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void spi_lcd_init(void)
{
  esp_err_t rv;

  // 1. config the bus
  spi_bus_config_t spi_bus_cfg = {
    .mosi_io_num = GPIO_NUM_7,// DIN (MOSI)
    .miso_io_num = -1,        // not used
    .sclk_io_num = GPIO_NUM_6,// CLK
    .quadwp_io_num = -1,
    .quadhd_io_num = -1,
    .max_transfer_sz = SPI_MAX_TX_BUFFER_SIZE
  };

  rv = spi_bus_initialize(SPI2_HOST, &spi_bus_cfg, SPI_DMA_CH_AUTO);
  if (rv == ESP_OK) {
    ESP_LOGI("", "SPI bus initialization successfully");
  } else {
    ESP_LOGE("", "SPI bus initialization failed with %d", rv);
    while(1);
  }

  // 2. config the device
  spi_device_interface_config_t devcfg = {
    .clock_speed_hz = 4 * 1000 * 1000,
    .mode = 0,
    .spics_io_num = GPIO_NUM_10, // CS Pin
    .queue_size = 1,
    .flags = 0,
    .command_bits = 0,
    .address_bits = 0,
    .dummy_bits = 0
  };

  rv = spi_bus_add_device(SPI2_HOST, &devcfg, &lcd_spi);
  if (rv == ESP_OK) {
    ESP_LOGI("", "SPI added device successfully");
  } else {
    ESP_LOGE("", "SPI adding device failed with %d", rv);
    while(1);
  }
}

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void spi_send(uint8_t byte)
{
    spi_transaction_t t = {
        .length = 8,
        .tx_buffer = &byte,
        .rx_buffer = NULL
    };

    spi_device_transmit(lcd_spi, &t);
}

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The typical transaction duration for one byte of data is given below.
    Interrupt Transaction via DMA: 34 µs.
    Interrupt Transaction via CPU: 32 µs.
    Polling Transaction via DMA: 17 µs.
    Polling Transaction via CPU: 15 µs.