Probem with PWM pairs

[aleandros]

I get a "RuntimeError: You must start() the PWM channel first" whenever I use two "contiguous" PWM pins. For example, If I want to use P9_29 and P9_31, which correspond to EHRPWM0A and EHRPWM0B (or P9_14 and P9_16), I can start them with PWM.start, and I can change the duty cycle of the first one without problems, but when I try to call PWM.set_duty_cycle on the second one, I get the aforementioned error.

Thanks in advance!

[jodyhi]

I think I have a work-around for this. It only happens for me if I set the frequency in the start() call. If I start both PWM pins with the default frequency (2kHz) and then do set_frequency() on both pins after that, I can set the duty cycle on both of them without error.

import Adafruit_BBIO.PWM as PWM
import time

green = "P8_19"
red = "P8_13"

PWM.start(red, 0)
PWM.start(green, 0)
PWM.set_frequency(red, 60.0)
PWM.set_frequency(green, 60.0)

def fade(colorA, colorB):
    for i in range(0, 101):
        PWM.set_duty_cycle(colorA, i)
        PWM.set_duty_cycle(colorB, 100-i)
        time.sleep(0.05)
    time.sleep(0.5)

try:
    while True:
        fade(red, green)
        fade(green, red)
except KeyboardInterrupt:
    PWM.cleanup()

[roadkell]

The problem is still there. Sometimes it happens, sometimes not. Start-stop-cleanup-start cycles occasionally help, but more often than not, the only way to get all PWMs up is to reboot and try again.

adafruit-beaglebone-io-python 0.0.30 (previously used 0.0.19, same problem)
Linux version 3.8.13-bone71 (root@a3-imx6q-wandboard-2gb) (gcc version 4.6.3 (Debian 4.6.3-14) ) #1 SMP Tue Mar 17 18:07:44 UTC 2015
Debian GNU/Linux 7.8 (wheezy)
BeagleBoard.org BeagleBone Debian Image 2014-04-23
BeagleBone Black Rev C (also tried Rev B, same problem)

[raunaqbhirangi]

Any updates on this issue? I just can't get around this.

[roadkell]

These instructions worked for me. Note that this includes applying ti_ehr_pwm.patch and building the 3.8-rt kernel. It seems to work for some kernel versions (Debian 7.5, 3.8.13-bone50), but fails for later versions (Debian 7.8, 3.8.13-bone70). I've also tried this solution, but it seems to fail without patching the kernel.
These instructions definitely won't work for 4.* kernels, since they've redesigned cape manager, device tree compiler and overlays. More info here and here.
I'll probably stick with 3.8 until there are clear solutions for later kernels.

[raunaqbhirangi]

The best and simplest solution to resolve this problem is running it on the default frequency of 2kHz. This seems to work.

[roadkell]

Go ahead and drive some servos with 2 kHz.

[pdp7]

It looks like this commit may have addressed the issue in the kernel:

commit 2543b5f335e2812369d0de2d21dc92526832b7d5
Author: Robert Nelson <robertcnelson@gmail.com>
Date:   Tue Sep 15 14:26:18 2015 -0500

    pwm: merge https://github.com/beagleboard/linux/pull/45

    Signed-off-by: Robert Nelson <robertcnelson@gmail.com>

 patch.sh                                    |  4 +++-
 patches/fixes/0011-PWM-period-control.patch | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 patches/fixes/0012-PWM-period-control.patch | 53 +++++++++++++++++++++++++++++++++++++++++++
 version.sh                                  |  2 +-
 4 files changed, 138 insertions(+), 2 deletions(-)

The Python example above does seem to be working in 3.8.13-bone80:

• Repo: http://github.com/RobertCNelson/bb-kernel
• Branch: am33x-v3.8

[pdp7]

@aleandros There have been several PWM improvements now merged from #108. Please try again and report if this is still an issue.