Difference between TX/RX and normal I/O with #use RS232 ???

chancaar · Joined: 23 Mar 2004 Posts: 6 Location: Belgium

Hello!

PIC = 16F877

I'll have to communicate with a device using baudrate 9600,stopbit=1, non parity.
When I connect this device I first do some tests using the special TX to transmit data. I can see it is working that way sending a specific string for my application. It works in 3 ways...

So I Define:
#use rs232(baud=9600,bits=8,parity=N,xmit=PIN_C6,rcv=PIN_C7)

// 3 ways:
//
// I send the caracters one by one with:
// for(i=0;i<n;i++) { putc('char');}
//
// or the entire string using puts("string");
//
// or I do is by printf("string")

Later I tried it using another normal (output declared) pin with the "#use rs232"...

So I Define for example (using PIN_B5 and PIN_D4):

(I configured PIN_B5 as an output, PIN_D4 as an input...)

#use rs232(baud=9600,bits=8,parity=N,xmit=PIN_B5,rcv=PIN_D4)

//and none of the 3 ways are working like mentioned above...

What's the different between normal I/O and TX/RX for sending and receiving signals when I define for both of them: the baudrate, parity bit, stop bit, transmit pin and receive pin???

I realise the TX/RX PIN is specially made for RS-232 communications but when I "talk" to a simular configured hyperterminal by using the normal I/O pins, it works! When I connect the device instead of the hyperterminal, it doesn't.

The TX/RX port is allready in use for another device in my project...

I hope somebody can help me with this problem.

Thanks for your time!

Yves
_________________
Yves Willemaers
Belgian student

dfnr2 · Joined: 26 Dec 2003 Posts: 2

What is your clock speed? The difference is that when you use the UART, the timing is generated in hardware, and for other pins, the timing is done in software. It is possible that for slow clock speeds, the software timing could be marginal.

I assume that you're using an apropriate conversion between RS2323 and chip logic levels.

D

chancaar · Joined: 23 Mar 2004 Posts: 6 Location: Belgium

I use a 20 MHz clock speed... and I'm using a MAX233 to transform the level.
_________________
Yves Willemaers
Belgian student

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

20MHz xtal or 20MHz ceramic resonator?

Get out your oscilloscope and x10 probe and check:

1) OSC2 should be 20MHz
2) Look at pin B5 while you transmit the value 0x55. Check that the pulse widths are correct. If they are more than a few percent slow or fast some RS232 devices will not work correctly
3) Look at the RS232 levels without your device connecteded. Generally the MAX233 swings about +/-9V.
4) Look at the RS232 levels WITH your device connected. Are the levels still valid? What levels is your device expecting? Eventhough something says it accepts RS232 input levels they may not mean anything from 3V to 15V, some cheap interface designs won't function properly without sufficient amplitude.
5) With your scope probe on pin B5 (your TX pin), force the PIC to reset. Are you seeing spurious transmissions? Could those lockup your receiving device? Most UARTs have timeouts built in so that if they do not detect the stop bits within a reasonable amount of time from the start bit, the received data is delcared invalid and the UART will watch for a new start condition.
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

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