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Pulse Width Modulation
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======================
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Pulse width modulation (PWM) is a way to get an artificial analog output on a
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digital pin. It achieves this by rapidly toggling the pin from low to high.
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There are two parameters associated with this: the frequency of the toggling,
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and the duty cycle. The duty cycle is defined to be how long the pin is high
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compared with the length of a single period (low plus high time). Maximum
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duty cycle is when the pin is high all of the time, and minimum is when it is
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low all of the time.
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On the ESP8266 the pins 0, 2, 4, 5, 12, 13, 14 and 15 all support PWM. The
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limitation is that they must all be at the same frequency, and the frequency
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must be between 1Hz and 1kHz.
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To use PWM on a pin you must first create the pin object, for example::
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>>> import machine
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>>> p12 = machine.Pin(12)
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Then create the PWM object using::
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>>> pwm12 = machine.PWM(p12)
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You can set the frequency and duty cycle using::
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>>> pwm12.freq(500)
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>>> pwm12.duty(512)
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Note that the duty cycle is between 0 (all off) and 1023 (all on), with 512
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being a 50% duty. Values beyond this min/max will be clipped. If you
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print the PWM object then it will tell you its current configuration::
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>>> pwm12
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PWM(12, freq=500, duty=512)
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You can also call the ``freq()`` and ``duty()`` methods with no arguments to
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get their current values.
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The pin will continue to be in PWM mode until you deinitialise it using::
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>>> pwm12.deinit()
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Fading an LED
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-------------
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Let's use the PWM feature to fade an LED. Assuming your board has an LED
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connected to pin 2 (ESP-12 modules do) we can create an LED-PWM object using::
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>>> led = machine.PWM(machine.Pin(2), freq=1000)
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Notice that we can set the frequency in the PWM constructor.
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For the next part we will use timing and some math, so import these modules::
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>>> import time, math
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Then create a function to pulse the LED::
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>>> def pulse(l, t):
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... for i in range(20):
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... l.duty(int(math.sin(i / 10 * math.pi) * 500 + 500))
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... time.sleep_ms(t)
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You can try this function out using::
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>>> pulse(led, 50)
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For a nice effect you can pulse many times in a row::
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>>> for i in range(10):
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... pulse(led, 20)
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Remember you can use ctrl-C to interrupt the code.
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Control a hobby servo
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---------------------
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Hobby servo motors can be controlled using PWM. They require a frequency of
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50Hz and then a duty between about 40 and 115, with 77 being the centre value.
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If you connect a servo to the power and ground pins, and then the signal line
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to pin 12 (other pins will work just as well), you can control the motor using::
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>>> servo = machine.PWM(machine.Pin(12), freq=50)
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>>> servo.duty(40)
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>>> servo.duty(115)
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>>> servo.duty(77)
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