I wanted to use timers in TM4C123GH6PM (in TIVA C series TM4C123G Launchpad evaluation kit). So I decided to use GPTM TimerA0 in periodic timer mode. I defined the address of the registers of GPTM and followed the steps given in the section 11.4.1 One-Shot/Periodic Timer Mode of the datasheet in page 722. I want a LED to blink after every 3 seconds (connected to PORT F-pin 1). But the LED is always ON. Are the address of the registers to which I am referring wrong? Or something else is the issue with the code?

//TIMER Registers
#define RCGCTIMER        (*((volatile unsigned long *)0x400FE604))
#define GPTMCTL                  (*((volatile unsigned long *)0x4003000C)) //timer zero
#define GPTMCFG                  (*((volatile unsigned long *)0x40030000))
#define GPTMTAMR                 (*((volatile unsigned long *)0x40030004))  
#define GPTMTAILR                (*((volatile unsigned long *)0x40030028))
#define GPTMRIS                (*((volatile unsigned long *)0x4003001C))
#define GPTMICR                (*((volatile unsigned long *)0x40030024))

//PORT F Registers
#define GPIO_PORTF_DATA_R       (*((volatile unsigned long*)0x400253FC))
#define GPIO_PORTF_DIR_R        (*((volatile unsigned long *)0x40025400))
#define GPIO_PORTF_AFSEL_R      (*((volatile unsigned long *)0x40025420))
#define GPIO_PORTF_DEN_R        (*((volatile unsigned long *)0x4002551C))
#define SYSCTL_RCGC2_R          (*((volatile unsigned long *)0x400FE108))
#define SYSCTL_RCGC2_GPIOF      0x00000020  // port F Clock Gating Control 

void initializeTimer()
    RCGCTIMER |= 0x00000001; //To use a GPTM, the appropriate TIMERn bit must be set in the RCGCTIMER. Here it is TIMER0

    //Periodic timer mode

    GPTMCTL &=0xFFFFFFFE; //TAEN is set 0. Timer A is disabled.
    GPTMCFG = 0x00000000; //Write the GPTM Configuration Register (GPTMCFG) with a value of 0x0000.0000
    GPTMTAMR |=0x00000002; GPTMTAMR &=0xFFFFFFFE; //TAMR is set 0x2. Periodic Timer mode is used (first bit1 is set 1 and then bit0 is set 0 in two statements)
    GPTMTAMR &= 0xFFFFFFEF; //TACDIR is set 0. The timer counts down.
    GPTMTAILR = 0x02DC6C00; //TAILR is set to 48,000,000 Hz
    GPTMCTL |=0x00000001; //TAEN is set 1. Timer A is enabled.

void initializePORTF()
  volatile unsigned long delay;
  SYSCTL_RCGC2_R |= 0x00000020;     // 1) F clock
  delay = SYSCTL_RCGC2_R;           // delay   
  GPIO_PORTF_DIR_R |= 0x02;         // PF1 output  
  GPIO_PORTF_AFSEL_R &= 0x00;       // No alternate function// 1) F clock
  GPIO_PORTF_DEN_R |= 0x02;         // Enable digital pins PF1  
  GPIO_PORTF_DATA_R |= 0x02;        //PF1 Set to 1. LED is ON

int main()

        //did TATORIS in GPTMRIS become 1??
        if((GPTMRIS | 0x00000001) == 1)
            GPTMICR |= 0x00000001; //Set 1 to TATOCINT. Writing a 1 to this bit clears the TATORIS bit in the GPTMRIS register and the TATOMIS bit in the GPTMMIS register. 
            GPIO_PORTF_DATA_R ^= 0x02; //Toggle PF1. Toggle LED
  • \$\begingroup\$ @Mahendra Gunawardena Pin1 of port F (red LED on the board) \$\endgroup\$ – user3219492 Jan 6 '17 at 9:47
  • 2
    \$\begingroup\$ Just a heads-up; If you intend on using the controllers of this series (or indeed manufacturer), check the Silicon Errata Sheet. Which comes separately. Because for many of the 123's it's 110+ pages. \$\endgroup\$ – Asmyldof Jan 6 '17 at 10:09
  • \$\begingroup\$ I have posted a potential solution. Also I have always used interrupts with times. I don't see you enabling interrupts. \$\endgroup\$ – Mahendra Gunawardena Jan 6 '17 at 10:41
  • \$\begingroup\$ @Asmyldof +100. Especially for TM4C123 you need to be extra careful when designing your own board. One single little issue in errata can be a reason to add dozens of additional components to your PCB (see GPIO#10 for instance)... \$\endgroup\$ – GAttuso Jan 6 '17 at 14:53
  • \$\begingroup\$ @GAttuso if TI's "modern" Processor Design branch goes bankrupt tomorrow, I won't loose much sleep over it. \$\endgroup\$ – Asmyldof Jan 6 '17 at 17:06

Also here is some example code for TIVA Timer setup. You might be able look at this and get some idea

