quick Help with RS232

vinniewryan · Joined: 29 Jul 2009 Posts: 154 Location: at work

The following code gets stuck in the 'System' function, before it ever reaches any of the 'output_high(pin_xx); commands. I'm basically trying to read the RS232 only when there's an available byte, in the simplest form possible. I've tested and my input is receiving bytes correctly, but when I use kbhit() and getc, the code gets stuck. I'm programming a PIC16f684 using compiler version 4.022

arunb · Joined: 08 Sep 2003 Posts: 492 Location: India

Use RDA interrupt to get the data instead of polling for received characters.

Ttelmah · Joined: 11 Mar 2010 Posts: 19401

What you are doing, is basically right, _except_ for the testing for changes, and what you test 'for', and a critical 'timing' issue.

Remember 'var' is a global variable. Once it receives a value, it'll keep holding it. You really only want to change the pins, if a 'new' value has arrived.

If you are using a terminal package to send the charcaters, 'enter', will send CR (13), and LF(10). So 'var' may well contain 10, and continuously set C4 high.

Code 20, is a 'DC4' control code, so unless you are deliberately sending this with the alt key, and the numeric code, this is never going to be seen. You have a remark '++', and '--', the codes for the +, and - signs, are 46, and 48 respectively is these are what you are trying to see.
Using interrupts does become 'better' if there are delays in the main code, but for a simple operation as shown, are not needed. However judging by the number of variables you are declaring, suggecting that a lot more is wanted in the future, may well be the batter way to go.
As a 'comment', you may well be 'wiser' to avoid using standard names like 'timer3', which is the name of a standard system timer, in your variable declarations.
The main reason for problems though is mentioned after the updated code.

vinniewryan · Joined: 29 Jul 2009 Posts: 154 Location: at work

Thanks for the replies. I will try using an RDA interrupt.

Ttelmah,

I want the variable 'var' to change constantly, because it's receiving a constant signal of data from a transmitter. I assumed that when there is no byte available, it will set var to 0? Or perhaps will crash the code causing my problem? I've been able to successfully receive data from the transmitter and respectively turn on and off the output pins, but when I add this code to the rest of my code, the rest of the code doesn't run because it all gets stuck in a loop of checking getc and setting outputs high. It never runs past that point.

If it would help I can post the rest of my code, but I'm going to try using the RDA interrupt first. My PIC does not have an onboard UART, or any form of communication. I do however have an I2C compatible PIC which may better suit my needs if I can't get this working correctly.

A third method I found was posted by PCM a while ago, and it includes using a PICBASIC snippet to count pulse width on an input. If I use this method, I can basically create my own type of serial encryption, as I'm only working with two different transmitted values.

I've created a timer system that allows me to generate delays without pausing the other running code, and without using a WDT or any hardware functions. It's a general timer code used for any section of the code, which is the reason for the general variable names like 'timer2'.
_________________
Vinnie Ryan

Ttelmah · Joined: 11 Mar 2010 Posts: 19401

No, when no byte is available, var will sit holding what was last received.
to set it to zero when nothing is available, you would need:

vinniewryan · Joined: 29 Jul 2009 Posts: 154 Location: at work

Thanks for all the help! Here's the final *working* code. I haven't yet, but will make the change you last suggested about nesting the "if(var=)" statements within the"if(kbhit())" function.

The code breakdown:

Receive an input from the RX pin, the value at the RX pin will either read 10 or 20. Park that. The code has a constant timing system that allows unlimited timers and functions to run without pausing or desynching anything else, which is necessary for the proper function of this code. Basically the PIC controls output to 3 motors, and constantly pulses them on and off really fast to give emulated speed control similar to a PWM. The RX input receives either a 10 (+), which increases the intensity of the VIBRATING motors, or 20 (-), which decreases the intensity. There is an LED connected to pin C3, one on C4, and one on A1, these simply indicate where the code is. A1 LED indicates that info is being received on the RS232, C2 and C3 determine whether it's a 10(+) or a 20(-).

This timer system can be used with any other code or pic, doesn't require interrupts, WDT's, or delays, and is very handy and easy to program. If requested, I'll post just the timer code so anyone out there can use it and easily get it working. It can accurately measure any timeframe, ns, ms, s, min, h, even days and months. All in a few lines. It's not really that exciting to the world of PICs, but it is to me because I use it so often and I made it.