16F88 and RS232

GDetienne · Joined: 20 Sep 2003 Posts: 47 Location: Brussel - Belgium

I have question about 16F88, RS232 and PCH 3.217.

I write the following test program (To do short, I don't give the LCD routine).

Ttelmah · Guest

When you change the RS232 definition, you automatically switch to using software RS232. Your original form is the 'correct' way to code to use the hardware UART. Understand, that if you code as originally written, and then never send anything to the transmit output, there is nothing to stop you using the pin for other things, and that no code is actually generated for the transmit pin, till it is used. By removing the reference to the transmit pin, you force the compiler to use the software RS232, so the code grows.
With the software RS232, 'int_rda', is never going to occur. Hence it will not work.
In the original code, I would change to buffering the serial, rather than using 'fgets'. The problem is that if garbage arrives at the RS232 input, that does not include a 'end of line' character, the code will remain hung inside the interrupt. Also if the string that arrives is larger than the string space you have allocated, the data will overrun, and overwrite other variables in memory, causing chaos. What you have will only work if the data is always 'right'....

Best Wishes

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Or to give a short answer, you can't split the hardware serial port.
It's all or nothing.

The 16F88 data sheet shows this in the section on the RCSTA register:

GDetienne · Joined: 20 Sep 2003 Posts: 47 Location: Brussel - Belgium

Thanks for your quick answer. That confirm what I think but with correct reasons.
Two questions :
- could you confirm that I can use the Tx port for Led control if I am sure not use it for transmit.
- I use fgets (cString) in my interrupt. Till now I try with caracters without garbage. I agree with you, it's better to change it for security. Do you think the best is a ring buffer or go out the interrupt after a fixed number of caracters ? What is your experience ? The length of my string is not constant but lower than 10 caracters.

Thanks a lot Ttelmah and PCM programmer (and others) for your advies.

Guy

Ttelmah · Guest

A ring buffer is a 'good' solution. However you might (assuming you want to always deal with a complete 'string' in the main code), consider a linear buffer. With this you just start adding characters to a string area in memory, and throw them away if the string gets too long (you lose data from the 'end', whereas with a ring buffer you normally lose it from the 'front' in the event of an overflow). When you see the terminator character, you set the flag to say you have a complete string, add a 'null' to the buffer, and it is just sitting there waiting in the buffer. With this you only need one counter. Effectively it'd behave exactly like the fgets, but with a limited size, and with the characters only added one at a time (to avoid sitting in the interrupt routine waiting for the string to arrive).
Yes, you can just use the output pin for another operation. Since the compiler does not actually generate the 'Tx' code, till you write something with putc, there is no problem. :-)

GDetienne · Joined: 20 Sep 2003 Posts: 47 Location: Brussel - Belgium

Thanks a lot. I have some directions to improve my self.

Thanks.