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Revision as of 07:24, 27 December 2007


RS232 serial communication is an ancient, low speed, reliable standard. Each of about 100 characters (a, b, c, 1, 2, 3...) has a one-byte ASCII code. These are transmitted as 8 consecutive low and high logic levels, with a few framing bits as well. Baud rate refers to the number of bits/second. A typical baud rate is 19200, which is about 10000 times slower than USB. The original teletype machines were 110 baud. Read more about RS232 on wikipedia.

All desktop and laptop computers used to have a serial or "COM" port, available through a male DB-9 connector. These are getting less common on newer computers. However you can get an inexpensive "USB to Serial Adapter" which makes a DB-9 COM port available, once you install some driver software.

The 18F4520 PIC has a built-in UART for transmitting and receiving characters on RS232 standard. After you make the hardware connection -- the PIC's transmit pin sends to the PC/laptop's receive pin and vice versa -- you can open a text window on the PC/laptop and see whatever your PIC program prints, and type characters that will be transmitted to your PIC. The CCS IDE has a built-in text window for this purpose, shown at the bottom of this page.


Do I need to hook up all 9 pins?

No, you only need three: transmit, receive, and ground. The rest have purposes but are seldom used any more. You do have to get the baud rate and a few other parameters matched, between whatever your two communicating partners are.

On a standard male DB-9 COM connector on a computer, 2=input to computer, 3=output from computer, 5=ground.

The connector you build will be female with the same pin numbers for the same purposes. Photo at right shows the pin numbering on the female DB-9.


Voltage problems: important!

PC/laptops use 12 volt signals The official RS232 standard uses high voltages, at least 12 volts. The PIC can only produce 5V logic levels, and it can be damaged by voltages above 5V. So although the PIC and your PC/laptop agree about the code for transmitting characters, they don't agree about the voltage levels that represent 0 and 1 bits.

Some devices use "5 volt RS232" Quite a lot of devices use 0 and 5V logic levels instead of the levels the RS232 standard specifies. The PIC does this, and so do Serial 2-line LCD Displays, which are very handy. So these can be connected directly.


Shifting voltage levels

For interfacing a PIC to a PC/laptop or to a Serial-to-USB Adapter, you need a level converter. The MAX232N is a lovely chip that serves this function, in both directions (PC to PIC and PIC to PC). In fact it has two channels in each direction, should you ever want that many. You would think that in order to produce 12V signals it would need a 12V power supply, but it doesn't, it produces the needed 12V internally from a convenient 5V supply.

Using the MAX232N

The chip needs five 1uF capacitors around it, most of which have to do with its ability to create 12V out of 5V. Connect MAX232N pins 13 & 14 (and ground) to a female DB-9 connector as shown, and plug into your PC/laptop's serial port. Connect MAX232N pins 11 & 12 to your PIC. Pin 11 is information from the PIC and pin 12 is information to the PIC. If you only want PIC output, you don't need need to hook up the PIC input.

 PC to PIC: DB-9 pin 3 --> MAX232N pin 13 --> MAX232N pin 12 --> PIC pin RC7/RX
 PIC to PC: PIN pin RC6/TX --> MAX232N pin 11 --> MAX232N pin 14 --> DB-9 pin 2 

Code on the PIC

#include <18f4520.h>
#use delay(clock=20000000)       // 20 MHz crystal on PCB
//#use rs232(baud=19200, xmit=PIN_A0, rcv=PIN_A1)    // you can use any pins for software uart, erratic tho...
#use rs232(baud=19200, UART1)       // hardware uart much better; uses  RC6/TX and RC7/RX 
// characters tranmitted faster than the pic eats them will cause it to fail, I think
int16 i;
int j;
void main() {
   while (TRUE) { 
      for (i=1; i<10000; i++) {
         printf("Hello world. %Lu %u\r\n", i, j);  // \r is carriage return, \n is scroll up
         if (kbhit()) j = getc();      // there is no scanf 


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