The PRU modules of the BeagleBone's Sitara controller have a timer.

In this short post, I'm trying if I can get it to work.



The future plan is to use it in an algorithm for stepper motor control.

I'd like to make a smart algorithm that recalculates acceleration/deceleration of the stepper motor while that motor is running.



PRU Timer


It's a component of the unit's Industrial Ethernet Peripheral but can be repurposed as a generic timer within the real time unit.

I took the example timer project from TI that sets the timer for 10 seconds, waits for it to expire and then exits.

I added two line of code: set an output low when the PRU starts, then toggle it after 10 seconds. That lets me validate the timer without a debugger.


The output I chose is P9_29. The only reason I chose that one is because I have a PRU cape with a LED on that pin.

This cape is not required. You could just put a +-1K resistor with a LED in series between that pin and GND. Or a scope probe or voltmeter.





The main part of the code is setting all registers and initialising the timer. The original TI code with license can be found here.


volatile register uint32_t __R30;
volatile register uint32_t __R31;

void main(void)
  /* Clear SYSCFG[STANDBY_INIT] to enable OCP master port */

  /* Disable counter */

  /* Reset Count register */
  CT_IEP.TMR_CNT = 0x0;

  /* Clear overflow status register */

  /* Set compare value */
  CT_IEP.TMR_CMP0 = 0x77359400; // 10 seconds @ 200MHz

  /* Clear compare status */

  /* Disable compensation */

  /* Enable CMP0 and reset on event */
  CT_IEP.TMR_CMP_CFG_bit.CMP_EN = 0x1;

  /* Clear the status of all interrupts */



Then I set the GPIO pin (all pins, this is just a tryout), wait for the timer to expire and toggle the GPIO pin.


#define PIN    1

// set all GPIOs low
    __R30 = 0x0;

  /* Enable counter */
  CT_IEP.TMR_GLB_CFG = 0x11;

  /* Poll until R31.31 is set */
  do {
    while ((__R31 & 0x80000000) == 0) {
    /* Verify that the IEP is the source of the interrupt */
  } while ((CT_INTC.SECR0 & (1 << 7)) == 0);

   __R30 ^= 1UL << PIN;


After that, the program clears up and stops. It's not a blinky, just an output that switches high after 10 seconds. Sorry.


  /* Disable counter */
  CT_IEP.TMR_GLB_CFG_bit.CNT_EN = 0x0;

  /* Disable Compare0 */

  /* Clear Compare status */

  /* Clear the status of the interrupt */
  CT_INTC.SECR0 = (1 << 7);

  /* Halt the PRU core */


Run It


I've reused the TI project PRU_IEP and just added my lines to the source. See this blog for getting started with PRU code compilation in CCS.

Once the code is compiled, move it over to the BB's Linux file system (I'm assuming you moved it to /home/debian/bin).

Then execute these commands to prepare the PRU:


# set pin 29 as PRU GPIO out
config-pin P9_29 pruout

#load the firmware to PRU
cp /home/debian/bin/PRU_IEP.out /lib/firmware/PRU_IEP.out
echo 'PRU_IEP.out' > /sys/class/remoteproc/remoteproc1/firmware

#start the program
echo 'start' > /sys/class/remoteproc/remoteproc1/state



image: capture of the timer controlled pin with an ocilloscope


If you want to retry, stop the PRU and start it again:


echo 'stop' > /sys/class/remoteproc/remoteproc1/state
echo 'start' > /sys/class/remoteproc/remoteproc1/state


When finished, set the BB back to default.


echo 'stop' > /sys/class/remoteproc/remoteproc1/state
config-pin P9_29 default