Date   : Wed, 27 Feb 2002 13:51:32 +0000 (GMT)
From   : Sprow <rsprowso@...>
Subject: Re: Circuits involving a SAA5050?

On Wed, 27 Feb 2002, Kris Adcock wrote:

> > -----Original Message-----
> > From: owner-bbc-micro@...
> > [mailto:owner-bbc-micro@...]On Behalf Of Chris Thornley

[snip Archimedes suggestion]

> > the chip is
> > connected to the CRTC controller chip in the BBC.
> >
> 
> The /output/ of the 5050 goes through the CRTC. 

Close,the output goes via the video ULA which chooses whether the CRTC or
SAA5050 is being shown.

> I'm more interested in what
> is providing the character bitpattern going /into/ the 5050, as I had always
> been under the impression that the characters were stored in the chip
> itself.

Indeed they are (see my earlier comment).

> Reading the SAA 5050 datasheet :
> 
> "The basic input to the SAA5050 is the character data from the teletext page
> memory. This is a 7 bit code. Each character code defines a dot matrix
> pattern."
> 
> Now, this seems to mean that the bitpattern for each character is supplied
> from elsewhere. Or does it mean that the input 7 bit code (ASCII) is used as
> an index into the data containing the dot matrix pattern?

The latter - you feed it ASCii data from a 1k ram,and in the case of a
podule you have a 2nd 1k of RAM which is the one you can write to.Then
switch between then on a flyback.

On the beeb this isn't an issue as the video and processor are synchronous
though running on the opposite phase of the clock.

Sprow.