[ KAY*FOG RBBS | INTE8251.ART | published 05/30/85 | 379 lines  13k ]

INTEL8251:  Programming the Intel 8251A USART
       by:  Ed Greenberg
            Kay*Fog
            CompuServe:    76703,1070
            MCIMAIL:       EDG

Preface
=======

This document was inspired by my collection of Intel data books.
Of the three databooks, only one contains the following
information.  Most microcomputer users do not have this
information in their computer manuals and do not have access to
the Intel manuals.  Murphy's law also states that the hobbyist
will not have the required manuals at midnight on Saturday when
he is busily trying to debug his communications program.
[Press ENTER key for more]


Introduction
============

In order to communicate via a serial port, a computer is equipped
with a device called a Universal Asynchronous Receiver/Transmitter
or UART.  This device converts a byte (written to an output port)
to a series of bits sent one at a time (serially) from a
communications port.  It also scans the input line on the serial
port and, detecting the beginning and end of each character,
assembles incoming characters and presents them on an input port.

Other names for this part are SIO's (serial input/output) or
Communications adapters and USARTS (Universal Synchronous/
Asynchronous Receiver/Transmitters).

This document deals with one common part - the Intel I8251a
USART. That usart is used in the North Star Horizon, the Morrow
Micro Decision, and other common computers.  This discussion deals
with the 8251A only in asynchronous mode.  It is the mode used
when communicating with a printer, a modem, a terminal and
usually, another microcomputer.


Using your USART
================

In order to use a communications port with a usart, one must
first initialize it.  By doing so, one sets parameters that tells
the USART how to do it's job.  One defines the parity, the number
of stop and data bits, and the divisor (if any) to be applied to
the incoming clock signal.

After the USART is initialized, one can read and write characters
on the data port of the usart. One can also check the status of
the usart on the status port.

Things to know before you start
===============================

Before you can write code for your usart, you must know the port
locations on which the usart appears.  There are two ports,
called the DATA port and the STATUS port.  Commands are written
to the status port, and the condition of the port is read from
it.  Actual characters are written to and read from the data

port.  A good place to find the port numbers is a published I/O
MAP in your manual.  Another place is in a program listing of the
BIOS or a communications program.  You will see something like
this:

STATUS    EQU       80H
DATA      EQU       81H

or the like.  Sometimes, the word MODEM is worked into the label
(e.g. MODDATP for MODem DATa Port.)  Be certain that you find the
correct set of ports in the case where your computer has more
than one usart.

A Note About Interrupts
=======================

Some computers use the facility called interrupts to signal the
processor that a character is ready.  In this case, it is
important for the amateur programmer to avoid messing up the
interrupt structure of the machine.  You should not read or write
your USART directly when it's interrupt is enabled.  Doing so may
cause spurious interrupts of the processor.  The result is that

nothing will work.  Determining whether your computer uses
interrupts on the communications line is beyond the scope of this
document.

Initializing the usart
======================

The following code fragment is taken from the MEX overlay for the
Morrow Micro Decision.  MEX is written and copyrighted by Robert
Fowler.  The comments are my own.

INITMOD:  ...
          ...
          MVI   A,087H          ;Take the usart out of
          OUT   MODCTL1         ; the condition for
          OUT   MODCTL1         ;  setting the mode (*)
          MVI   A,40H           ;Put it back into that
          OUT   MODCTL1         ; condition.  This resets
                                ;  just about everything for
                                ;   new commands.
INITMOD1: MVI   A,4EH           ;This is the MODE BYTE (*)
          OUT   MODCTL1         ;Send it to the  control/status port

          MVI   A,37H           ;This is the COMMAND BYTE (*)
          OUT   MODCTL1         ;Send it to the control/status port
          IN    PORT            ;Clear out the DATA port

          RET                   ;Return

(*)  See the definition below

Input and Output of Characters
==============================

Below are sample routines for input and output of characters on
the 8080 (or Z80) using an 8251A.

;Input character routine.  Character is returned in A.
INPUT:    IN   STATUS         ;Get the status of the usart
          ANI  2              ;turn off all bits but RxR (**)
          JZ   INPUT          ;go back and check again because
                              ; there is no character ready
          IN   DATA           ;There is a character ready,
                              ; so go get it.
;OPTIONAL STEP

          ANI  7FH            ;Remove the high bit

          RET                 ;Go back to the caller

;Output character routine.  Character is provided in C.
OUTPUT:   IN   STATUS         ;Get the status of the usart
          ANI  1              ;turn off all bits but TxR (**)
          JZ   OUTPUT         ;Go back and check again because
                              ; the USART isn't ready for another
                              ;  character.
          MOV  A,C            ;The USART is ready, so get the
                              ; character in A for output
          OUT  DATA           ;Now output it and...
          RET                 ;Return to the caller.

(**)  See the definition of the Status byte below

The remainder of this document is concerned with defining the
actual bytes sent to (and received from) the usart.

