MC68B50 trouble

[Wawavoun]

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

 

[per]

Datasheet lists 1500 KHz as maximum data clock source for this particular model, so you're overclocking it just a little. You might have snagged on a edge-case timing case internal to the chip which you might not have hit at 2000 KHz E clock.

 

[Wawavoun]

On 6502.org forum a user say his MC68B50 work with the same configuration as mine (with the same 1843.2 KHz signal on E pin and rx-tx clock pins)...

I will continue to search !

 

[Hugo Holden]

On 6502.org forum a user say his MC68B50 work with the same configuration as mine (with the same 1843.2 KHz signal on E pin and rx-tx clock pins)...

I will continue to search !

But, it might pay to consider that if you are on the edge of a spec, there will be some specimens of the same type number IC which will work, and some that will not.

The only way you would know would be if the person on 6502.org sent you his chip and you tried it in your setup, whether it was your setup that is in some way different, or your particular specimen of the chip cannot handle the high range data clock rate.

If you are searching to explain a marginal condition, it may well be a very long search.

 

[Wawavoun]

Yes I agree...

I just want to be the most possible sure that the problem is not elsewhere. I am a very distracted so check and recheck is my only option.

 

[Wawavoun]

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.