Em 20/02/2014 12:49, Mike snyder escreveu:
> Hi Isacc,
>
> Thanks for the info - looks like the switching time is pretty short so it
> should work - I will work on trying this out.. (to your point it definite=
ly
> looks like it would simplify the code significantly...)
>
> To answer your questions
>
> Do you use interrupts on the RX pin?
>
> No interrupts in use today on RX pin  but I agree it would make sense to
> move to an interrupt based scheme for the RX routine...


Perhaps you know already the method of doing bit-banged UART with
interrupts but I will explain here for the benefit of others that may
not know it:


Reception:

1) Set an interrupt on the falling edge of RXD pin;

2) In the RXD interrupt handler:
    a) Set a timer to overflow in exactly half a bit time and enable its
interrupt;
    b) Disable the RXD pin interrupt;
    c) Set a global bit counter to zero;
    d) Return;

3) In the timer interrupt handler:
    a) Sample the pin and save its value;
    b) Reprogram the timer to overflow in exactly one bit time;
    c) If the bit counter is zero:
        i) Check whether the received bit is a valid start bit (logic
level zero)
            - If not, it was a glitch, disable the timer and its
interrupt and re-enable the RXD pin interrupt;
            - if yes, increment the bit counter
        ii) Return;
    d) If the bit counter is between one and eight:
        i) Shift the bit to a static variable;
        ii) Increment the bit counter;
        ii) Return;
    e) If the bit counter is nine:  =20
        i) Check whether the bit is a valid stop bit (logic level one)
            - If not, re-enable RXD pin interrupt;
            - If yes, save the byte;
        ii) Zero the bit counter;
        iii) Disable the timer interrupt;
        iv) Re-enable the RXD pin interrupt;
        v) Return.


Transmission is much simpler, just set-up a timer to overflow in exactly
a bit time and shift the bits out on each timer interrupt.

=20
> What is the baud-rate?
>
> Its an odd baud rate 20600 bps


At this rate,, using interrupts a PIC32 running at 40MHz should spend 5
to 10% of CPU time in the UART routines. Perhaps less if you write them
in assembly.
I think it is perfectly doable and you still have almost 90% of CPU time
for the rest of the application.


> What is the processor?
>
> PIC32


That's great, because the PIC32  port of my SimpleRTOS is the most
stable, you could start using it right away.



Best regards,

Isaac

--=20
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