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)

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()
 

ports/esp32/esp32_rmt.c

@@ -52,6 +52,8 @@ typedef struct _esp32_rmt_obj_t {
     uint8_t channel_id;
     gpio_num_t pin;
     uint8_t clock_div;
+    uint16_t carrier_duty_percent;
+    uint32_t carrier_freq;
     mp_uint_t num_items;
     rmt_item32_t *items;
 } esp32_rmt_obj_t;
@@ -61,6 +63,8 @@ STATIC mp_obj_t esp32_rmt_make_new(const mp_obj_type_t *type, size_t n_args, siz
         { MP_QSTR_id,        MP_ARG_REQUIRED | MP_ARG_INT, {.u_int = -1} },
         { MP_QSTR_pin,       MP_ARG_REQUIRED | MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = mp_const_none} },
         { MP_QSTR_clock_div,                   MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 8} }, // 100ns resolution
+        { MP_QSTR_carrier_duty_percent,        MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 50} },
+        { MP_QSTR_carrier_freq,                MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0} },
     };
     mp_arg_val_t args[MP_ARRAY_SIZE(allowed_args)];
     mp_arg_parse_all_kw_array(n_args, n_kw, all_args, MP_ARRAY_SIZE(allowed_args), allowed_args, args);
@@ -68,6 +72,16 @@ STATIC mp_obj_t esp32_rmt_make_new(const mp_obj_type_t *type, size_t n_args, siz
     gpio_num_t pin_id = machine_pin_get_id(args[1].u_obj);
     mp_uint_t clock_div = args[2].u_int;
 
+    bool carrier_en = false;
+    mp_uint_t carrier_duty_percent = 0;
+    mp_uint_t carrier_freq = 0;
+
+    if (args[4].u_int > 0) {
+        carrier_en = true;
+        carrier_duty_percent = args[3].u_int;
+        carrier_freq = args[4].u_int;
+    }
+
     if (clock_div < 1 || clock_div > 255) {
         mp_raise_ValueError(MP_ERROR_TEXT("clock_div must be between 1 and 255"));
     }
@@ -77,6 +91,8 @@ STATIC mp_obj_t esp32_rmt_make_new(const mp_obj_type_t *type, size_t n_args, siz
     self->channel_id = channel_id;
     self->pin = pin_id;
     self->clock_div = clock_div;
+    self->carrier_duty_percent = carrier_duty_percent;
+    self->carrier_freq = carrier_freq;
 
     rmt_config_t config;
     config.rmt_mode = RMT_MODE_TX;
@@ -85,11 +101,11 @@ STATIC mp_obj_t esp32_rmt_make_new(const mp_obj_type_t *type, size_t n_args, siz
     config.mem_block_num = 1;
     config.tx_config.loop_en = 0;
 
-    config.tx_config.carrier_en = 0;
+    config.tx_config.carrier_en = carrier_en;
     config.tx_config.idle_output_en = 1;
     config.tx_config.idle_level = 0;
-    config.tx_config.carrier_duty_percent = 0;
+    config.tx_config.carrier_duty_percent = self->carrier_duty_percent;
-    config.tx_config.carrier_freq_hz = 0;
+    config.tx_config.carrier_freq_hz = self->carrier_freq;
     config.tx_config.carrier_level = 1;
 
     config.clk_div = self->clock_div;
@@ -103,8 +119,14 @@ STATIC mp_obj_t esp32_rmt_make_new(const mp_obj_type_t *type, size_t n_args, siz
 STATIC void esp32_rmt_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
     esp32_rmt_obj_t *self = MP_OBJ_TO_PTR(self_in);
     if (self->pin != -1) {
-        mp_printf(print, "RMT(channel=%u, pin=%u, source_freq=%u, clock_div=%u)",
+        mp_printf(print, "RMT(channel=%u, pin=%u, source_freq=%u, clock_div=%u",
             self->channel_id, self->pin, APB_CLK_FREQ, self->clock_div);
+        if (self->carrier_freq > 0) {
+            mp_printf(print, ", carrier_freq=%u, carrier_duty_percent=%u)",
+                self->carrier_freq, self->carrier_duty_percent);
+        } else {
+            mp_printf(print, ")");
+        }
     } else {
         mp_printf(print, "RMT()");
     }