30us PWM signals not working on Navio2

Navio / Edge Hardware
1

Hi,

I am trying to send 30us PWM signals to the KISS 24A ESCs, but the pulses are not recognized by the ESC. The tolerance is +/- 2us. However, 900us for arming, 1000us for start and up to 2000us work just fine.

Any suggestions on how to get 30us PWM signals on Navio2?

I am using Navio2, a Raspberry Pi 3, and the examples provided in the Navio2 emlid repo. The servo frequency is 400Hz.
The 30us PWM signals worked on Navio+ which uses PCA9685 for PWM output.

Edit: Figured I could update the rcio code via emlidtool and see if it fixes the issue with the 30us PWM signals. This gives an error, because /sys/kernel/rcio/status/crc is not existing. I am using a freshly installed raspbian version 4.4.36-a7765e7-emlid-v7+. I need Jessie instead of Stretch, because of other dependencies, e.g. libcrypto1.0.0.

Edit2: Tried to compile rcio-dkms-0.8.3 and also 0.9.0 on the pi, which both failed with

include/linux/ratelimit.h:31:9: error: initializer element is not constant
struct ratelimit_state name =
^

Edit3: switched to raspbian stretch emlid image. Have now the latest rcio firmware v1.0 and the rcio kernel module 0.9.0 installed. Still, no PWM pulses with 30ns possible!

2

Out of curiosity, how do you inspect the PWM-signal?

3

Well, to tell the truth, I didn’t use an oscilloscope or a logic analyzer.

The KISS ESC answers to a 30us PWM signal with a serial telemetry message, which is received on the raspberry pi 3 via Navio2 UART Rx. As I said, the code worked on Navio+ (of course with emlid’s Navio+ raspbian image instead of the Navio2 raspbian image). And since the ESC did not answer, I figured that it did not receive the 30us pulse.

I found the line .rc_min = 900 in rcio-dkms/src/rcio_pwm.c at master · emlid/rcio-dkms · GitHub . But I guess that this is only used for receiving remote control pulses. Nevertheless, I recompiled with .rc_min = 20, just to be sure.

I also checked the PWM values under /sys/class/pwm/pwmchip0/pwm0/duty_cycle and for

pwm.set_duty_cycle(0, (float)30/1000.0);

they are 29999 and

pwm.set_duty_cycle(0, (float)31/1000.0);

results in 30999.

With the latter 31us and also with 32us, I finally got the ESCs to send telemetry back. So it seems that the PWM timing on the STM32F103 is inaccurate?

4

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