esp32/machine_pwm: Implement duty_u16() and duty_ns() PWM methods.

The methods duty_u16() and duty_ns() are implemented to match the existing
docs.  The duty will remain the same when the frequency is changed.
Standard ESP32 as well as S2, S3 and C3 are supported.

Thanks to @kdschlosser for the fix for rounding in resolution calculation.

Documentation is updated and examples expanded for esp32, including the
quickref and tutorial.  Additional notes are added to the machine.PWM docs
regarding limitations of hardware PWM.

1 files changed, 86 insertions, 20 deletions

diff --git a/docs/esp32/tutorial/pwm.rst b/docs/esp32/tutorial/pwm.rst
index 0c1afb213b..12d10a86b9 100644
--- a/docs/esp32/tutorial/pwm.rst
+++ b/docs/esp32/tutorial/pwm.rst
@@ -1,4 +1,4 @@
-.. _esp32_pwm: 
+.. _esp32_pwm:
 
 Pulse Width Modulation
 ======================
@@ -11,7 +11,7 @@ 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.
 
-More comprehensive example with all 16 PWM channels and 8 timers::
+* More comprehensive example with all 16 PWM channels and 8 timers::
 
     from machine import Pin, PWM
     try:
@@ -29,21 +29,87 @@ More comprehensive example with all 16 PWM channels and 8 timers::
             except:
                 pass
 
-Output is::
-
-    PWM(pin=15, freq=100, duty=64, resolution=10, mode=0, channel=0, timer=0)
-    PWM(pin=2, freq=100, duty=128, resolution=10, mode=0, channel=1, timer=0)
-    PWM(pin=4, freq=200, duty=192, resolution=10, mode=0, channel=2, timer=1)
-    PWM(pin=16, freq=200, duty=256, resolution=10, mode=0, channel=3, timer=1)
-    PWM(pin=18, freq=300, duty=320, resolution=10, mode=0, channel=4, timer=2)
-    PWM(pin=19, freq=300, duty=384, resolution=10, mode=0, channel=5, timer=2)
-    PWM(pin=22, freq=400, duty=448, resolution=10, mode=0, channel=6, timer=3)
-    PWM(pin=23, freq=400, duty=512, resolution=10, mode=0, channel=7, timer=3)
-    PWM(pin=25, freq=500, duty=576, resolution=10, mode=1, channel=0, timer=0)
-    PWM(pin=26, freq=500, duty=640, resolution=10, mode=1, channel=1, timer=0)
-    PWM(pin=27, freq=600, duty=704, resolution=10, mode=1, channel=2, timer=1)
-    PWM(pin=14, freq=600, duty=768, resolution=10, mode=1, channel=3, timer=1)
-    PWM(pin=12, freq=700, duty=832, resolution=10, mode=1, channel=4, timer=2)
-    PWM(pin=13, freq=700, duty=896, resolution=10, mode=1, channel=5, timer=2)
-    PWM(pin=32, freq=800, duty=960, resolution=10, mode=1, channel=6, timer=3)
-    PWM(pin=33, freq=800, duty=1023, resolution=10, mode=1, channel=7, timer=3)
+  Output is::
+
+    PWM(Pin(15), freq=100, duty=64, resolution=10, mode=0, channel=0, timer=0)
+    PWM(Pin(2), freq=100, duty=128, resolution=10, mode=0, channel=1, timer=0)
+    PWM(Pin(4), freq=200, duty=192, resolution=10, mode=0, channel=2, timer=1)
+    PWM(Pin(16), freq=200, duty=256, resolution=10, mode=0, channel=3, timer=1)
+    PWM(Pin(18), freq=300, duty=320, resolution=10, mode=0, channel=4, timer=2)
+    PWM(Pin(19), freq=300, duty=384, resolution=10, mode=0, channel=5, timer=2)
+    PWM(Pin(22), freq=400, duty=448, resolution=10, mode=0, channel=6, timer=3)
+    PWM(Pin(23), freq=400, duty=512, resolution=10, mode=0, channel=7, timer=3)
+    PWM(Pin(25), freq=500, duty=576, resolution=10, mode=1, channel=0, timer=0)
+    PWM(Pin(26), freq=500, duty=640, resolution=10, mode=1, channel=1, timer=0)
+    PWM(Pin(27), freq=600, duty=704, resolution=10, mode=1, channel=2, timer=1)
+    PWM(Pin(14), freq=600, duty=768, resolution=10, mode=1, channel=3, timer=1)
+    PWM(Pin(12), freq=700, duty=832, resolution=10, mode=1, channel=4, timer=2)
+    PWM(Pin(13), freq=700, duty=896, resolution=10, mode=1, channel=5, timer=2)
+    PWM(Pin(32), freq=800, duty=960, resolution=10, mode=1, channel=6, timer=3)
+    PWM(Pin(33), freq=800, duty=1023, resolution=10, mode=1, channel=7, timer=3)
+
+* Example of a smooth frequency change::
+
+    from utime import sleep
+    from machine import Pin, PWM
+
+    F_MIN = 500
+    F_MAX = 1000
+
+    f = F_MIN
+    delta_f = 1
+
+    p = PWM(Pin(5), f)
+    print(p)
+
+    while True:
+        p.freq(f)
+
+        sleep(10 / F_MIN)
+
+        f += delta_f
+        if f >= F_MAX or f <= F_MIN:
+            delta_f = -delta_f
+
+  See PWM wave at Pin(5) with an oscilloscope.
+
+* Example of a smooth duty change::
+
+    from utime import sleep
+    from machine import Pin, PWM
+
+    DUTY_MAX = 2**16 - 1
+
+    duty_u16 = 0
+    delta_d = 16
+
+    p = PWM(Pin(5), 1000, duty_u16=duty_u16)
+    print(p)
+
+    while True:
+        p.duty_u16(duty_u16)
+
+        sleep(1 / 1000)
+
+        duty_u16 += delta_d
+        if duty_u16 >= DUTY_MAX:
+            duty_u16 = DUTY_MAX
+            delta_d = -delta_d
+        elif duty_u16 <= 0:
+            duty_u16 = 0
+            delta_d = -delta_d
+
+  See PWM wave at Pin(5) with an oscilloscope.
+
+Note: the Pin.OUT mode does not need to be specified.  The channel is initialized
+to PWM mode internally once for each Pin that is passed to the PWM constructor.
+
+The following code is wrong::
+
+    pwm = PWM(Pin(5, Pin.OUT), freq=1000, duty=512)  # Pin(5) in PWM mode here
+    pwm = PWM(Pin(5, Pin.OUT), freq=500, duty=256)  # Pin(5) in OUT mode here, PWM is off
+
+Use this code instead::
+
+    pwm = PWM(Pin(5), freq=1000, duty=512)
+    pwm.init(freq=500, duty=256)