[Coco] 6551 at 115200 bps

[Roger Taylor]

At 05:20 PM 1/18/2009, you wrote:
>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.

With the 1.8432 mhz crystal connected at XTAL1 and XTAL2 where it's 
supposed to be, setting the baud rate to 0000 achieves the same 
thing, or am I missing something?

Are you saying the E signal could be sent to TxC and RxC to achieve 
57600 bps and 115200 bps based on whether the CoCo is at normal or 
double speed?

Also, if the 6551 has an internal crystal oscillator, why is the 
1.8432 crystal needed?  I keep seeing questions like this being asked 
about the 6551 and how to get faster or tweaked bitrates from it, 
mainly from old Apple and Commodore folks.








>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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>
>
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-- 
Roger Taylor

http://www.wordofthedayonline.com