Issue with PRU cape LED hello world program

Hi everyone,

I’m a beaglebone/PRU newbie and I’m stuck on getting a basic hello world program up and running on the PRU. My goal is to use load a simple PRU assembly program to light an LED on the PRU development cape. The light corresponds to pin P9.28. I’m using the uio_pruss kernel module.

Rather than using an overlay, I’m opting to use config-pin since my application will eventually require a fair bit of dynamic pin reconfiguration. I note in this post that the PRU is disabled in the default overlay. Using the method suggested in the post, I modify the default overlay I’m using (am335x-boneblack-emmc-overlay.dtb) to change the status of the pruss@4a300000 fragment from “disabled” to “okay”. To make sure that my board is now using my modified overlay, I’ve also disabled the heartbeat LED. When I reboot, the heartbeat flash is gone and when I use dtc -I fs /sys/firmware/devicetree/base to look at the current device tree, the fragment is as follows:


pruss@4a300000 {
compatible = “ti,pruss-v2”;
ti,pintc-offset = <0x20000>;
ti,hwmods = “pruss”;
ti,deassert-hard-reset = “pruss”, “pruss”;
status = “okay”;
interrupt-parent = <0x1>;
interrupts = <0x14 0x15 0x16 0x17 0x18 0x19 0x1a 0x1b>;
phandle = <0xa5>;
reg = <0x4a300000 0x80000>;
linux,phandle = <0xa5>;


The cape seems to be working fine since “config-pin P9.28 hi” and “config-pin P9.28 low” turns the LED on an off.

My PRU assembly code, lighton.p, is:


.origin 0
.entrypoint start

set r30.t3
jmp rept


and my app loader code, loader.c, is:


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <prussdrv.h>
#include <pruss_intc_mapping.h>

#define PRU_NUM 0

static int pru_cleanup(void) {
int rtn = 0;

if((rtn = prussdrv_pru_disable(PRU_NUM)) != 0) {
fprintf(stderr, “prussdrv_pru_disable() failed\n”);
rtn = -1;

if((rtn = prussdrv_exit()) != 0) {
fprintf(stderr, “prussdrv_exit() failed\n”);
rtn = -1;

return rtn;

int main(int argc, char **argv) {
int rtn;

if((rtn = prussdrv_init()) != 0) {
fprintf(stderr, “prussdrv_init() failed\n”);
return rtn;

if((rtn = prussdrv_open ( PRU_EVTOUT_0 )) != 0) {
fprintf(stderr,“prussdrv_open() failed\n”);
return rtn;

if((rtn = prussdrv_exec_program(PRU_NUM, “./lighton.bin”)) < 0) {
fprintf(stderr, “prussdrv_exec_program() failed\n”);
return rtn;

getchar(); /* Pause to let the program execute. */

return pru_cleanup();


The following is a screen capture of what I’m doing to try and get all of this to run:


root@beaglebone:~/pru-helloworld# uname -r
root@beaglebone:~/pru-helloworld# lsmod
Module Size Used by
spidev 7466 0
c_can_platform 6486 0
c_can 9942 1 c_can_platform
can_dev 12174 1 c_can
pwm_tiehrpwm 4920 0
spi_omap2_mcspi 10167 0
pwm_tiecap 3801 0
tieqep 7575 0
omap_aes 13012 0
omap_sham 21488 0
crypto_engine 5939 1 omap_aes
uio_pruss 4928 0
omap_rng 4572 0
rng_core 7340 1 omap_rng
evdev 10035 1
tps65217_charger 4723 0
omap_wdt 4159 0
leds_gpio 3308 0
uio_pdrv_genirq 3437 0
uio 8614 2 uio_pruss,uio_pdrv_genirq
cpufreq_dt 4805 0
8021q 18006 0
garp 5788 1 8021q
mrp 7092 1 8021q
stp 2117 1 garp
llc 5021 2 garp,stp
cpufreq_powersave 1567 0
cpufreq_conservative 3649 0
root@beaglebone:~/pru-helloworld# ls /dev/ui*
/dev/uinput /dev/uio0 /dev/uio1 /dev/uio2 /dev/uio3 /dev/uio4 /dev/uio5 /dev/uio6 /dev/uio7
root@beaglebone:~/pru-helloworld# pasm -b lighton.p

PRU Assembler Version 0.87
Copyright © 2005-2013 by Texas Instruments Inc.

Pass 2 : 0 Error(s), 0 Warning(s)

Writing Code Image of 3 word(s)
root@beaglebone:~/pru-helloworld# gcc -o loader -ggdb loader.c -lprussdrvd
root@beaglebone:~/pru-helloworld# config-pin -q P9.28
P9_28 Mode: gpio Direction: out Value: 0
root@beaglebone:~/pru-helloworld# ./loader
File ./lighton.bin open passed



but the LED remains dark.

Any suggestions would be greatly appreciated! Thanks in advance for your help and apologies in advance if this is something super obvious that I’ve missed.



If you use PRU’s “r30”, then you need to mux your pin to “pruout” mode. The “gpio” mode is for the “classic” slow GPIOs that are accessed through the L3 bus.

I haven’t tested, but you may want to try the following command:
$ config-pin P9.28 pruout


Yes! That was it, thanks so much. I should have spotted that since that mode shows up as an option when I type the command config-pin -q P9.28. After I typed config-pin P9.28 pout, the LED turned on when I loaded the PRU assembly code as described above.

Thanks again!