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  • Author Author: Jan Cumps
  • Date Created: 12 Aug 2019 6:23 PM Date Created
  • Views 8057 views
  • Likes 9 likes
  • Comments 26 comments
  • timer
  • pru
  • BeagleBone
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BeagleBone PRU - Timer Functionality

Jan Cumps
Jan Cumps
12 Aug 2019

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.

 

image

 

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.

 

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Code

 

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 */
  CT_CFG.SYSCFG_bit.STANDBY_INIT = 0;

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

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

  /* Clear overflow status register */
  CT_IEP.TMR_GLB_STS_bit.CNT_OVF = 0x1;

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

  /* Clear compare status */
  CT_IEP.TMR_CMP_STS_bit.CMP_HIT = 0xFF;

  /* Disable compensation */
  CT_IEP.TMR_COMPEN_bit.COMPEN_CNT = 0x0;

  /* Enable CMP0 and reset on event */
  CT_IEP.TMR_CMP_CFG_bit.CMP0_RST_CNT_EN = 0x1;
  CT_IEP.TMR_CMP_CFG_bit.CMP_EN = 0x1;

  /* Clear the status of all interrupts */
  CT_INTC.SECR0 = 0xFFFFFFFF;
  CT_INTC.SECR1 = 0xFFFFFFFF;

 

 

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);

  //toggle
   __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 */
  CT_IEP.TMR_CMP_CFG = 0x0;

  /* Clear Compare status */
  CT_IEP.TMR_CMP_STS = 0xFF;

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

  /* Halt the PRU core */
  __halt();
}

 

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

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

.

Attachments:
bb_PRU_TIMER.zip
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Top Comments

  • Jan Cumps
    Jan Cumps over 6 years ago +3
    @self-documentation on the part where the timer timeout is detected: This part: /* Poll until R31.31 is set */ do { while ((__R31 & 0x80000000 ) == 0 ) { } The __R31 bit 31 is local interrupt 1: and "…
  • shabaz
    shabaz over 6 years ago +2
    Hi Jan, This is very useful information! It's new to me that the PRU could access a 32-bit timer clocked at 200MHz. That's fantastic.
  • grondak
    grondak over 6 years ago +2
    I last was interested in these PRUs over 5 years ago. They are just as you said: the key to unlock determinism in a Linux system. My application needed to start the timer, then later read the timer value…
  • Jan Cumps
    Jan Cumps over 5 years ago in reply to Jan Cumps

    The PRU in action:

     

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  • Jan Cumps
    Jan Cumps over 5 years ago

    I just retried the project and it's still working. Now tested with Debian Buster, CCS 10, TI Compiler TI.v2.3.3

    All commands executed with sudo powers.

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  • Jan Cumps
    Jan Cumps over 5 years ago in reply to jaytiru

    jaytiru  wrote:

     

    How to configure interrupts for R31 bit. i am using Beagle Bone AI and unable to get interrupt on R31. 31.

    Please help

    Hopefully someone who has a BB AI and is interested in PRU C programming can chime in.

     

    In the mean time: how do you know that you don't get the interrupt after 10 seconds?

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  • Jan Cumps
    Jan Cumps over 5 years ago in reply to jaytiru

    I don't set an interrupt on the pin in this example.

     

    I poll until  the interrupt bit _R31.31 is set ( _R31 == 0x80000000.).

    This happens after 10 seconds when the timer expires.

    The PRU process flow is not actually interrupt driven, you have to poll interrupt flags.

    For a real time unit that makes sense - because a real interrupt would break the deterministic real time mechanism.

     

      /* Clear the status of all interrupts */  
      CT_INTC.SECR0 = 0xFFFFFFFF;  
      CT_INTC.SECR1 = 0xFFFFFFFF;  
    
      /* 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); 

     

     

    Look at my other blog on how to set the pins to PRU functionality: BeagleBone Control Stepper Motors with PRU - Part 2: Test Driving Outputs

    I can't confirm if this works on a BB AI. I don't have one.

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  • jaytiru
    jaytiru over 5 years ago

    How to configure interrupts for R31 bit. i am using Beagle Bone AI and unable to get interrupt on R31. 31.

    Please help

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