[Coco] 6551 at 115200 bps

[Mike Pepe]

Roger,

If you were to take the 1.8432MHz clock and feed it right into the
6551's TxC and RxC (clock) pins and set the chip to external clock mode,
you'd get 115,200 bps out of it.

It's guaranteed up to 125k bps, meaning the fastest clock it can accept
on those pins would be 2MHz.

I'd guess the internal crystal oscillator isn't capable of reliably
operating at 3.68MHz- but it may be possible to feed it with an external
can oscillator. The guts of the thing are indeed capable of 115,200.


> -----Original Message-----
> From: coco-bounces at maltedmedia.com [mailto:coco-
> bounces at maltedmedia.com] On Behalf Of Roger Taylor
> Sent: Sunday, January 18, 2009 9:31 AM
> To: CoCoList for Color Computer Enthusiasts
> Subject: [Coco] 6551 at 115200 bps
> 
> 
> 
> Has anyone noticed on Sock's website he mentions that doubling the
> 6551 crystal value to 3.6864 mhz can be done but setting the 6551 to
> the ExternalClock/16 mode would yield 115200 bps, then he says:  "
> this *almost* works! "   Why wouldn't it work?
> 
> We've proven that the CoCo can send and receive through the BITBANGER
> port at 115.2 bps, and believe it or not - 230.4 kbps.
> 
> Maybe Sock means that IRQ can't be triggered that fast and the CoCo
> keep up?  Why not?  The 6551's IRQ goes to the CART signal which is
> tied to FIRQ.  A circular buffer in the FIRQ service routine could do
> a quick FIFO store to a buffer and it should work?  If not, the
> crystal should probably be dropped to 1.8432 / 2  to yield a 57600
> bps mode which should work.
> --
> Roger Taylor
> 
> http://www.wordofthedayonline.com
> 
> 
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