stm32,esp32: In machine_i2s, send null samples in underflow situations.

miketeachman · dpgeorge · commit 0be3b91f11e8 · 2021-11-12T16:50:15.000+11:00
Eliminate noise data from being sent to the I2S peripheral when the
transmitted sample stream is stopped.

Signed-off-by: Mike Teachman &lt;mike.teachman@gmail.com&gt;
diff --git a/ports/esp32/machine_i2s.c b/ports/esp32/machine_i2s.c
@@ -448,6 +448,7 @@ STATIC void machine_i2s_init_helper(machine_i2s_obj_t *self, size_t n_pos_args,
     i2s_config.dma_buf_count = get_dma_buf_count(mode, bits, format, self->ibuf);
     i2s_config.dma_buf_len = DMA_BUF_LEN_IN_I2S_FRAMES;
     i2s_config.use_apll = false;
+    i2s_config.tx_desc_auto_clear = true;
 
     // I2S queue size equals the number of DMA buffers
     check_esp_err(i2s_driver_install(self->port, &i2s_config, i2s_config.dma_buf_count, &self->i2s_event_queue));
diff --git a/ports/stm32/machine_i2s.c b/ports/stm32/machine_i2s.c
@@ -509,6 +509,9 @@ STATIC void feed_dma(machine_i2s_obj_t *self, ping_pong_t dma_ping_pong) {
         if (self->bits == 32) {
             reformat_32_bit_samples((int32_t *)dma_buffer_p, SIZEOF_HALF_DMA_BUFFER_IN_BYTES / (sizeof(uint32_t)));
         }
+    } else {
+        // underflow.  clear buffer to transmit "silence" on the I2S bus
+        memset(dma_buffer_p, 0, SIZEOF_HALF_DMA_BUFFER_IN_BYTES);
     }
 
     // flush cache to RAM so DMA can read the sample data

Original file line number	Diff line number	Diff line change
`@@ -509,6 +509,9 @@ STATIC void feed_dma(machine_i2s_obj_t *self, ping_pong_t dma_ping_pong) {`
`509`	`509`	`if (self->bits == 32) {`
`510`	`510`	`reformat_32_bit_samples((int32_t *)dma_buffer_p, SIZEOF_HALF_DMA_BUFFER_IN_BYTES / (sizeof(uint32_t)));`
`511`	`511`	`}`
	`512`	`+ } else {`
	`513`	`+ // underflow. clear buffer to transmit "silence" on the I2S bus`
	`514`	`+ memset(dma_buffer_p, 0, SIZEOF_HALF_DMA_BUFFER_IN_BYTES);`
`512`	`515`	`}`
`513`	`516`
`514`	`517`	`// flush cache to RAM so DMA can read the sample data`