Lesson 10: Introduction to the GPIO Port Control Register (GPIOPCTL)

• In the PA1 row, U0Tx sits under the 0x1 column → the encoding is 0x1.
• PA1 is pin 1 → field PMC1 → bits 7:4 → shift = 4 (_PCTL_PIN1).
• Enable the alternate function, write the mux code into PMC1, then enable digital I/O:

// Route PA1 -> U0Tx (encoding 0x1)
GPIOA->AFSEL = _PIN1;                  // pin 1 = alternate function
GPIOA->PCTL  = (0x1 << _PCTL_PIN1);    // PMC1 = 0x1
GPIOA->DEN   = _PIN1;                  // enable digital I/O

1. Turn on the GPIO port clock (RCGCGPIO) and the peripheral clock (e.g., RCGCUART).
2. Set the pin's bit in GPIOAFSEL.
3. Write the correct 4-bit code into the matching PMCn field of GPIOPCTL.
4. Set GPIODEN (digital enable) for the pin.
5. For the special-consideration pins — PC0–PC3 (JTAG/SWD) and PD7 on both chips, plus PF0 on TM4C123G / PE7 on TM4C1294 — you must first unlock them via GPIOLOCK and un-commit via GPIOCR before any of the above will take effect.

In our labs the port initialization lives in a Setup_GPIO() function built from a fixed 8-step comment scaffold. Step 4 writes the PCTL mux code you looked up, and Step 5 raises AFSEL to hand the pin to the peripheral. The example below routes PA0 and PA1 to UART0 (U0Rx / U0Tx), mux code 0x1.

void Setup_GPIO(void)
{
    // Configure GPIOs
    // 1. Enable Clock to the GPIO Modules (SYSCTL->RCGCGPIO |= (_PORTs);)
    SYSCTL->RCGCGPIO |= (_PORTA);
    // allow time for clock to stabilize (SYSCTL->PRGPIO & (_PORTs))
    while ((SYSCTL->PRGPIO & (_PORTA)) != (_PORTA)) {};

    // 2. Unlock GPIO only PD7, PF0 on TM4C123G; PD7, PE7 on TM4C1294 (GPIOx->LOCK = 0x4C4F434B; and GPIOx->CR = _PINs;)
    //    -> PA0 / PA1 are ordinary pins: no unlock needed here.

    // 3. Set Analog Mode Select bits for each Port (GPIOx->AMSEL = _PINs;)
    //    -> UART is a DIGITAL function: leave AMSEL cleared (do nothing).

    // 4. Set Port Control Register for each Port (GPIOx->PCTL = PMCn << _PCTL_PINn, check the PCTL table)
    GPIOA->PCTL = (0x1 << _PCTL_PIN0)    // PA0 -> U0Rx
                | (0x1 << _PCTL_PIN1);   // PA1 -> U0Tx

    // 5. Set Alternate Function Select bits for each Port (GPIOx->AFSEL = _PINs;)
    GPIOA->AFSEL = _PIN0 | _PIN1;

    // 6. Set the output pins for each port only (Direction of the Pins: GPIOx->DIR = _PINs;)
    //    -> Not needed: once AFSEL routes the pins to UART0, the peripheral owns their direction.

    // 7. Set PUR bits (internal Pull-Up Resistor), PDR (Pull-Down Resistor), ODR (Open Drain) for each Port
    //    -> Not needed for a push-pull UART. (I2C would set ODR on its SDA pin here.)

    // 8. Set Digital ENable register on all digital port.pins (GPIOx->DEN = _PINs;)
    GPIOA->DEN = _PIN0 | _PIN1;
}

Once a pin is handed to a peripheral through PCTL (step 4) and AFSEL (step 5), most of the "GPIO personality" registers stop mattering — but one still does. Here is the rule for each:

• Step 6 — DIR (direction): Not needed. When AFSEL = 1, the peripheral hardware drives the pin's direction internally. GPIODIR only applies to plain-GPIO pins; setting it on an alternate-function pin has no effect. So skip step 6 for peripheral pins — use it only for the GPIO output pins in the same port.
• Step 7 — PUR / PDR (pull-up / pull-down): Usually not needed. Digital peripherals such as UART, SSI/SPI, PWM, and Timer are push-pull — they actively drive the line, so no pull resistor is required. You would only add one to give a defined idle level to a floating input line, which is an electrical choice, not a mux requirement.
• Step 7 — ODR (open drain): Only for open-drain buses. The classic case is I2C: you set ODR on the SDA pin (SCL stays standard/push-pull) and rely on pull-ups, usually external. For UART/SSI/PWM you leave ODR cleared.
• Step 8 — DEN (digital enable): Always needed. DEN switches on the pin's digital I/O pad. Without it the pin carries nothing — not a GPIO signal and not the peripheral signal you just muxed in. Every digital peripheral pin must have its DEN bit set. The only time you leave DEN cleared is for an analog function (ADC input, analog comparator), where you set AMSEL in step 3 instead.

Rule of thumb: for a digital peripheral pin you touch only PCTL (4), AFSEL (5), and DEN (8) — plus ODR (7) if, and only if, the peripheral is I2C.

TM4C123G MCU

TM4C1294 MCU