feat(rmt): Add simple callback encoder

examples/peripherals/rmt/led_strip_simple_encoder/CMakeLists.txt

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+# The following lines of boilerplate have to be in your project's CMakeLists
+# in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.16)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(led_strip_simple_encoder)

examples/peripherals/rmt/led_strip_simple_encoder/README.md

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+| Supported Targets | ESP32 | ESP32-C3 | ESP32-C6 | ESP32-H2 | ESP32-P4 | ESP32-S2 | ESP32-S3 |
+| ----------------- | ----- | -------- | -------- | -------- | -------- | -------- | -------- |
+# RMT Transmit Example -- LED Strip
+
+(See the README.md file in the upper level 'examples' directory for more information about examples.)
+
+Almost any waveform can be generated by RMT peripheral, as long as a proper encoder is implemented. In this example, the simple callback RMT encoder is used to convert RGB pixels into format that can be recognized by hardware.
+
+This example shows how to drive an addressable LED strip [WS2812](https://cdn-shop.adafruit.com/datasheets/WS2812B.pdf) by implementing a callback that can be used by the simple callback RMT encoder.
+
+## How to Use Example
+
+### Hardware Required
+
+* A development board with any supported Espressif SOC chip (see `Supported Targets` table above)
+* A USB cable for Power supply and programming
+* A WS2812 LED strip
+
+Connection :
+
+```
+                              --- 5V
+                               |
+                               +
+RMT_LED_STRIP_GPIO_NUM +------ +---|>| (WS2812 LED strip)
+                        DI     +
+                               |
+                              --- GND
+```
+
+The GPIO number used in this example can be changed according to your board, by the macro `RMT_LED_STRIP_GPIO_NUM` defined in the [source file](main/led_strip_example_main.c). The number of LEDs can be changed as well by `EXAMPLE_LED_NUMBERS`.
+
+### Build and Flash
+
+Run `idf.py -p PORT flash monitor` to build, flash and monitor the project.
+
+(To exit the serial monitor, type ``Ctrl-]``.)
+
+See the [Getting Started Guide](https://docs.espressif.com/projects/esp-idf/en/latest/get-started/index.html) for full steps to configure and use ESP-IDF to build projects.
+
+## Console Output
+
+```
+I (302) cpu_start: Starting scheduler on PRO CPU.
+I (0) cpu_start: Starting scheduler on APP CPU.
+I (323) example: Create RMT TX channel
+I (343) example: Create simple callback-based encoder
+I (353) example: Start LED rainbow chase
+```
+
+
+After you seeing this log, you should see a rainbow chasing demonstration pattern. To change the chasing speed, you can update the `EXAMPLE_ANGLE_INC_FRAME` value in [source file](main/led_strip_example_main.c). To change the density of colors, you can change `EXAMPLE_ANGLE_INC_LED` in the same file.
+
+## Troubleshooting
+
+For any technical queries, please open an [issue](https://github.com/espressif/esp-idf/issues) on GitHub. We will get back to you soon.

examples/peripherals/rmt/led_strip_simple_encoder/main/CMakeLists.txt

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+idf_component_register(SRCS "led_strip_example_main.c"
+                       INCLUDE_DIRS ".")

