TM4C123G LED Example

LED connection

The LEDs are connected to the GPIO port F pins of the microcontroller.
To use the LEDs, the values of the GPIO port F need to be set –> address of GPIO PF needed.

Bit-wise setting of the LED pins

LED	Pin number	Binary	Hex
Red	1	`0010`	`0x2`
Blue	2	`0100`	`0x4`
Green	3	`1000`	`0x8`
All	1, 2, 3	`1110`	`0xE`

Using bit shift operations , the first, second and third bits can be defined as

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#define LED_RED 	(1U << 1)
#define LED_BLUE 	(1U << 2)
#define LED_GREEN	(1U << 3)

This is a cleaner alternative to the hex values above.

All the LEDs together would be

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ALL_LEDS = LED_RED | LED_BLUE | LED_GREEN

Memory map

The two different address ranges are because the GPIO are connected to two different peripheral busses :

the APB (advanced peripheral bus) –default, but older and slower
the AHB (advanced high-performance bus) – newer, faster

Initially, the Port F address range contains no data due to clock gating .

RCGCGPIO (Register for Clock Gating Control of GPIO) must be set to enabled for the GPIO module to be activated
- The GPIO module gets a clock signal
- Access to the GPIO module registers are allowed
From the datasheet:
Note that each GPIO module clock must be enabled before the registers can be programmed.
There must be a delay of 3 system clocks after the GPIO module clock is enabled before any GPIO module registers are accessed.

Remove clock gating

The register address is the base + offset:
0x400F’E000 + 0x608 = 0x400F’E608
This register contains 32 bits = 4 bytes of data = 2 hex words

Set the fifth bit to wake up Port F:

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unsigned int *RCGCGPIO = (unsigned int *)0x400FE608U;	
*RCGCGPIO |= (1U << 5);

Now clock gating is removed for GPIO Port F
But the GPIO-F bits still need to be configured as digital outputs

Switch to AHB

Use GPIOHBCTL (GPIO High-Performance Bus Control) and set the corresponding bit for Port F (5th bit).

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unsigned int *GPIOHBCTL = (unsigned int *)0x400FE06CU;
*GPIOHBCTL |= (1U << 5);

Configuring GPIO-F

Set pins as outputs

Use GPIODIR (GPIO direction) with offset 0x400.
Setting a bit in GPIODIR sets the corresponding pin to be an output.

Perform bit setting for all LEDs

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unsigned int *GPIODIR = (unsigned int *)0x4005D400U;
*GPIODIR |= ALL_LEDS;

Set pins to be digital outputs

Default is tristate –> need to change to digital.
Use GPIODEN (GPIO digital enable) with offset 0x51C

Perform bit setting for all LEDs

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unsigned int *GPIODEN = (unsigned int *)0x4005D51CU;
*GPIODEN |= ALL_LEDS;

Setting the LED pins

Read-modify-write

Use GPIO_PORTF_AHB_DATA_R from the header file, which is defined to be

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#define GPIO_PORTF_AHB_DATA_R (*((volatile unsigned long *)0x4005D3FC))

Hex 0x3FC corresponds to $1111'1111'00_{2}$, which, using bit masking , corresponds to ‘opening’ up all the Port F pins for modification, i.e. setting all eight bits to be writable.

Initialising the blue light to be on:

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GPIO_PORTF_AHB_DATA_R = LED_BLUE;

Bit setting of a different LED without affecting the other bits (here: without changing the state of the blue LED):

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GPIO_PORTF_AHB_DATA_R |= LED_RED; 	// red on
GPIO_PORTF_AHB_DATA_R &= ~LED_RED; 	// red off

In the above, assembly carries out a read-modify-write sequence:
LDR -> (ORR.W, etc.) -> STR

This sequence is necessary because the GPIO pins data are stored within a single byte, hence, at a single memory address.
What would be better: separate access of each bit at its own unique address
- Single atomic write operation to this unique address.
- True independent bit manipulation.
- In interrupt routines , GPIO bits may be modified.
  - If a read-modify-write cycle for GPIO bit setting is used, and the ISR happens after the read and before the modify, the changes made by the ISR will be discarded.
  - This behaviour may not be desired, hence the read-modify-write cycle is often replaced with the single write operation
- –> Bit masking

Bit masking

Uses the bits of the address bus as a mask.
Allows modifications of individual GPIO pins without affecting the others, in a single instruction.
Is interrupt -safe.

GPIO bits are connected to CPU by a bus (address lines + data lines)
Each GPIO bit is connected to its down
- data line
- address line
The first two bits of the address line are unused, as the hardware requires all addresses to be divisible by four.
Each bit combination has its own address – therefore, this hardware design requires many registers with unique addresses.
The GPIO bit can only be modified when the corresponding address line has been set, regardless of the value of the data line.

In this example, there are 256 x 32-bit DATA registers for GPIO Port F.

Address arithmetic

e.g. for the red LED

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/*calculate the address*/
LED_RED = 1U << 1;
GPIO_PORTF_BASE = 0x40005D00U;

LED_RED_ADDRESS = GPIO_PORTF_BASE + (LED_RED << 2)

/*assign the value*/
unsigned long volatile *p_LED_RED = (unsigned long volatile *)LED_RED_ADDRESS;
*p_LED_RED = LED_RED;

Note: the 2 bit shifts are to skip the unused address lines

Pointer arithmetic, or array indexing

Alternatively, array indexing of the defined macro GPIO_PORTF_AHB_DATA_BITS_R can be used.

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GPIO_PORTF_AHB_DATA_BITS_R[LED_RED] = LED_RED;

Final code

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#include "lm4f120h5qr.h"
#include "delay.h"

#define PORT_F 		(1U << 5);

#define LED_RED 	(1U << 1)
#define LED_BLUE 	(1U << 2)
#define LED_GREEN	(1U << 3)
#define ALL_LEDS	(LED_RED | LED_BLUE | LED_GREEN)

int main(){
	SYSCTL_RCGCGPIO_R |= PORT_F;	// enable clock
	SYSCTL_GPIOHBCTL_R |= PORT_F; 	// enable AHB
	
	// set LED GPIO to digital outputs
	GPIO_PORTF_AHB_DIR_R |= ALL_LEDS;
	GPIO_PORTF_AHB_DEN_R |= ALL_LEDS;
	
	// set blue LED to on
	GPIO_PORTF_AHB_DATA_BITS_R[LED_BLUE] = LED_BLUE;
	
	while(1){
		GPIO_PORTF_AHB_DATA_BITS_R[LED_RED] = LED_RED;
		delay();
		
		GPIO_PORTF_AHB_DATA_BITS_R[LED_RED] = 0;
		delay();		
	}
	
	return 0;
}

led-example

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