This posts describes how to check the state of a single input pin of PIC microcontroller in C as well as assembly language.
Note: For this tutorial I am using MPLAB X v5.0 but you can use any version upto v5.35. Version 5.35 is last version that included MPASM i.e. the assembler we require to compile the assembly code used in this tutorial. Code given here will not work for later versions.
How to Check a Single Input Pin of PIC Microcontroller in Assembly
The status of a single bit can be checked by using bit-oriented instructions BTFSS and BTFSC. I have already discussed how to use these instructions in a post titled “BTFSS and BTFSC Instructions for PIC Microcontroller“.
Lets look at few examples of using BTFSS and BTFSC to monitor an input pins and perform different actions.
Example: Check the status of a pin and perform an action if it is ‘0’
The following code uses BTFSC to monitor Pin 0 of PORTC continuously until it becomes ‘0‘. 0x55 is sent to PORTD when it becomes ‘0‘.
BSF TRISC, 0 ; Make Pin 0 of PORTC an input Port
CLRF PORTD ; Make PORTD an output Port
CHECK
BTFSC PORTC, 0 ; Check Pin 0 of PORTC, Skip next instruction if it is '0'
BRA CHECK ; Code comes here if Pin 0 of PORTC is '1'. Check it again.
MOVLW 0x55 ; Copy 0x55 to WREG if Pin 0 of PORTC is '0'
MOVWF PORTD ; Copy WREG (0x55) to PORTDExample: Check the status of a pin and copy its value to a selected bit of a file register
The following code uses BTFSS to monitor pin 5 of PORTC continuously and copies its value to bit ‘0‘ of RAM location 0x01.
BSF TRISC, 5 ; Make Pin 5 of PORTC an input Port
CHECK
BTFSS PORTC, 5 ; Check Pin 5 of PORTC, Skip next instruction if it is '1'
BCF 0x01, 0 ; Code comes here if Pin 5 of PORTC is '0'. Clear Bit 0 of RAM location 0x01.
BSF 0x01, 0 ; Code comes here if Pin 5 of PORTC is '1'. Set Bit 0 of RAM location 0x01.Example: Check the status of a pin and send its value to another pin
The following code uses BTFSS to monitor a switch connected to pin 5 of PORTC continuously and sends its status to an LED connected to pin ‘3‘ of PORTD.
BSF TRISC, 5 ; Make Pin 5 of PORTC an input Port
BCF TRISD, 3 ; Make Pin 3 of PORTD an output Port
CHECK
BTFSS PORTC, 5 ; Check Pin 5 of PORTC, Skip next instruction if it is '1'
BRA LED_OFF ; Code comes here if Pin 5 of PORTC is '0'. Jump to label LED_OFF
BSF PORTD, 3 ; Code comes here if Pin 5 of PORTC is '1'. Set Pin 3 of PORTD
BRA CHECK ; Check again
LED_OFF
BCF PORTD, 3 ; Clear Pin 3 of PORTD
BRA CHECK ; Check againHow to Check a Single Input Pin of PIC Microcontroller in C
Individual bit x of TRISy can be accessed by writing TRISybits.TRISyx. To access pin Ryx, you need to write PORTybits.Ryx. Using this syntax and comparison operators in C, we can check the status of a single pin and perform desired actions based on the status.
Example: Check the status of a pin and perform an action if it is ‘0’
The following code monitors Pin 0 of PORTC continuously until it becomes ‘0‘. 0x55 is sent to PORTD when it becomes ‘0‘.
#include <xc.h>
void main(void)
{
TRISCbits.TRISC0 = 1; // Make RC0 an input port
TRISD = 0x00; // Make PORTD an output port
while (PORTCbits.RC0 == 1); // Monitor RC0
PORTD = 0x55; // Send 0x55 to PORTD when RC0 becomes 0
while (1);
}Example: Check the status of a pin and send its value to another pin
The following code monitors a switch connected to pin 5 of PORTC continuously and sends its status to an LED connected to pin ‘3‘ of PORTD.
#include <xc.h>
void main(void)
{
TRISCbits.TRISC5 = 1; // Make RC5 an input port
TRISDbits.TRISD3 = 0; // Make RD3 an output port
while(1)
{
if (PORTCbits.RC5 == 1)
{
PORTDbits.RD3 = 1; // Set RD3, if RC5 = 1
}
else
{
PORTDbits.RD3 = 0; // Clear RD3, if RC5 = 0
}
}
}
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