6551 "user" speed

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Joined: Aug 4 2005
Posts: 13

Is it possible to get 115200 bps out of a 6551?

Looking at the documentation on the 6551, there is a 4 bit field in the control register to set the baud rate. The docs specify using an external 1.8432MHz crystal in order to get the usual 1980s baud rates for codes 0001 to 1111, but code 0000 is listed as "16x external clock".

Now, I doubt they actually have a PLL or anything in there to actually multiply the external rate. I suspect what was meant was something like "the external clock will be 16x the effective baud rate". This would mesh w/the way the 6850 actually does a divide-by-16 to center the bit sampling correctly.

If this is true, then 1.8432MHz divided down by 16 would give a baud rate of 115.2kbps - has anyone actually done this? At N81 that's only 86.8usec/char, so clearly a tight loop of assembly code would be required to handle it, but the question is, can the 6551 actually do it? Or what DOES it do at 0000?