Peripheral Programming

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Programming the GPIO in TM4C123

In this article we are going to write our first program for accessing GPIOs on the EK-TM4C123GXL LaunchPad Board.

Memory Map

The ARM Cortex-M4 has 4GB (Giga bytes) of memory space. It uses memory mapped I/O, which means that the I/O peripheral ports are mapped into the 4GB memory space.

The TI EK-TM4C123GXL LaunchPad uses the TM4C123GH6PM microcontroller, which has 256K bytes (256KB) of on-chip Flash memory for code, 32KB of on-chip SRAM for data, and a large number of on-chip peripherals.

	Allocated size	Allocated address
Flash	256 KB	0x0000.0000 To 0x0003.FFFF
SRAM	32 KB	0x2000.0000 To 0x2000.7FFF
I/O	All the peripherals	0x4000.0000 to 0x400F.FFFF

GPIO & Special Purpose I/O

While memory holds code and data for the CPU to process, the I/O ports are used by the CPU to access input and output devices. In the microcontroller we have two types of I/O :

• General Purpose I/O (GPIO)

  The GPIO ports are used for interfacing devices such as LEDs, Switches, LCD, Keypad, and so on.

• Special purpose I/O

  These I/O ports have designated function such as ADC (Analog-to-Digital), Timer, UART, PWM and so on.

TM4C123GH6PM Block Diagram

TM4C123GH6PM Micro controller

ARM chip used in TI Tiva LaunchPad is Tiva C series TM4C123GH6PM microcontroller.

The pins are designated as:
➢ PA0 – PA7
➢ PB0 – PB7
➢ PC0 – PC7
➢ PD0 – PD7
➢ PE0 – PE5
➢ PF0 – PF4

Port-E and Port-F do not have all the 8 pins implemented.

GPIO APB & AHB Bus

The ARM chips have two buses:

1. Advanced Peripheral Bus (APB) and
2. Advanced High-Performance Bus (AHB).

The AHB bus is much faster than APB. The AHB allows one clock cycle access to the peripherals. The APB is slower and its access time is minimum of two clock cycles.

GPIO APB Memory Map

The I/O ports addresses assigned to the Port A – Port F for APB are as follow:

• GPIO Port A : 0x4000.4000
• GPIO Port B : 0x4000.5000
• GPIO Port C : 0x4000.6000
• GPIO Port D : 0x4000.7000
• GPIO Port E : 0x4002.4000
• GPIO Port F : 0x4002.5000

GPIO AHB Memory Map

The Base addresses for the GPIOs of AHB is as follow:

• GPIO Port A : 0x4005.8000
• GPIO Port B : 0x4005.9000
• GPIO Port C : 0x4005.A000
• GPIO Port D : 0x4005.B000
• GPIO Port E : 0x4005.C000
• GPIO Port F : 0x4005.D000

4K bytes of memory space is assigned to each of the GPIO port. Each GPIO has a large number of special function registers (SFR) associated with it and the GPIO DATA register supports bit-banding. The GPIO DATA register is 8-bit wide. With bit-banding, it will need 256 words (4 bytes each, 1 KB total). There are many registers associated with each of the above I/O ports and they have designated addresses in the memory map. The above addresses are the Base addresses meaning that within that base address we have registers associated with that port.

Digital I/O Pads

The TM4C123GH6PM microcontroller contains six ports and thus six of these physical GPIO blocks. Note that not all pins are implemented on every block. Some GPIO pins can function as I/O signals for the on-chip peripheral modules.

GPIO Register Map

Name	Offset	Tivaware Name	Description
GPIODATA	0x000	GPIO_PORTn_DATA_R	GPIO Data
GPIODIR	0x400	GPIO_PORTn_DIR_R	GPIO Direction
GPIOIS	0x404	GPIO_PORTn_IS_R	GPIO Interrupt Sense
GPIOIBE	0x408	GPIO_PORTn_IBE_R	GPIO Interrupt Both Edges
GPIOIEV	0x40C	GPIO_PORTn_IEV_R	GPIO Interrupt Event
GPIOIM	0x410	GPIO_PORTn_IM_R	GPIO Interrupt Mask
GPIORIS	0x414	GPIO_PORTn_RIS_R	GPIO Raw Interrupt Status
GPIOMIS	0x418	GPIO_PORTn_MIS_R	GPIO Masked Interrupt Status
GPIOICR	0x41C	GPIO_PORTn_ICR_R	GPIO Interrupt Clear
GPIOAFSEL	0x420	GPIO_PORTn_AFSEL_R	GPIO Alternate Function Select
GPIODR2R	0x500	GPIO_PORTn_DR2R_R	GPIO 2-mA Drive Select
GPIODR4R	0x504	GPIO_PORTn_DR4R_R	GPIO 4-mA Drive Select
GPIODR8R	0x508	GPIO_PORTn_DR8R_R	GPIO 8-mA Drive Select
GPIOODR	0x50C	GPIO_PORTn_ODR_R	GPIO Open Drain Select
GPIOPUR	0x510	GPIO_PORTn_PUR_R	GPIO Pull-Up Select
GPIOPDR	0x514	GPIO_PORTn_PDR_R	GPIO Pull-Down Select
GPIOSLR	0x518	GPIO_PORTn_SLR_R	GPIO Slew Rate Control Select
GPIODEN	0x51C	GPIO_PORTn_DEN_R	GPIO Digital Enable
GPIOLOCK	0x520	GPIO_PORTn_LOCK_R	GPIO Lock
GPIOCR	0x524	GPIO_PORTn_CR_R	GPIO Commit
GPIOAMSEL	0x528	GPIO_PORTn_AMSEL_R	GPIO Analog Mode Select
GPIOPCTL	0x52C	GPIO_PORTn_PCTL_R	GPIO Port Control
		where n = A or B or C or D or E or F

Accessing GPIO of this microcontroller includes much more steps as this microcontroller provides a lot of feature in a single board, hence requires lots of configuration.

For accessing and configuring the microcontroller pins, we have to load some values in the registers, associated with that particular pin.

Initialization of an I/O port

To initialize an I/O port for general use:

1. Activate the clock for the port in the Run Mode Clock Gating Control Register 2 (RCGC2).
2. Unlock the port (LOCK = 0x4C4F434B). This step is only needed for pins PC0-3, PD7 and PF0 on TM4C123GXL LaunchPad.
3. Disable the analog function of the pin in the Analog Mode Select register (AMSEL), because we want to use the pin for digital I/O. If this pin is connected to the ADC or analog comparator, its corresponding bit in AMSEL must be set as 1. In our case, this pin is used as digital I/O, so its corresponding bit must be set as 0.
4. Clear bits in the port control register (PCTL) to select regular digital function. Each GPIO pin needs four bits in its corresponding PCTL register. Not every pin can be configured to every alternative function.
5. Set its direction register (DIR). A DIR bit of 0 means input, and 1 means output.
6. Clear bits in the alternate Function Select register (AFSEL).
7. Enable digital port in the Digital Enable register (DEN).

• Please note that we need to add a short delay between activating the clock and setting the port registers.
• PC0-PC3 is used for JTAG connections to the debugger on the LaunchPad. So we’d better do not use these pins normally.