68661 EPCI woes. Can anyone help?

dwight elvey dkelvey at hotmail.com
Mon Aug 13 10:10:13 CDT 2007

>From: "river" <river at zip.com.au>
>I've connected a 68661 EPCI (Enhanced Programmable Comms Interface) chip to
>an 8085A system. It is the "C" variant, so it requires the 5.0688Mhz Xtal.
>It's all connected right, and I checked with the CRO and it is being
>selected and the 5.0688Mhz TTL signal is accurate and clean.
>However, the terminal it connects to just gets garbage or sometimes doesn't
>show anything at all. I put a CRO on the Tx line and the signal is RS232
>compliant and data is going out. I tried the terminal with a working system
>and it works fine.
>I think I may be programming the chip wrong. I assume, according to the
>literature, that I set up the Mode register for Internal clock x1 internal
>clock for async comms. I then program the necessary 4-bit code for the 
>I want. However, the datasheet shows the speeds with x16 clock. So, I tried
>programming it with x16 internal clock, but still no luck. Also, the data
>sheet says when using async that you use the x1 clock - which I did
>originally - yet the sheet shows the speeds with x16 clock. I'm very
>All I want is 2400bps, 7data, no parity, one stop bit async.
>Has anyone used the 68661?

For async you should be using
X16 and the correct divide settings for x16. 1X is only for sync com.
You need to compensate the divide when doing X16. It is different
than 1X.
If you have a scope, you can calculate what the rate is.

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