Hi All,

I have a serial interface for a home build Flex computer made with a MC68B50.

This chip need a bus timing sync clock named E and a separate rx-tx clock.

The rx-tx clock can be provided by an output of a 6522 via (CB1). In this case I use a clock at 153600 Hz and a 16 divider in the acia giving a 9600 baud transmission.
In this case everything works well.

I can also use the E (bus timing sync) signal as rx-tx clock. This signal run at 1843,2 KHz. Using the same divider this should give a 115200 baud transmission. This did not work.

I have already use this specific MC68B50 at this transmission speed (1843.2 KHz rx-tx clock and 16 divider) but with a higher E frequency (2 MHz).

I cant see where is the problem except that using the same clock for both functions is problematic but I dont find anything on this point into the datasheet.

Any opinions and may be solutions ?

Thanks and regards.
Philippe

I have used 68B50 without issue at 115,200 using the same ph2 clock at 1.8432MHz applied to E, RXCK, and TXCK (pins 14, 3, and 4). I've also used the same ph2 to tristate the processor bus and controls in the same setup, again without issue. So I am at a loss to explain your issue.

Are you able to read the ACIA registers?

Neil

I assume the parallel bus part of the system is working because I can use the acia at 9600 baud.
Change the rx-tx clock dont change anything to this part.

May be I must add that I use a small uart ttl to usb interface. But again I have already use this interface at 115200 baud so the problem could not be here.

Well.... Crystal frequency is 4xE so E is not 1843.2 KHz but 921.6 KHz --> 57600 baud.

Problem solved !

Thanks to those that try to help.