#define TIMER0_CFG_R            (*((volatile unsigned long *)0x40030000))
#define TIMER0_TAMR_R           (*((volatile unsigned long *)0x40030004))
#define TIMER0_CTL_R            (*((volatile unsigned long *)0x4003000C))
#define TIMER0_IMR_R            (*((volatile unsigned long *)0x40030018))
#define TIMER0_MIS_R            (*((volatile unsigned long *)0x40030020))
#define TIMER0_ICR_R            (*((volatile unsigned long *)0x40030024))
#define TIMER0_TAILR_R          (*((volatile unsigned long *)0x40030028))
#define TIMER0_TAPR_R           (*((volatile unsigned long *)0x40030038))
#define TIMER0_TAR_R            (*((volatile unsigned long *)0x40030048))
#define TIMER_CFG_16_BIT        0x00000004  // 16-bit timer configuration,
                                            // function is controlled by bits
                                            // 1:0 of GPTMTAMR and GPTMTBMR
#define TIMER_TAMR_TAMR_PERIOD  0x00000002  // Periodic Timer mode
#define TIMER_CTL_TAEN          0x00000001  // GPTM TimerA Enable
#define TIMER_IMR_TATOIM        0x00000001  // GPTM TimerA Time-Out Interrupt
                                            // Mask
#define TIMER_ICR_TATOCINT      0x00000001  // GPTM TimerA Time-Out Raw
                                            // Interrupt
#define TIMER_TAILR_TAILRL_M    0x0000FFFF  // GPTM TimerA Interval Load
                                            // Register Low
#define SYSCTL_RCGC1_R          (*((volatile unsigned long *)0x400FE104))
#define SYSCTL_RCGC1_TIMER0     0x00010000  // timer 0 Clock Gating Control

void DisableInterrupts(void); // Disable interrupts
void EnableInterrupts(void);  // Enable interrupts
long StartCritical (void);    // previous I bit, disable interrupts
void EndCritical(long sr);    // restore I bit to previous value
void WaitForInterrupt(void);  // low power mode
void (*PeriodicTask)(void);  // user function
// ***************** Timer0A_Init ****************
// Activate Timer0A interrupts to run user task periodically
// Inputs:  task is a pointer to a user function
//          period in usec
// Outputs: none
void Timer0A_Init(void(*task)(void), unsigned short period){ 
  SYSCTL_RCGC1_R |= SYSCTL_RCGC1_TIMER0; // 0) activate timer0
  PeriodicTask = task;             // user function 
  TIMER0_CTL_R &= ~0x00000001;     // 1) disable timer0A during setup
  TIMER0_CFG_R = 0x00000004;       // 2) configure for 16-bit timer mode
  TIMER0_TAMR_R = 0x00000002;      // 3) configure for periodic mode
  TIMER0_TAILR_R = period;         // 4) reload value
  TIMER0_TAPR_R = 49;              // 5) 1us timer0A
  TIMER0_ICR_R = 0x00000001;       // 6) clear timer0A timeout flag
  TIMER0_IMR_R |= 0x00000001;      // 7) arm timeout interrupt
  NVIC_PRI4_R = (NVIC_PRI4_R&0x00FFFFFF)|0x40000000; // 8) priority 2
  NVIC_EN0_R |= NVIC_EN0_INT19;    // 9) enable interrupt 19 in NVIC
  TIMER0_CTL_R |= 0x00000001;      // 10) enable timer0A
void Timer0A_Handler(void){
  TIMER0_ICR_R = TIMER_ICR_TATOCINT;// acknowledge timer0A timeout
  (*PeriodicTask)();                // execute user task

On the TIVA, TM4C123GH6PM micro controller Port F is a special port. PF0 on TIVA need to be unlock.

enter image description here
Click on image for a larger version of the image.

Below is example code.

void PortF_Init(void){ volatile unsigned long delay;
  SYSCTL_RCGC2_R |= 0x00000020;     // 1) activate clock for Port F
  delay = SYSCTL_RCGC2_R;           // allow time for clock to start
  GPIO_PORTF_LOCK_R = 0x4C4F434B;   // 2) unlock GPIO Port F
  GPIO_PORTF_CR_R = 0x1F;           // allow changes to PF4-0
  // only PF0 needs to be unlocked, other bits can't be locked
  GPIO_PORTF_AMSEL_R = 0x00;        // 3) disable analog on PF
  GPIO_PORTF_PCTL_R = 0x00000000;   // 4) PCTL GPIO on PF4-0
  GPIO_PORTF_DIR_R = 0x0E;          // 5) PF4,PF0 in, PF3-1 out
  GPIO_PORTF_AFSEL_R = 0x00;        // 6) disable alt funct on PF7-0
  GPIO_PORTF_PUR_R = 0x11;          // enable pull-up on PF0 and PF4
  GPIO_PORTF_DEN_R = 0x1F;          // 7) enable digital I/O on PF4-0

Please take note of

GPIO_PORTF_LOCK_R = 0x4C4F434B; // 2) unlock GPIO Port F

Also I suggest you sign up for Embedded Systems - Shape The World. They use TIVA and it is great semi self-paced course.


  • \$\begingroup\$ Thank you for the reply. Unfortunately, UNLOCKING the PORT F made no difference. I also made use of the registers you provided (except that I chose to Poll instead of interrupt) and again the red LED is always ON. Thank you so much for the reference links (especially users.ece.utexas.edu/~valvano/arm) \$\endgroup\$ – user3219492 Jan 6 '17 at 19:50
  • \$\begingroup\$ @user3219492 Your are most welcome, I usually follow the course each year for fun. So identify yourself in the forums. Good luck. Also I notice that you are trying to use Pin 1, not Pin 0, only pin 0 in port F is locked. \$\endgroup\$ – Mahendra Gunawardena Jan 6 '17 at 23:30
  • \$\begingroup\$ Okay. I have posted the question in TI Community forums. Let's wait for that. So the unlocking statement in not going to make any effect as I am not using PF0. \$\endgroup\$ – user3219492 Jan 7 '17 at 5:45

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