examples/peripherals/rmt/led_strip_simple_encoder/main/led_strip_example_main.c

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+/*
+ * SPDX-FileCopyrightText: 2021-2024 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#include <string.h>
+#include <math.h>
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "esp_log.h"
+#include "driver/rmt_tx.h"
+
+#define RMT_LED_STRIP_RESOLUTION_HZ 10000000 // 10MHz resolution, 1 tick = 0.1us (led strip needs a high resolution)
+#define RMT_LED_STRIP_GPIO_NUM      8
+
+#define EXAMPLE_LED_NUMBERS         24
+
+#define EXAMPLE_FRAME_DURATION_MS   20
+#define EXAMPLE_ANGLE_INC_FRAME     0.02
+#define EXAMPLE_ANGLE_INC_LED       0.3
+
+static const char *TAG = "example";
+
+static uint8_t led_strip_pixels[EXAMPLE_LED_NUMBERS * 3];
+
+static const rmt_symbol_word_t ws2812_zero = {
+    .level0 = 1,
+    .duration0 = 0.3 * RMT_LED_STRIP_RESOLUTION_HZ / 1000000, // T0H=0.3us
+    .level1 = 0,
+    .duration1 = 0.9 * RMT_LED_STRIP_RESOLUTION_HZ / 1000000, // T0L=0.9us
+};
+
+static const rmt_symbol_word_t ws2812_one = {
+    .level0 = 1,
+    .duration0 = 0.9 * RMT_LED_STRIP_RESOLUTION_HZ / 1000000, // T1H=0.9us
+    .level1 = 0,
+    .duration1 = 0.3 * RMT_LED_STRIP_RESOLUTION_HZ / 1000000, // T1L=0.3us
+};
+
+//reset defaults to 50uS
+static const rmt_symbol_word_t ws2812_reset = {
+    .level0 = 1,
+    .duration0 = RMT_LED_STRIP_RESOLUTION_HZ / 1000000 * 50 / 2,
+    .level1 = 0,
+    .duration1 = RMT_LED_STRIP_RESOLUTION_HZ / 1000000 * 50 / 2,
+};
+
+static size_t encoder_callback(const void *data, size_t data_size,
+                               size_t symbols_written, size_t symbols_free,
+                               rmt_symbol_word_t *symbols, bool *done, void *arg)
+{
+    // We need a minimum of 8 symbol spaces to encode a byte. We only
+    // need one to encode a reset, but it's simpler to simply demand that
+    // there are 8 symbol spaces free to write anything.
+    if (symbols_free < 8) {
+        return 0;
+    }
+
+    // We can calculate where in the data we are from the symbol pos.
+    // Alternatively, we could use some counter referenced by the arg
+    // parameter to keep track of this.
+    size_t data_pos = symbols_written / 8;
+    uint8_t *data_bytes = (uint8_t*)data;
+    if (data_pos < data_size) {
+        // Encode a byte
+        size_t symbol_pos = 0;
+        for (int bitmask = 0x80; bitmask != 0; bitmask >>= 1) {
+            if (data_bytes[data_pos]&bitmask) {
+                symbols[symbol_pos++] = ws2812_one;
+            } else {
+                symbols[symbol_pos++] = ws2812_zero;
+            }
+        }
+        // We're done; we should have written 8 symbols.
+        return symbol_pos;
+    } else {
+        //All bytes already are encoded.
+        //Encode the reset, and we're done.
+        symbols[0] = ws2812_reset;
+        *done = 1; //Indicate end of the transaction.
+        return 1; //we only wrote one symbol
+    }
+}
+
+void app_main(void)
+{
+    ESP_LOGI(TAG, "Create RMT TX channel");
+    rmt_channel_handle_t led_chan = NULL;
+    rmt_tx_channel_config_t tx_chan_config = {
+        .clk_src = RMT_CLK_SRC_DEFAULT, // select source clock
+        .gpio_num = RMT_LED_STRIP_GPIO_NUM,
+        .mem_block_symbols = 64, // increase the block size can make the LED less flickering
+        .resolution_hz = RMT_LED_STRIP_RESOLUTION_HZ,
+        .trans_queue_depth = 4, // set the number of transactions that can be pending in the background
+    };
+    ESP_ERROR_CHECK(rmt_new_tx_channel(&tx_chan_config, &led_chan));
+
+    ESP_LOGI(TAG, "Create simple callback-based encoder");
+    rmt_encoder_handle_t simple_encoder = NULL;
+    const rmt_simple_encoder_config_t simple_encoder_cfg = {
+        .callback = encoder_callback
+        //Note we don't set min_chunk_size here as the default of 64 is good enough.
+    };
+    ESP_ERROR_CHECK(rmt_new_simple_encoder(&simple_encoder_cfg, &simple_encoder));
+
+    ESP_LOGI(TAG, "Enable RMT TX channel");
+    ESP_ERROR_CHECK(rmt_enable(led_chan));
+
+    ESP_LOGI(TAG, "Start LED rainbow chase");
+    rmt_transmit_config_t tx_config = {
+        .loop_count = 0, // no transfer loop
+    };
+    float offset = 0;
+    while (1) {
+        for (int led = 0; led < EXAMPLE_LED_NUMBERS; led++) {
+            // Build RGB pixels. Each color is an offset sine, which gives a
+            // hue-like effect.
+            float angle = offset + (led * EXAMPLE_ANGLE_INC_LED);
+            const float color_off = (M_PI * 2) / 3;
+            led_strip_pixels[led * 3 + 0] = sin(angle + color_off * 0) * 127 + 128;
+            led_strip_pixels[led * 3 + 1] = sin(angle + color_off * 1) * 127 + 128;
+            led_strip_pixels[led * 3 + 2] = sin(angle + color_off * 2) * 117 + 128;;
+        }
+        // Flush RGB values to LEDs
+        ESP_ERROR_CHECK(rmt_transmit(led_chan, simple_encoder, led_strip_pixels, sizeof(led_strip_pixels), &tx_config));
+        ESP_ERROR_CHECK(rmt_tx_wait_all_done(led_chan, portMAX_DELAY));
+        vTaskDelay(pdMS_TO_TICKS(EXAMPLE_FRAME_DURATION_MS));
+        //Increase offset to shift pattern
+        offset += EXAMPLE_ANGLE_INC_FRAME;
+        if (offset > 2 * M_PI) {
+            offset -= 2 * M_PI;
+        }
+    }
+}