View source for PWM support

This tutorial uses the [[A new kernel and device tree: rd235a|rd235a]] kernel.

In fact pwm is not currently configured in the standard device tree provided with NTC's standard kernel (March 12 2016).

PWM0 can be activated in this way:
<pre>
cd /sys/class/pwm/pwmchip0
sudo echo 0 >export
</pre>

Now the new directory pwd0 provides all the meta-files to set up the waveform.
<pre>
cd pwm0
sudo echo normal > polarity 
sudo echo 1 >enable 
sudo echo 20000000 > period 
sudo echo 1500000 > duty_cycle 
</pre>

Now if you connect an oscilloscope to the pin PWM0 (U13 pin 18) you'll see a square wave, period 20ms and a pulse width of 1.5ms.

[[File:pwmrenzo.jpg|500px]]

This is exactly the signal needed to keep a servo motor in its central position (or stop a rotational 360 degree servo motor).

If you have a servo motor you can plug it (using a separate power supply, power spikes from a motor can cause chip to reset and reboot):
<pre>
Servo black wire <-> power supply - <-> CHIP ground (e.g.U13 pin 1)
Servo white wire <-> CHIP PWM0 (U13 pin 18)
Servo red wire <-> power supply +
</pre>

Now it is possible to change the pulse width, for example:
<pre>
sudo echo 1400000 > duty_cycle 
sudo echo 1600000 > duty_cycle 
sudo echo 1000000 > duty_cycle 
sudo echo 2000000 > duty_cycle 
sudo echo 1500000 > duty_cycle
</pre> 
move the servo slighly counterclockwise, slightly clockwise, completely counterclockwise (-90 degrees), completely clockwise (90 degree) and back in its central position, respectively.
(if the servo is a rotational one, rotates slowly backwards, slowly forward, fullspeed backwards, fullspeed forward,and stop again).

For other applications it is possible to set different values to period, duty_cycle, polarity.

At the end disable pwm0 in this way:
<pre>
sudo echo 0 > enable
</pre>

and unexport it:
<pre>
sudo echo 0 > /sys/class/pwm/pwmchip0/unexport
</pre>