Date   : Tue, 26 Feb 2002 10:42:43 -0000
From   : "Kris Adcock" <krisa@...>
Subject: Re: Circuits involving a SAA5050?

> -----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?
>
>
>
> On Mon, 25 Feb 2002, Kris Adcock wrote:
>
> > 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
>
> Getting the timing right on an SAA5050 looks a real pig of a
> job.I'd have
> thought it was easier to get a high speed ROM and put the
> character set in
> it yourself then have 2x1k banks of RAM.
>
> 1k is the one being output by video engine which blindly looks up the
> result in ROM (with some latches in the state machine to take
> into account
> when you're in double height/blink/etc mode)
>
> 1k is one that the Risc PC can write to over the podule bus
>
> The two RAMs can then be switched between during a VSync
> flyback.Quite an
> interesting project I think.
>

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.