i2s: update examples for the preload feature

examples/peripherals/i2s/i2s_basic/i2s_std/main/i2s_std_example_main.c

@@ -53,6 +53,10 @@ static void i2s_example_read_task(void *args)
     uint8_t *r_buf = (uint8_t *)calloc(1, EXAMPLE_BUFF_SIZE);
     assert(r_buf); // Check if r_buf allocation success
     size_t r_bytes = 0;
+
+    /* Enable the RX channel */
+    ESP_ERROR_CHECK(i2s_channel_enable(rx_chan));
+
     /* ATTENTION: The print and delay in the read task only for monitoring the data by human,
      * Normally there shouldn't be any delays to ensure a short polling time,
      * Otherwise the dma buffer will overflow and lead to the data lost */
@@ -88,7 +92,16 @@ static void i2s_example_write_task(void *args)
         w_buf[i + 7] = 0xF0;
     }
 
-    size_t w_bytes = 0;
+    size_t w_bytes = EXAMPLE_BUFF_SIZE;
+
+    /* (Optional) Preload the data before enabling the TX channel, so that the valid data can be transmit immediately */
+    while (w_bytes == EXAMPLE_BUFF_SIZE) {
+        /* Here we load the target buffer repeatedly, until all the DMA buffers are preloaded */
+        ESP_ERROR_CHECK(i2s_channel_preload_writing_data(tx_chan, w_buf, EXAMPLE_BUFF_SIZE, &w_bytes));
+    }
+
+    /* Enable the TX channel */
+    ESP_ERROR_CHECK(i2s_channel_enable(tx_chan));
     while (1) {
         /* Write i2s data */
         if (i2s_channel_write(tx_chan, w_buf, EXAMPLE_BUFF_SIZE, &w_bytes, 1000) == ESP_OK) {
@@ -203,11 +216,7 @@ void app_main(void)
     i2s_example_init_std_simplex();
 #endif
 
-    /* Step 3: Enable the tx and rx channels before writing or reading data */
-    ESP_ERROR_CHECK(i2s_channel_enable(tx_chan));
-    ESP_ERROR_CHECK(i2s_channel_enable(rx_chan));
-
-    /* Step 4: Create writing and reading task */
+    /* Step 3: Create writing and reading task, enable and start the channels */
     xTaskCreate(i2s_example_read_task, "i2s_example_read_task", 4096, NULL, 5, NULL);
     xTaskCreate(i2s_example_write_task, "i2s_example_write_task", 4096, NULL, 5, NULL);
 }