UART software versus hardware and Interrupts?

[aaea]

Hi

I have two simple questions:
(a) Can a software UART transmit at 57.6kBaud using a 12MHz Xstal? and What is the fastest that the software UART can transmit with a 12MHz Xstal?
(b) If I have various interrupts, what will occur with the UART transmission?

Thank you in advance.
a.

[niktab]

i strongly recommend, do not use software uart simulation, especially if you use any interrupts. this probably will cause real mess.

[Ttelmah]

If you search the forum, you will find that some time ago, the 'instructions per bit' for the software UART was posted. I think 57600, is 'pushing it'. My memory was somewhere around 60 instructions 'worst case'. At 57600, on a processor at 3MIPS, you have 52 instructions per bit.
On interrupts, you _cannot_ have interrupts enabled while transmitting, especially at a rate like this (it takes more time than a single bit, to get into, and out of an interrupt handler, even 'doing nothing' in the handler). You can though use the 'DISABLE_INTS' option in the use RS232 declaration, so that they are disabled on a 'per character' basis, which will allow your interrupts to occur 'between' the characters (if you have an older compiler, you can instead encapsulate the transmission to do this).

Best Wishes

[bkamen]

I agree. Software UARTs in a pinch only.

Hardware UARTs are good.

Interrupts are fabulous.
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Dazed and confused? I don't think so. Just "plain lost" will do. :D

[SherpaDoug]

Software UARTs are perfectly legitimate. They do take a lot of CPU time and a little ROM space. But for a project that has a specific "setup" or "download" phase when the CPU would be idle anyway, software UARTs allow you to use PICs that don't have hardware UARTs, or not enough UARTs.

Also software UARTs are sometimes a lifesaver for debug.
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The search for better is endless. Instead simply find very good and get the job done.

[bkamen]

I still only use them in a pinch.

So many PIC's come with HW UART's now...

what the decision making process comes down to this:

Will the PIC be busy enough where you can't afford to miss RS232 data and you can't afford to stop doing other things.

If the answer is yes, then hardware UARTs.

If the answer is no, then software is ok.

-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D

[Ttelmah]

Remember though that the original poster, was asking about transmit, not receive. So the comments about 'missing data', don't apply. Software UART's, used with care are fine for transmit. For receive, they are much more of a problem. Though 90% of PICs 'come with a UART'. if you have multiple serial streams involved, a software UART may be useful.

Best Wishes

[bkamen]