Much Help needed with 8251 USART Problem.......

[analopconst201]

Hi friends.

I have been trying to debug an 8251 UART for 5 days now. In fact 3 of them and they all error the same way. I got 1 from a different source, and the last 2 from a single source.

The problem is when receiving characters from my PC, into a simple system with a CPU + UART.

Most characters are received correctly, but 1 out of 10 or so chars are junk.

I have set up the USART for asynchronous communications, 8 data bits, 1 stop bit, no parity.

I configure Putty for the exact same setting.

I have even tried to test the USART on a breadboard, and do it manually. I still get the same issue.

I am initializing the USART correctly I think, the mode byte is 0x4D, and command is 0x37. This is exactly for 8 data bits, 1 stop, no parity, etc.


I am out of ideas, and this is driving me nuts. The issue was happening with my first 8251, so after a while I assumed it was damaged. Bought another 2. Same issue.

Same issue whether when USART used with CPU, or breadboard.

The only thing I can think of is that I am initializing it incorrectly, but that would be unlikely. I have tried many many times.


If someone has any ideas please let me know. I am getting desperate now.

Thank you and best wishes

Paul

 

[bertus]

Hello,

What is the speed you are using?
This page tells me that asynchronous mode the speed is lower as 20 Kbits/s :
https://www.eeeguide.com/serial-communication-interface-8251/

Bertus

 

[analopconst201]

Hello,

What is the speed you are using?
This page tells me that asynchronous mode the speed is lower as 20 Kbits/s :
https://www.eeeguide.com/serial-communication-interface-8251/

Bertus

Hello Bertus. I posted this on EEVBlog Forum and a guy gave the answer. Basically it wont work at 1x baud. Here is the link: https://www.eevblog.com/forum/projects/much-help-needed-with-8251-usart-problem/

Thank you so much Bertus. Best.

 

[bertus]

Hello,

Thanks for the link. It might also help others with the same problem.

Bertus

 

[Harald Kapp]

8251, from 1980, really?

The chip is, as its name says, an asynchrounous transceiver. Meaning that the asynchronous data needs to be synchronized. This is where the 16 x oversampling comes into play. A standard technique with these UARTs.