We use a Jetson Nano on a JN30 carrier board, running Ubuntu 18.04.2 LTS.
We have connected a ASB0305HP fan.
Setting fan PWM via /sys/devices/pwm-fan/target_pwm works. However, we cannot get a readout from /sys/devices/pwm-fan/rpm_measured, it always displays 0.
The JN30 carrier board, has a FAN_TACH (208) pin described as “tachometer from the fan (open drain input with 100k pull-up to 1.8V)”, which we have connected.
Measuring the FAN_TACH out pin from the fan with an oscilloscope, we do get a frequency output. For example, the images attached show the fan set to 100 and 200 PWM. The frequency changes, but rpm_measured still always displays 0.
Any input on this? Did someone ever get this to work? There is a lot of posts about setting the PWM, but none about reading the RPM.