Doubt in the following program regarding the value of GPIO_PORTF_DATA_R

Question

The following code is related to tiva c series tm4c123gh6pm board

    #include <stdint.h>
    #include "inc/tm4c123gh6pm.h"
int main(void)
{
    unsigned int value;

    SYSCTL_RCGCGPIO_R |= 0x20; /* enable clock to GPIOF */
    GPIO_PORTF_DIR_R = 0x0E; /* set PORTF 1, 2 &amp; 3 pins as output (LED) pins */
    GPIO_PORTF_DEN_R = 0x1E; /* set PORTF pins 1, 2, 3, and 4 as digital pins */
    GPIO_PORTF_PUR_R = 0x10; /* enable pull up for pin 4 */

    while (1)
    {
        value = GPIO_PORTF_DATA_R; /* read data from PORTF */
        value = ~value; /* switch is low active; LED is high active */
        value = value &gt;&gt; 1; /* shift it right to display on the red LED */
        GPIO_PORTF_DATA_R = value; /* put it on the red LED */
    }
}

According to my understanding, when switch is pressed, the pin4 corresponding to PORTF is activated. Since the switch is active low, the corresponding value becomes 0 in GPIO_PORTF_DATA_R making the value of this register as 0b xxx0 xxxx. I'm unable to understand how this value is becoming 0x02 when all bits are inverted and shifted to right by one bit. I tried to dump this code in the board and it's working perfectly(i.e., when switch is pressed, red led is glowing).

Answer 1

Debugging the code from the question

Based upon the information in the question, I assume the code is being run in a EK-TM4C123GXL. The Tiva™ C Series TM4C123G LaunchPad Evaluation Board User's Guide contains the following table which shows the connections for the LEDs and switches on the board:

I added the code from the question into a CCS 12 project for a TM4C123GH6PM so could debug by single stepping. Before stepping into the while loop the LEDs on the board are all off. The TM4C123GH6PM datasheet description of the GPIO Data (GPIODATA) register shows that the register is initialised to zero on reset.

The result of stepping through the while loop after starting the program while not pressing any button is:

Value read from GPIO_PORTF_DATA_R	Value written to GPIO_PORTF_DATA_R	Comment
0x00000014	0x7FFFFFF7	RGB LED changes from Off to Blue+Red On (Magenta)
0x00000016	0x7FFFFFF4	RGB LED changes from Blue+Red On to Blue On
0x00000014	0x7FFFFFF5	RGB LED remains at Blue On
0x00000014	0x7FFFFFF5	The shift register on GPIO_PORTF_DATA_R is now stable

The SW1 button was pressed and continued stepping:

Value read from GPIO_PORTF_DATA_R	Value written to GPIO_PORTF_DATA_R	Comment
0x00000004	0x7FFFFFFD	RGB LED changes from Blue On to Green+Blue On (Cyan)
0x0000000C	0x7FFFFFF9	RGB LED changes from Green+Blue On to Green On
0x00000008	0x7FFFFFFB	RGB LED changes from Green On to Green+Red On (Yellow)
0x0000000A	0x7FFFFFFA	RGB LED remains at Green+Red On
0x0000000A	0x7FFFFFFA	The shift register on GPIO_PORTF_DATA_R is now stable

So, effectively the code is creating a shift register on some bits of the GPIO_PORTF_DATA_R register, where the output bits for the LEDs can be read back on the next iteration around the while loop.

Consider changing from Direct Register Modification (DRM) only programming to using TivaWare

The [FAQ] FAQs for TM4C Arm Cortex-M4F Microcontrollers on the Texas Instruments forum contains the following:

4) Avoid use of Direct Register Modification (DRM) only programming, and instead use TivaWare.

One of the primary purposes of TivaWare has been to remove the need for individual users to learn the ins and outs of DRM programming. For this reason, we will not be able to offer support for any questions that only pertain to DRM programming.

Where the code from the question is using DRM programming. If the program was re-written to use TivaWare functions it could be clearer. E.g. the GPIOPinWrite function takes the ui8Pins argument which is a bit-mask of which GPIO port output bit(s) to update. By changing the code to use TivaWare GPIOPinWrite could be used to only update the PF1 state for the RGB LED (Red) on the board.

If required, could modify this answer to add an example of how the DRM code in the question could be converted to TivaWare.