it IIC or parallel - and something to repeatedly feed 
the SAA5050 data (the job that the CRTC does normally).

The timing still looks like a pig,as I commented earlier.

Note that the input to the SAA5050 is only 7 bits,whereas (for example) to
select double height mode you do a VDU141 on the beeb.
This is because VDU13 (which is what the SAA5050 actually sees) is
swallowed up by the VDU driver as a carriage return.
This is also the reason why the whole character set is aliased from 160
upwards...
Sprow.