Gavin Jackson <PICLIST@MITVMA.MIT.EDU> wrote:

> I am using a 74A for some async TX/RX and would like to know what
> the best way of configuring BRGH is? Either high speed or low speed
> as I understand that there is a bug in one of the settings.

    Gavin:

    The bug is with BRGH = 1 (high speed); if you can get away with
    it in your application, set BRGH to 0 (low speed).

> I also wrote some code and ran it in the MPLAB ver 3.22.02. The
> code is for the 74A is as follows:
>
> ORG     00H
> BSF     STATUS,RP0
> CLRF    TRISA
> CLRF    TRISE
> BCF     STATUS,RP0
> BSF     PORTA,0
> BSF     PORTA,1
> BSF     PORTA,4
> BSF     PORTE,0
>         END
>
> Non of these instructions seem to set the bits, or that's what the
> simulator indicates. The only bit which sets is Port A bit 4. Is it
> something to do with their alternate functions?

    Yes.  RA0-3, RA5, and RE0-2 default to ANALOG INPUTS on power-up
    (see Section 13 and Figure 13-2 in the 16C74 Data Book).  You
    can make the pins outputs by writing to the TRIS registers (as
    you've done in your program), but when your program (or the
    simulator) reads from those pins, they'll always read low
    because the input side of each pin is still connected to the A/D
    circuitry.

    Note that the pins actually ARE going high; the simulator just
    can't tell that they are, because it always sees "0" when it
    looks at an output pin that's still configured as an analog
    input.

    To make the pins digital outputs AND digital inputs, so the
    simulator will accurately show the state of the output pins,
    write "xxxxx11x" to ADCON1 (register 0x9F).  The following
    modification to your code will do it:

        ORG     00H
        BSF     STATUS,RP0
        CLRF    TRISA
        CLRF    TRISE
        MOVLW   00000110B       ;New line.
        MOVWF   ADCON1          ;New line.
        BCF     STATUS,RP0
        BSF     PORTA,0
        BSF     PORTA,1
        BSF     PORTA,4
        BSF     PORTE,0
        END

    Also, note that RA4 is an open-collector output... If you really
    want it to go high in your circuit, you'll need to ad a pullup
    resistor to it.

    -Andy

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