PIC18F4520 – Serial communication(USART)

The USART module of PIC 18F4520 uses pins RC7 & RC6 for reception & transmission respectively. The mini development board can communicate with PC/external modules through

USB port via CP2102
Serial port via MAX232
RMC connector K15 in 5V level
RMC connector K17 in 3.3V level

Besides, USART is also used for interfacing Zigbee & RFID reader.

Schematic

Note:

3V3 USART communication through K17 is possible only if the jumpers J13 and J14 (XBTX, XBRX) are shorted.
For communicating through K15 & K17, remove jumpers J3 & J4

Sample Code

The sample code to check USART module is given below. Initially the letter ‘A’ gets transmitted. Then, upon receiving a character, the same gets retransmitted.

Value in SPBRG is calculated as follows
Baud Rate = Fosc / (16 (X + 1)
The desired baudrate is 9600 bps and crystal frequency, Fosc is 20 MHz. This gives X as 129 (81 in hexadecimal) which is loaded in SPBRG. Any change in crystal frequency/baudrate changes this value as per the above equation.

/**********************************************************************
                            HEADER FILE
***********************************************************************/
#include<pic18.h>		

/**********************************************************************
                           MAIN FUNCTION
***********************************************************************/
void main()			        		
{
 TRISC = 0x80;         /* RC6=0 (O/P) RC7=1(I/P)                      */			
 SPBRG = 0x81;         /* Serial Port Baud Rate Generator for 9600    */  
 TXSTA = 0X24;         /* TXEN=1, SYNC=0, BRGH=1                      */
 RCSTA = 0X90;         /* Reception Enable (SPEN=1,CREN=1)            */
 TXREG = 'A';          /* Load the character to be transmitted        */ 
 while(!TRMT);         /* Wait here till transmission is complete     */
 while(1)
 {
  if(RCIF==1)
  {
   RCIF=0;             /* Clear Receive interrupt flag bit            */
   TXREG = RCREG;      /* Retransmit the received character           */
   while(!TRMT);       /* Wait here till transmission is complete     */
  }
 }
}

/************************  END OF PROGRAM  ****************************/

/**********************************************************************

HEADER FILE

***********************************************************************/

#include<pic18.h>

/**********************************************************************

MAIN FUNCTION

***********************************************************************/

void main()

{

TRISC = 0x80; /* RC6=0 (O/P) RC7=1(I/P) */

SPBRG = 0x81; /* Serial Port Baud Rate Generator for 9600 */

TXSTA = 0X24; /* TXEN=1, SYNC=0, BRGH=1 */

RCSTA = 0X90; /* Reception Enable (SPEN=1,CREN=1) */

TXREG = 'A'; /* Load the character to be transmitted */

while(!TRMT); /* Wait here till transmission is complete */

while(1)

{

if(RCIF==1)

{

RCIF=0; /* Clear Receive interrupt flag bit */

TXREG = RCREG; /* Retransmit the received character */

while(!TRMT); /* Wait here till transmission is complete */

}

/************************ END OF PROGRAM ****************************/

Output

The following screenshots explain how to test the sample code using RealTerm (click to download).

Step 1: Open RealTerm
Step 2: RealTerm opens as shown below
Step 3: Go to ‘Port’ option, set correct baudrate (which is set as 9600 in the sample code) and correct port
Step 4: Click ‘Change’ (encircled in red) to apply the changes. Now check the status of Port. If it is closed, click ‘Open’ button (encircled in green) to open it.
Step 5: To check reception, go to Send option, type the string in the space provided(encircled in green) and click Send ASCII button. The first “hello” in green colour is transmitted from PC & that in yellow colour is retransmitted by the controller