Date   : Tue, 24 Feb 1987 20:55:42 GMT
From   : Herman Schuurman <herman@ti-csl.csnet>
Subject: Re: Fact sheet or info in the 8251 USART

in article <1260@beta.UUCP>, dzzr@beta.UUCP (Douglas J Roberts) says:
> 
>   Does anybody know the commands for the 8251 USART?
> I would like to tell the USART in my Z-80 CPM machine to
> send serial data with 1 start bit, 7 data bits, 1 parity bit, and
> one stop bit.
> 
>   The initialization routine in my bios sends 0CEH to the serial status 
> port which sets the UART to 2 stop bits, 16xclock, 8 data bits, no parity.
> 
>   If anybody knows the other control bytes, mucho appreciado!
>   
> -- 
> Douglas J. Roberts, dzzr@lanl.gov
> Los Alamos National Laboratory
> Box 1663, F602
> Los Alamos, N. M. 87544   

               Mode format - async mode

       0-1     Clock rate: 00 = sync mode, 01 = 1x, 10 = 16x, 11 = 64x
       2-3     Char length: 00 = 5, 01 = 6, 10 = 7, 11 = 8 bits
        4      Parity enable: 0 = disabled, 1 = enabled
        5      Parity: 0 = odd, 1 = even
       6-7     Stop bits: 00 = invalid, 01 = 1, 10 = 1.5, 11 = 2

               Command instruction

        0      Transmit enable: 1 = enable
        1      Data Terminal Ready: 1 = ~DTR is forced low
        2      Receive enable: 1 = enable
        3      Send break character: 1 = TxD "low", 0 = normal operation
        4      Error reset: 1 = Reset PE, OE and FE flags
        5      Request to send: 1 = ~RTS is forced low
        6      Internal reset: 1 = enter mode instruction format
        7      Enter HUNT mode: 1 = enable search for sync chars

               Status byte

        0      TxRDY: 1 = Transmitter ready for next character
        1      RxRDY: 1 = Receiver ready - character waiting
        2      TxEMPTY: 1 = Transmit buffer empty
        3      PE: Parity Error
        4      OE: Overrun Error
        5      FE: Framing Error
        6      SynDet: 1 = Sync character detected
        7      DSR: 1 = Data Set Ready

Bit #0 is the least significant bit; #7 is the most significant bit.

The mode byte for 1 start bit, 7 data bits, odd parity, and 1 stop bit
would be 05Ah (which includes the 16x clock factor too).


-- 
Herman Schuurman       ARPA:  herman%TI-CSL@CSNET-RELAY.ARPA
Texas Instruments Inc. CSNET: herman@TI-CSL
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