Electronic – Basic C Programming Question On Microcontroller

cledmicrocontrollerprogrammingswitches

I think this question is ok in the electronics stack and not the programming stack (I hope).

I have a simple code written in C, on a TM4C123 evaluation board. It uses the two on board switches to control the colour of the on board RGB LED.

The switches are configured in software with pull up resistors and the PORTF configurations are all correct. I had the same functionality working using switch case statements, but tried to re-create it with if else statements but it doesn't work as it is supposed to.

Below is the original working code:

#include "include/tm4c123gh6pm.h"
#define RED_LED (1U << 1)
#define BLUE_LED (1U << 2)
#define GREEN_LED (1U << 3)

int main()
{
    SYSCTL_RCGCGPIO_R = 0x20U;
    GPIO_PORTF_LOCK_R = 0x4C4F434BU;
    GPIO_PORTF_CR_R = 0xFFU;
    GPIO_PORTF_DIR_R = 0x0EU;
    GPIO_PORTF_PUR_R = 0x11U;
    GPIO_PORTF_DEN_R = 0x1FU;


        while(1)
        {
        GPIO_PORTF_DATA_R &= (0x011U);

            switch(GPIO_PORTF_DATA_R & 0x11U)
            {
                case 0x11:
                GPIO_PORTF_DATA_R |= RED_LED;
                break;

                case 0x01:
                GPIO_PORTF_DATA_R |= BLUE_LED;
                break;

                case 0x10:
                GPIO_PORTF_DATA_R |= GREEN_LED; 
                break;

                default:

                break;
            }

        }

}

Here is the troublesome code:

#include "include/tm4c123gh6pm.h"
#define RED_LED (1U << 1)
#define BLUE_LED (1U << 2)
#define GREEN_LED (1U << 3)

    int main()
{
    SYSCTL_RCGCGPIO_R = 0x20U;
    GPIO_PORTF_LOCK_R = 0x4C4F434BU;
    GPIO_PORTF_CR_R = 0xFFU;
    GPIO_PORTF_DIR_R = 0x0EU;
    GPIO_PORTF_PUR_R = 0x11U;
    GPIO_PORTF_DEN_R = 0x1FU;

        while(1)
        {
        GPIO_PORTF_DATA_R &= (0x011U);

            if(GPIO_PORTF_DATA_R & 0x01U)
                {
                    GPIO_PORTF_DATA_R |= RED_LED;
                }   

            if(GPIO_PORTF_DATA_R & 0x10U)
                {   
                    GPIO_PORTF_DATA_R |= BLUE_LED;
                }

            if(GPIO_PORTF_DATA_R & 0x011U)
                {
                    GPIO_PORTF_DATA_R |= GREEN_LED; 
                }

            else
                {
                    GPIO_PORTF_DATA_R &= 0x0U; 
                }
            }
}

The switches are stored in PORTF bits 0 and 4. And the red, blue and green LED is stored in port F bits 1, 2 and 3 respectively.

On line 63 I have an 'AND' assignment to check if bits 0 or 4 is high (button(s) been pressed).

Next the data register is logical 'ANDED' with 1's on bits 0 and 4 to check if it is a 1.

Then the IF statements should switch on the LED's accordingly.

Operation should be:

No switches pressed (bits 4 and 0 = 00) , no LED on

Switch 1 pressed (bits 4 and 0 = 01), green LED on

Switch 2 pressed (bits 4 and 0 = 10), blue LED on

Both switches pressed (bits 4 and 0 = 11), both LED's on

When I step through the code with the de-bugger, the IF statements all appear true with the red, blue and green LED all coming on in order. The else statement is appearing false.

Am I making a mistake that with the pull up resistors, the bits 4 and 0 will be active low (1 when not pressed, and a 0 when pressed)?

Thanks for any replies.

Best Answer

As you noted, you have a logic flaw where several of your if statements might be true simultaneously. This should be solved with the equivalent of switch, namely if() else if() .... This is the least of the problems though.

The root of your problems seems to be the lack of "debouncing", which is mandatory for all forms of buttons and switches. This is absolutely necessary unless you have external RC filters, which is most often not the case, since HW designers count on software to deal with it. I won't describe "debouncing" here since it's typically addressed early on in any beginner embedded course.

Your switch statement reads the GPIO register once, so it seems to work, by luck. But the if-else reads the same register in multiple places, over time.

Another problem is that in general, you should not treat hardware registers like ordinary variables. They are volatile qualified, meaning that each access to them is unique. The compiler is not allowed to optimize the access, so several reads over time might produce different results.

Therefore best practice is to isolate access read/write to registers on lines of their own. Don't mix such access with complex expressions.

What you should do no matter if-else or switch, is to read the register once, store the result in a temporary local variable and then work with that variable when determining which button that was pressed.

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