RS232 can't receive Character occasionaly?

stefsun · Joined: 23 May 2007 Posts: 22

I use the pic16F877's RS232 Communication with PC,
After acquisiting the V of A0,when delay is long,PIC16F877 can't receive the single character,(for example the character Q,pic16F877 is still acqisiting)

printf( "delay_ms(300)\n" );
delay_ms(300);

when delay is 300ms,PIC can receive Q and stop acquisiting,

when delay is 3000ms(3s),PIC can't receive Q.

the code is

Douglas Kennedy · Joined: 07 Sep 2003 Posts: 755 Location: Florida

RS232 is asynchronous. That means when you code is executing an incoming character can arrrive at anytime. Anytime often meanss right in the middle of any line of code. Code that takes the most time Ex delays is then the most vunerable. The only way to guarantee the char can be received is with the hardware UART and an interrupt service routine. Sure if the baud rate is low and you main code has no delays you can make it work providing your rs232 polling loop executes in the order of a bit time ( approx 1/10 th of the baud rate) or less. This issue has been asked and answered often on this board. Again rs232 is asynchronous the chars don't show up when you read them so your code must be there ( at kbhit() ) when they arrive. The reason that some chars are seen by your code is probably just luck that your code got to kbhit in time to see the start bit.

Ttelmah · Guest

With the hardware UART, there are two characters of input buffering, plus the input shiift regoster (giving just under 3 characters of total buffer). If more than this number of characters are received, without calling code to retrieve the characters from the hardware, the hardware raises the 'OERR' flag (overrun error), _and disables the UART_.
Now at 9600bps, a character can basically arrive every mSec. You don't say how the characters are being sent, but with the 3 second delay, whatever is sending the characters, is sending more characters than can be held in the hardware, so the UART gets disabled...
First, add the keyword 'ERRORS' to your #use RS232 line. This adds code to the handler, which will automatically reset this overrun error if it occurs. Data will still have been lost, but at least the system has the ability to recover.
Then 'rethink' how you work. Look at making the receive task interrupt driven, so that a larger buffer can be used. Then look at other ways of doing delays. Generally, long software 'delays' inside code, are relatively poor programming, on systems that want to perform tasks. Instead look at using one of the hardware timers, to generate a system 'tick' (this is how the PC works, with a 'tick' of 10mSec, or 18mSec, depending on the OS). Then when you want to delay for 3 seconds, simply 'count' ticks, while still looping, and checking for other data to arrive.

Best Wishes