Hi @spatra,
Thank you for the detailed descriptions. Now I am pretty sure that there is an upper limitation on the pwm-2 (pin-33). :-)
Before moving to the conclusion, I would love to share the minimal-test-procedures (for some other fellows) who want to investigate this issue.
Step0. Reboot a Jetson Nano.
sudo reboot
Step1. Enable pwm0 (pin-32) and pwm2 (pin-33) using a command:
sudo /opt/nvidia/jetson-io/jetson-io.py
Step2. Check the PWM status.
sudo cat /sys/kernel/debug/pwm
It will probably look like as follows:
platform/7000a000.pwm, 4 PWM devices
pwm-0 ((null) ): period: 0 ns duty: 0 ns polarity: normal
...
pwm-2 ((null) ): period: 0 ns duty: 0 ns polarity: normal
...
Step3. Export pwm-0 (pin-32) and pwm-2 (pin-33) using a command:
echo 0 > /sys/class/pwm/pwmchip0/export
echo 2 > /sys/class/pwm/pwmchip0/export
Step4. Check the PWM status.
sudo cat /sys/kernel/debug/pwm
For this time, it will probably look like as follows:
platform/7000a000.pwm, 4 PWM devices
pwm-0 (sysfs ): requested period: 0 ns duty: 0 ns polarity: normal
...
pwm-2 (sysfs ): requested period: 0 ns duty: 0 ns polarity: normal
...
Step5. Freely change the period and duty_cycle and apply it on the pin.
For example, if you want to set:
*Target: pwm-2 (pin-33 of J41)
*Frequency : 100 kHz
*Duty cycle: 20 %
Please note that the unit of period and duty_cycle is nano-seconds, you just need to type:
frequency 100 kHz (=100000 Hz)
→ period: 1/100000 seconds (=10000 nano-seconds)
→ duty_cycle: 2000 nano-seconds (20% of 10000 nano-seconds)
echo 10000 > /sys/class/pwm/pwmchip0/pwm2/period
echo 2000 > /sys/class/pwm/pwmchip0/pwm2/duty_cycle
echo 1 > /sys/class/pwm/pwmchip0/pwm2/enable
Step6. Investigate the PWM Frequency Upper Limitation.
Firstly, we would need to set the duty_cycle as 0 %.
echo 0 > /sys/class/pwm/pwmchip0/pwm2/duty_cycle
Secondly, we would need to decrease the period. For me, I could see an -bash: echo: write error: Invalid argument error between 5333 and 5334 nano-seconds of a period value.
period: 5334 nano-seconds
→ period: 5.334e-6 seconds
→ frequency: 187 kHz (=1/5.334e-6 Hz)
echo 5334 > /sys/class/pwm/pwmchip0/pwm2/period
(No messages)
echo 5333 > /sys/class/pwm/pwmchip0/pwm2/period
-bash: echo: write error: Invalid argument error
Analysis
I would think that the PWM Frequency Upper Limit (on Jetson Nano) is around 187 kHz.