Electronic – MSP430 Prevent Bouncing

cdebouncemicrocontrollermsp430

I'm currently working on a homework that does: the first press to the button at P1.1 opens the LED at P1.0, the second press to the same button blinks the LED and the third press closes the LED.

I wrote a code for that in Code Composer Studio but the microcontroller does not work properly. It does the opening, blinking, and closing operations but when the LED starts to blink, I have to press the button a few times to close the LED. I think that causes from bouncing (What I mean by bouncing is in the picture below) or the for loop that I wrote for blink operation.

I added a for loop (for (i=10000; i>0; i--)) to prevent that but that didn't work.

Here is my code:


int main(void) {
    WDTCTL = WDTPW | WDTHOLD;   // stop watchdog timer

    P1DIR |= 0x01;              // Configuring P1.0 as output
    P1DIR &= 0xFD;              // Configuring P1.1 as input

    P1REN |= 0x02;              // Enable resistor for P1.1
    P1OUT |= 0x02;              // Setting the resistor as Pull-Up and P1.0 as low
    P1OUT &= 0xFE;
    
    volatile unsigned int i;
    int j = 0;

    while(1) {

        for (i=10000; i>0; i--);                // Delay loop to avoid bouncing
        if ( (~P1IN & 0x02) && j==0 ) {         // Check if the button is pressed and j is 0 for validating it's the 1st press
            P1OUT ^= 0x01;                      // If the button is pressed, then toggle P1.0 (LED is ON)
            j = 1;                              // Change the value of "j" to "1"
        }

        for (i=10000; i>0; i--);                // Delay loop to avoid bouncing
        if ( (~P1IN & 0x02) && j==1) {          // Check if the button is pressed and j is 1 for validating it's the 2nd press
            while(1) {                          // Infinite loop to run the code below until break (3rd press)
                P1OUT ^= 0x01;                  // Toggle P1.0 (LED is blinking)
                for (i=45000; i>0; i--);        // Wait before closing or opening the LED
                if (~P1IN & 0x02) {             // If the button is pressed for the 3rd time, then break the loop
                    break;
                }
            }
            j = 2;                              // If the loop is broken, then change the value of "j" to "2"
        }

        for (i=10000; i>0; i--);                // Delay loop to avoid bouncing
        if ( (~P1IN & 0x02) && j==2) {          // Checking if the button is pressed and j is 2 for validating it's the 3rd press
            P1OUT ^= 0x01;                      // If the button is pressed, then toggle P1.0 (LED is OFF)
            j = 0;                              // Change the value of "j" to "0" for returning to the beginng of the main loop (1st press)
        }
    }
}

How can I fix this issue? My code covers all the thing that we learn until now. So I shouldn't use any codes, variables etc. apart from my code. Thanks in advance!

Best Answer

You have to use the delay after spotting the first flank, not at some random location in the code. The most crude version would be this (pseudo code):

uint8_t pin=0;
uint8_t prev=0;

while(1)
{
  pin = PORTX;
  if(pin != prev) // edge detected
    break;
  prev = pin;
}

busy_delay();  // some 5 to 20 ms usually
pin = PORTX;  // read when the switch is done bouncing
// now you can use the value of pin

The more professional version involves a circular interrupt which reads the port at even time intervals and compares with previous read and stores the result if it's the same.

Related Solutions

Electronic – Msp430 Launchpad Using Motor to Control Motor

My suggestion would be get rid of the delay function and use the built in if you are using IAR or CCS."__delay_cycles (cycles);" using the built ins, will give you more predicable results and better timing. You could probably just replace your delay for the built in, to get closer to the results you are looking for.

personally I would go about the whole thing using interrupts and timers to pull the whole thing off and use the peripherals that the msp430 has. I would suggest using the WDT as an interval timer, doing this would leave the other timer for your motor control. p1.3 is used as the wake up pin and p1.0 is the led pin.

example code

volatile int counter;
void main(void) {
    WDTCTL = WDTPW + WDTHOLD;                 // Stop watchdog timer
    P1DIR = 0x01;                             // P1.0 output, else input
    P1OUT =  BIT3;                            // P1.3 set, else reset
    P1REN |= BIT3;                            // P1.3 pullup
    P1IE |= BIT3;                             // P1.3 interrupt enabled
    P1IES |= BIT3;                            // P1.3 Hi/lo edge
    P1IFG &= ~BIT3;                           // P1.3 IFG cleared
    _BIS_SR(GIE);
}
// Port 1 interrupt service routine
#pragma vector=PORT1_VECTOR
__interrupt void Port_1(void)
{
    WDTCTL = WDT_MDLY_64; //enables intervals of 64mS
    IE1 |= WDTIE;  // set WDT interrupt
}
#pragma vector=WDT_VECTOR
__interrupt void watchdog_timer (void)
{
    ++counter;
    P1OUT |= BIT0;
    if (counter == 245) {  //245 x 64mS = 15.6 seconds
        P1OUT &= ~BIT0;
        WDTCTL = WDTPW + WDTHOLD; 
    }
}

Not complete nor has this code been tested, but it should turn the led on for 15 seconds every time the P1.3 button is pushed.

Electronic – How to exit from the if statemet which is inside for loop and go back to the void loop in Arduino

Are you sure you need to exit at ANY time wherever the execution pointer is? Normally, you could just check the button press at the end of the while and for loops and call return when it is pressed.

Sometimes we think our program has to do something and break out of execution, but upon careful examination, we conclude that it can finish one loop interation and then quit.

Could you tell us what's done inside the loops, so we can make sure what's needed?

Edit: As I suspected, the problem with your program is that your delay intervals are too long, e.g. delay(1000) and delay(500), and you can't check for button presses while the program is waiting.

To fix that, create the following function, that will replace your calls to delay:

// waits for the specified miliseconds 
// or return (almost) immediately if button is pressed
// return value indicates whether button was pressed or not
boolean waitOrButtonPressed(int miliseconds) {
  for (int i = 0; i < miliseconds / 10; i++) {
    if (readButtons(5)) == 5)
      return true;
    delay(10);
  }
  return false;
}

Then, inside your for loop, replace your calls to delay(1000) by the following code:

if (waitOrButtonPressed(1000)) return;

The function I created will make your program wait almost like delay(), but will return true if a button was pressed during that time. If that happens, the if statement above will exit loop() which will be called again by the Arduino generated code.

Remember to keep the miliseconds to wait when you replace your calls to delay() to keep your program waiting behavior unchanged.

Edit 2: Ramblings

Cases like this remind me of the movie Tron, in which the main character is dragged into a computer, where time passes much slowlier than in real life. So, like in the movie, 1000 miliseconds (1s) is a LOT OF TIME for the MCU. You can do a whole lot more than waiting in that time. Try and break up the long Waits and do useful stuff with your MCU cycles.

Also, learn how to use interrupts, like others mentioned in their comments and answers. It's just another (a bit more complicated) way of doing stuff while you wait. In your case, interrupts could be use to detect the button press without the need to check it everytime everywhere. Here's a place to start learning about interrupts with Arduino.

Best Answer

Related Solutions

Electronic – Msp430 Launchpad Using Motor to Control Motor

Electronic – How to exit from the if statemet which is inside for loop and go back to the void loop in Arduino

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