Example code using a timer with the BeagleBone Black's PRU microcontroller. This code generates 5 pulses with 100ns width.

main.c

/*
 * Demonstration of the BeagleBone's PRU, using a timer.
 * Ken Shirriff, http://righto.com
 */
#include <stdint.h>

#define DELAY_NS 100 // Use 500000000 for 0.5 second delay
#define WRITE_PIN 15 /* P8_11, Ethernet output data, pr1_PRU0_pru_r30_15 */

// PRU Interrupt control registers
#define PRU_INTC 0x00020000 // Start of PRU INTC registers TRM 4.3.1.2
#define PRU_INTC_GER ((volatile uint32_t *)(PRU_INTC + 0x10)) // Global Interrupt Enable, TRM 4.5.3.3
#define PRU_INTC_SICR ((volatile uint32_t *)(PRU_INTC + 0x24)) // Interrupt, TRM 4.5.3.6
#define PRU_INTC_GPIR ((volatile uint32_t *)(PRU_INTC + 0x80)) // Interrupt, TRM 4.5.3.11

// PRU ECAP control registers (i.e. PWM used as a timer)
#define ECAP 0x00030000 // ECAP0 offset, TRM 4.3.1.2
// Using APWM mode (TRM 15.3.2.1) to get timer (TRM 15.3.3.5.1)
#define ECAP_TSCTR ((volatile uint32_t *)(ECAP + 0x00)) // 32-bit counter register, TRM 15.3.4.1.1
#define ECAP_APRD ((volatile uint32_t *)(ECAP + 0x10)) // Period shadow, TRM 15.3.4.1.5, aka CAP3
#define ECAP_ECCTL2 ((volatile uint32_t *)(ECAP + 0x2a)) // Control 2, TRM 15.3.4.1.8
#define ECAP_ECEINT ((volatile uint16_t *)(ECAP + 0x2c)) // Enable interrupt, TRM 15.3.4.1.9
#define ECAP_ECCLR ((volatile uint16_t *)(ECAP + 0x30)) // Clear flags, TRM 15.3.4.1.11

// R30 is the GPIO register
// R31 is the interrupt register
volatile register unsigned int __R31, __R30;

// Forward definitions
void init_pwm();
void wait_for_pwm_timer();

void main() {
    init_pwm();
    int i;
    for (i = 0; i < 5; i++) {
        wait_for_pwm_timer();
        __R30 = 1 << WRITE_PIN;
        wait_for_pwm_timer();
        __R30 = 0;
    }
    __R31 = 35;  // Interrupt to host
    __halt();
}

// Initializes the PWM timer, used to control output transitions.
// Every DELAY_NS nanoseconds, interrupt 15 will fire
inline void init_pwm() {
    *PRU_INTC_GER = 1; // Enable global interrupts
    *ECAP_APRD = DELAY_NS / 5 - 1; // Set the period in cycles of 5 ns
    *ECAP_ECCTL2 = (1<<9) /* APWM */ | (1<<4) /* counting */;
    *ECAP_TSCTR = 0; // Clear counter
    *ECAP_ECEINT = 0x80; // Enable compare equal interrupt
    *ECAP_ECCLR = 0xff; // Clear interrupt flags
}

// Wait for the PWM timer to fire.
// see TRM 15.2.4.26
inline void wait_for_pwm_timer() {
    while (!(__R31 & (1 << 30))) {} // Wait for timer compare interrupt
    *PRU_INTC_SICR = 15; // Clear interrupt
    *ECAP_ECCLR = 0xff; // Clear interrupt flags
}

Hello, how can I compile and run this snippet on my Beaglebone Black? I'm sorry for posing such a novice question. I've tried searching your blog, but couldn't find any solutions. Thanks

I'm not the author but you essentially download http://www.ti.com/tool/PRU-CGT and use a makefile from one of the PRU example such as those examples in the sdk http://www.ti.com/tool/PROCESSOR-SDK-AM335X