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Javed Shafi
Joined: 15 Mar 2004
Posts: 6
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| Using software simulated UART |
 Posted: Thu Dec 01, 2005 9:25 am |
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Just a question regarding the use of directive
#use rs232(baud=100, xmit=PIN_C6, rcv=PIN_C7)
I have successfully used this directive before for PIC16LF877 and PIC16LF872 using port C pins 6 and 7. These PICS have inbuild UARTS.
I have inherited a project which uses a PIC16C505 and I note I can still use the #use rs232 even though this device does not have a UART.
Anybody have any experience or issues of using software simulated UARTs that I should be aware of as I never done this before.
Any help on this greatly appreciated.
Best regards
Javed
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Javed Shafi
email: [email protected] |
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Ttelmah
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| Posted: Thu Dec 01, 2005 11:20 am |
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There are a lot of differences.
The first, is that there is no buffering, so characters wil only be received, if the code is sitting 'waiting' for them to arrive.
The second, is that operation is 'half duplex' at best. You cannot both receive and send at the same time.
The worst one though, is that because all the timings are in software, the processor, can only do the serial transmit/receive, when nothing else is happening. If you have interrupt driven events for anything else, these will interfere with the timings. At low baud rates, it is possible to 'get away' with this, but at higher baud rates, it implies all interrupts must be disabled for the duration of a character being sent/received. I have in the past, posted a way of 'encapsulating' the transmission, so that this is automatically done on a 'per character' basis.
Best Wishes |
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Javed Shafi
Joined: 15 Mar 2004
Posts: 6
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| Posted: Thu Dec 01, 2005 11:50 am |
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Hi Ttelmah,
Many thanks for your reply.
You mention that
"I have in the past, posted a way of 'encapsulating' the transmission, so that this is automatically done on a 'per character' basis"
Could you tell me where I can find this link for more info.
Best regards
Javed
_________________
Javed Shafi
email: [email protected] |
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Ttelmah
Guest
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| Posted: Thu Dec 01, 2005 3:29 pm |
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I'll repeat it, because it is easy to do.
Basically, assuming you are using 'putc' for output, then write:
| Code: |
void intoff_putc(int val) {
disable_interrupts(global);
putc(val);
enable_interrupts(global);
}
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Then for printf, use:
printf(intoff_putc,"What you want to send");
or for putc itself, use:
intoff_putc('c');
If you remember that individual interrupt flags will still get set when the global interrupts are disabled, you can send a string of text, and the transmission will send a single character, then check interrupts, then send the next character etc..
Provided no interrupt event 'cares' if it's handler is delayed for up to one character time, things still work OK.
For low baud rates like yours, you may well be OK with interrupts running.
You can also generate an interrupt based receive, by connecting the serial receive line an interrupt input (interrupt on the signal going low), then as soon as the handler is called, execute the 'getc' function, and save the data received.
Best Wishes |
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