[Coco] Bitbanging limits

[Roger Taylor]

I changed the topic since this is getting into sort of a project to 
see just what the limit is for the CoCo's bitbanger port (with a 6809).

894886.25 / 8 cycles per bit yields ~111861 bits per second.  I'm not 
bothering with 6309 code which *would* give faster possibilities but 
not everybody has the upgrade.

In 8 cycles you can LSR <34 which is 65314, pushing bit #0 into the 
carry bit, followed by a RORA which pushes the carry bit into the 
data byte.  But, to prevent graphics distortion on a CoCo 1/2, you 
might have to first set up the PIA's DDR so that bits 1-7 are input 
bits during the receive routines.  This way, an LSR to that register 
shouldn't corrupt anything.

You'd need ~80 cycles for a Start-8-Stop character format.  ~88 for 
an extra stop bit which should give you time to loop back and check 
for a new start bit.

The truth is, 57600/115200 bps is probably our best bet for 
compatibility with other PC's.  OS-9 would get the higher speed since 
it's in ~2mhz mode.


-- 
Roger Taylor