esp32/esp32_rmt: Extend RMT to support carrier feature.

The ESP32 RMT peripheral has hardware support for a carrier frequency, and
this commit exposes it to Python with the keyword arguments carrier_freq
and carrier_duty_percent in the constructor.  Example usage:

    r = esp32.RMT(0, pin=Pin(2), clock_div=80, carrier_freq=38000, carrier_duty_percent=50)

1 files changed, 14 insertions, 3 deletions

diff --git a/docs/library/esp32.rst b/docs/library/esp32.rst
index 33b779a21..715afddde 100644
--- a/docs/library/esp32.rst
+++ b/docs/library/esp32.rst
@@ -151,6 +151,11 @@ used to transmit or receive many other types of digital signals::
 
     r = esp32.RMT(0, pin=Pin(18), clock_div=8)
     r  # RMT(channel=0, pin=18, source_freq=80000000, clock_div=8)
+
+    # To use carrier frequency
+    r = esp32.RMT(0, pin=Pin(18), clock_div=8, carrier_freq=38000)
+    r  # RMT(channel=0, pin=18, source_freq=80000000, clock_div=8, carrier_freq=38000, carrier_duty_percent=50)
+
     # The channel resolution is 100ns (1/(source_freq/clock_div)).
     r.write_pulses((1, 20, 2, 40), start=0)  # Send 0 for 100ns, 1 for 2000ns, 0 for 200ns, 1 for 4000ns
 
@@ -164,6 +169,9 @@ define the pulses.
 multiplying the resolution by a 15-bit (0-32,768) number. There are eight
 channels (0-7) and each can have a different clock divider.
 
+To enable the carrier frequency feature of the esp32 hardware, specify the
+``carrier_freq`` as something like 38000, a typical IR carrier frequency.
+
 So, in the example above, the 80MHz clock is divided by 8. Thus the
 resolution is (1/(80Mhz/8)) 100ns. Since the ``start`` level is 0 and toggles
 with each number, the bitstream is ``0101`` with durations of [100ns, 2000ns,
@@ -174,17 +182,20 @@ For more details see Espressif's `ESP-IDF RMT documentation.
 
 .. Warning::
    The current MicroPython RMT implementation lacks some features, most notably
-   receiving pulses and carrier transmit. RMT should be considered a
+   receiving pulses. RMT should be considered a
    *beta feature* and the interface may change in the future.
 
 
-.. class:: RMT(channel, \*, pin=None, clock_div=8)
+.. class:: RMT(channel, \*, pin=None, clock_div=8, carrier_freq=0, carrier_duty_percent=50)
 
     This class provides access to one of the eight RMT channels. *channel* is
     required and identifies which RMT channel (0-7) will be configured. *pin*,
     also required, configures which Pin is bound to the RMT channel. *clock_div*
     is an 8-bit clock divider that divides the source clock (80MHz) to the RMT
-    channel allowing the resolution to be specified.
+    channel allowing the resolution to be specified. *carrier_freq* is used to
+    enable the carrier feature and specify its frequency, default value is ``0``
+    (not enabled).  To enable, specify a positive integer.  *carrier_duty_percent*
+    defaults to 50.
 
 .. method:: RMT.source_freq()