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