PIC16F1937 and UART : keep led ON while receiving - c

Question

I'm trying to keep a led ON while PIC is receiving a character (while a button is pressed), the led being OFF by default, but I can't seem to find a solution.
I'm 3 months in learning using microcontrollers (specifically PIC) coding with MPLABX IDE and started understanding UART communication. At first I tried turning a led ON/OFF when a charater is received, it went pretty good and worked, but right now I'm trying to keep a led active while a button is pressed but can't really get over this problem. I have my code in a interrupt function like this :
//#define LED RB5
void __interrupt () ISR (void)
{
if(RCIF) // data is received
{
data = RCREG; // get the value
if (data == '1') // if value received is '1'
LED = 1; //turn ON led
RCIF = 0;
}
LED = 0; // turn OFF led
}
Using the above code makes so the led turns on/off really fast while I keep pressing a button and isn't really what I wanted.I hope someone can help me understand what I need to do. Thanks !

Answer 1

The LED goes off quickly because you switch if off immediately after switching it on:
void __interrupt () ISR (void)
{
if(RCIF) // data is received
{
data = RCREG; // get the value
if (data == '1') // if value received is '1'
LED = 1; //turn ON led
RCIF = 0;
}
LED = 0; // turn OFF led <<=== This is executed unconditionally.
}
You might put LED = 0; in an else branch if you don't want it to be executed everytime.
Maybe like this:
void __interrupt () ISR (void)
{
if(RCIF) // data is received
{
data = RCREG; // get the value
if (data == '1') // if value received is '1'
LED = 1; //turn ON led
else
LED = 0; // turn OFF led
RCIF = 0;
}
}
Depending on your logic, the else might be placed after the end of the outer if block.

Answer 2

The LED goes off quickly after receiving any character.
RCIF flag will only set if UART data is received. So to turn off the LED you should start a timer of certain millisec and unless your reception is completed keep on restarting the timer.
void __interrupt () ISR (void)
{
if(RCIF) // data is received
{
data = RCREG; // get the value
if (data == '1') // if value received is '1'
LED = 1; //turn ON led
RCIF = 0;
start_timer_interrupt(x_ms); //set x_ms optimum timing so as to LED ON is visable with UART Rx
flag_rx_timer=1;
}
if(TIMER_OVERFLOW) //TIMER_OVERFLOW is an example keyword used here. Pleas add exact timer overflow flag
{
if(flag_rx_timer) //check if timer overflow is because of timer started during UART Rx
{
LED = 0;
flag_rx_timer=0;
}
}
}```

Answer 3

I suggest you create a state machine for this in app.c.
Define some states with an enum and a struct to hold the state and place them in app.h, include app.h in files where you wish to use this module (e.g. your system_interrupt.c file).
For example you could do this:
typedef enum{
IDLE,
START_RECEIVING,
STILL_RECEIVING,
}uart_state_t;
typedef struct{
uart_state_t current_state;
}uart_module_t
volatile uart_module_t uart_module = {0}; // initial state = IDLE, needs to be volatile because it will be updated via interrupt
Then create a function to service your state machine. This will handle the state that it is currently in and transition to other states as desired. For instance, the state machine starts in the IDLE state but once your interrupt has fired and RCIF bit is set then you'll transition in to the START_RECEIVING state which will turn the LED on then transition to the STILL_RECEIVING state where it will poll the RCIF bit until cleared. That would look something like this:
void uartFSM(void){
switch(uart_module.current_state){
case IDLE:
{
break;
}
case START_RECEIVING:
{
LED = 1; // Turn LED on
uart_module.current_state = STILL_RECEIVING; // state update
break;
}
case STILL_RECEIVING:
{
if(!RCIF){
// done receiving
LED = 0; // Turn LED off
uart_module.current_state = IDLE; // state update
}
break;
}
default:
{
// whoops
break;
}
}
}
Now your interrupt will look like this:
void __interrupt () ISR (void)
{
if(RCIF) // data is received
{
data = RCREG; // get the value
// if value received is '1'
if (data == '1') uart_module.current_state = START_RECEIVING; // state update
}
}
Now you just have to make sure that you call the uartFSM() some where in APP_Tasks so that the state machine gets serviced.