-----------------------------------------------------------------


Format of the Mode Byte
=======================

  7    6    5    4    3    2    1    0
+----+----+----+----+----+----+----+----+
|s(2)|s(1)| ep |pen |l(2)|l(1)|b(2)|b(1)|
+----+----+----+----+----+----+----+----+

Content of the Mode byte
========================

s(2) and s(1) - the stop bit indicators:
----------------------------------------

        s(2)    s(1)    meaning
        ----    ----    -------
         0       0      Invalid
         0       1      1 stop bit
         1       0      1.5 stop bits
         1       1      2 stop bits

ep - the parity bit

--------------------

        ep              meaning
        --              -------
         0              Odd parity is generated and checked
         1              Even parity is generated and checked

                Note: this bit is only active when pen is set
                to 1.

pen - the parity enable bit
---------------------------

        pen             meaning
        ---             -------
         0              parity disabled
         1              parity enabled

l(2) and l(1) - the word length bits
------------------------------------

        l(2)    l(1)    meaning

        ----    ----    -------
         0       0      5 bits
         0       1      6 bits
         1       0      7 bits
         1       1      8 bits

                Note:  For ASCII, we usually use only 7 or 8 bits.
                Hams and TTY/TDD (for the deaf) use 5 bits.  The only
                thing that 6 bits is used for is a colloquialism for
                seventy five cents.

b(2) and b(1) - the baud rate divisor bits
------------------------------------

        b(2)    b(1)    meaning
        ----    ----    -------
         0       0      synchronous
         0       1      1x
         1       0      16x
         1       1      64x

                Note:  This should be left alone.  Whatever

                your system comes equipped for... that's it.

Format of the Command Byte
==========================

  7    6    5    4    3    2    1    0
+----+----+----+----+----+----+----+----+
|EH  |IR  |RTS |ER  |SBRK|RxE |DTR |TxEN|
+----+----+----+----+----+----+----+----+

Content of the Command Byte
===========================

EH - the Enter Hunt Mode bit
----------------------------

This bit has no effect in async mode!

IR - Internal Reset
-------------------

        1 Returns 8251A to Mode Instruction format.


RTS - Request to Send
---------------------

        1 will force RTS high on the RS-232 connector.

        Note:  This assumes that the designer hooked all
        the signals up on the PC board.  This is not
        necessarily true.

        On some computers, where the port is wired backwards,
        this will control CTS rather than RTS.

ER - Error Reset
----------------

        1 Will reset error flags in the status word
        (PE OE and FE will be reset.)  See the definition
        of the status word below.

SBRK - Send a break
-------------------


        1 Will send a break

        Note: Usually, one sends a command with this bit set
        and then, after a delay that equals the length of a
        break, one sends another command that does not have
        the break bit on.

RxE - Receive Enable
--------------------

        1 will enable the receiver.  If you are going to disable
        the receiver for any reason, you should have a data book
        in front of you and know what you're doing.  This bit
        should almost always be on for any command you send.

DTR - Data Terminal Ready
-------------------------

        1 will turn on Data Terminal Ready at the RS-232 connector.

        Note:  This assumes that the designer hooked all

        the signals up on the PC board.  This is not
        necessarily true.

        On some computers, where the port is wired backwards,
        this will control DSR rather than DTR.

TxE - Transmitter Enable
------------------------

        1 will enable the transmitter.  If you are going to disable
        the transmitter for any reason, you should have a data book
        in front of you and know what you're doing.  This bit should
        almost always be on for any command you send.

Format of the Status Byte
=========================

  7    6    5    4    3    2    1    0
+----+----+----+----+----+----+----+----+
|DSR |SDET|RE  |OE  |PE  |TxE |RxR |TxR |
+----+----+----+----+----+----+----+----+


Content of the Status Byte
==========================

DSR - Data Set Ready
--------------------

        1 Indicates that DSR is low!  That's right, Low!

        Note that this bit may be (a) not wired at all or (b)
        wired to DTR or some other pin on the RS-232 connector.
        The only way to tell for sure is to look at a schematic.

SDET - SYNC Detect/ BREAK Detect
--------------------------------
(In the Intel documentation this is called SYNDET, not SDET.)

In async mode, this bit "will go high whenever an all zero word
of the programmed length (including start bit, data bit, parity
bit and *one* stop bit) is received." (***)  In other words, when
the other end sends *you* a break.

This bit stays high until a reset command is issued.


FE - Framing Error
------------------

This bit is set when "A valid stop bit is not detected at the end of
every character."

This bit stays high until a reset command is issued.

OE - Overrun Error
------------------

This bit is set when a character has been pending on the USART buffer
and another character comes in before the first one has been read.
The first character is lost and this bit is set to alert the CPU to
the problem.

This bit stays high until a reset command is issued.

PE - Parity Error
-----------------


This bit is set when a parity error is detected.

This bit stays high until a reset command is issued.

TxEMPTY - Transmit Buffer Empty
-------------------------------

"When the 8251A has no characters to transmit, the TxEMPTY ...
[bit will be set to 1.]  ... " (***)

RxRDY - Receiver Ready
----------------------

"This ... [bit set to 1] ... indicates that the 8251A contains
a character that is ready to be input to the CPU." (***)

This is the usual bit to test to see if a character is available.
Usually one sees ANI 2 (on 8080) when this bit is tested.

TxRDY - Transmitter Ready
-------------------------


"This ... [bit set to a 1] ... signals the CPU that the transmitter
is ready to accept a data character."

This is the usual bit to test to see if a character may be sent.
Usually one sees ANI 1 (on 8080) when this bit is tested.

----------------------- End of INTE8251.ART Text -----------------------
 