98 lines
3.0 KiB
Markdown
98 lines
3.0 KiB
Markdown
# PWM's
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This document describes some basics for developers.
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## Check the PWM features
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Example for Tinkerboard:
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```sh
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# ls -la /sys/class/pwm/
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total 0
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drwxr-xr-x 2 root root 0 Apr 24 14:11 .
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drwxr-xr-x 66 root root 0 Apr 24 14:09 ..
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lrwxrwxrwx 1 root root 0 Apr 24 14:09 pwmchip0 -> ../../devices/platform/ff680000.pwm/pwm/pwmchip0
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lrwxrwxrwx 1 root root 0 Apr 24 14:09 pwmchip1 -> ../../devices/platform/ff680010.pwm/pwm/pwmchip1
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lrwxrwxrwx 1 root root 0 Apr 24 14:09 pwmchip2 -> ../../devices/platform/ff680030.pwm/pwm/pwmchip2
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```
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```sh
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# ls -la /sys/class/pwm/pwmchip2/
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total 0
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drwxr-xr-x 3 root root 0 Apr 24 14:17 .
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drwxr-xr-x 3 root root 0 Apr 24 14:17 ..
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lrwxrwxrwx 1 root root 0 Apr 24 14:17 device -> ../../../ff680030.pwm
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--w------- 1 root root 4096 Apr 24 14:17 export
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-r--r--r-- 1 root root 4096 Apr 24 14:17 npwm
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drwxr-xr-x 2 root root 0 Apr 24 14:17 power
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lrwxrwxrwx 1 root root 0 Apr 24 14:17 subsystem -> ../../../../../class/pwm
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-rw-r--r-- 1 root root 4096 Apr 24 14:17 uevent
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--w------- 1 root root 4096 Apr 24 14:17 unexport
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```
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## General PWM tests
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Connect an oscilloscope or at least a meter to the pin (used pin32 for example with Tinkerboard).
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Switch to root user by "su -".
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### Investigate state of PWMs
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For Tinkerboard:
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* ff680000 and ff680010 seems to be not usable (unknown, which functionality make use of that, maybe fan?)
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* if there is no ff680020, ff680030, this can be activated. See section [Change available features](README.md#change-available-features)
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### Creating pwm0 on pwmchip2
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```sh
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echo 0 > /sys/class/pwm/pwmchip2/export
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```
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investigate result:
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```sh
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# ls /sys/class/pwm/pwmchip2/
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device export npwm power pwm0 subsystem uevent unexport
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# ls /sys/class/pwm/pwmchip2/pwm0/
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capture duty_cycle enable period polarity power uevent
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# cat /sys/class/pwm/pwmchip2/pwm0/period
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0
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# cat /sys/class/pwm/pwmchip2/pwm0/duty_cycle
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0
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# cat /sys/class/pwm/pwmchip2/pwm0/enable
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0
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# cat /sys/class/pwm/pwmchip2/pwm0/polarity
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inversed
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```
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### Initialization of pwm0
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```sh
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echo 10000000 > /sys/class/pwm/pwmchip2/pwm0/period
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echo "normal" > /sys/class/pwm/pwmchip2/pwm0/polarity
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echo 3000000 > /sys/class/pwm/pwmchip2/pwm0/duty_cycle # this means 30%
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echo 1 > /sys/class/pwm/pwmchip2/pwm0/enable
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```
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> Before writing the period, all other write actions will cause an error "-bash: echo: write error: Invalid argument".
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> The "period" is in nanoseconds (or a frequency divider for 1GHz), 1000 will produce a frequency of 1MHz, 1000000 will
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> cause a frequency of 1kHz. For the example 10000000 we have 100Hz.
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Now we should measure a value of around 1V with the meter, because the basis value is 3.3V and 30% leads to 1V.
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Try to inverse the sequence:
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`echo "inversed" > /sys/class/pwm/pwmchip2/pwm0/polarity`
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Now we should measure a value of around 2.3V with the meter, which is the difference of 1V to 3.3V.
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If we have attached an oscilloscope we can play around with the values for period and duty_cycle and see what happen.
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## Links
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* <https://docs.kernel.org/driver-api/pwm.html>
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