Add 24-bit mode to I2S (esp8266#7835)

Add basic 24 bit mode to the I2S API with a i2s_set_bits() call.
By default 16b mode is still used, but if i2s_set_bits(24) is run
before i2s_begin() then the HW will drive 24-bits of data.  This
data must be left-aligned (i.e. bits 31..8) in 4-byte samples.

Fixes esp8266#5244 (the HW doesn't support 8 or 32 bits, only 16 or 24).

Author: earlephilhower

cores/esp8266/core_esp8266_i2s.cpp

@@ -73,6 +73,7 @@ static i2s_state_t *tx = NULL;
 
 // Last I2S sample rate requested
 static uint32_t _i2s_sample_rate;
+static int _i2s_bits = 16;
 
 // IOs used for I2S. Not defined in i2s.h, unfortunately.
 // Note these are internal GPIO numbers and not pins on an
@@ -84,6 +85,14 @@ static uint32_t _i2s_sample_rate;
 #define I2SI_BCK  13
 #define I2SI_WS   14
 
+bool i2s_set_bits(int bits) {
+  if (tx || rx || (bits != 16 && bits != 24)) {
+    return false;
+  }
+  _i2s_bits = bits;
+  return true;
+}
+
 static bool _i2s_is_full(const i2s_state_t *ch) {
   if (!ch) {
     return false;
@@ -441,7 +450,7 @@ void i2s_set_rate(uint32_t rate) { //Rate in HZ
   }
   _i2s_sample_rate = rate;
 
-  uint32_t scaled_base_freq = I2SBASEFREQ/32;
+  uint32_t scaled_base_freq = I2SBASEFREQ / (_i2s_bits * 2);
   float delta_best = scaled_base_freq;
 
   uint8_t sbd_div_best=1;
@@ -483,6 +492,9 @@ void i2s_set_dividers(uint8_t div1, uint8_t div2) {
   // div1, div2 = Set I2S WS clock frequency.  BCLK seems to be generated from 32x this
   i2sc_temp |= I2SRF | I2SMR | I2SRMS | I2STMS | (div1 << I2SBD) | (div2 << I2SCD);
 
+  // Adjust the shift count for 16/24b output
+  i2sc_temp |= (_i2s_bits == 24 ? 8 : 0) << I2SBM;
+
   I2SC = i2sc_temp;
 
   i2sc_temp &= ~(I2STXR); // Release reset
@@ -549,6 +561,9 @@ bool i2s_rxtxdrive_begin(bool enableRx, bool enableTx, bool driveRxClocks, bool
 
   // I2STXFMM, I2SRXFMM=0 => 16-bit, dual channel data shifted in/out
   I2SFC &= ~(I2SDE | (I2STXFMM << I2STXFM) | (I2SRXFMM << I2SRXFM)); // Set RX/TX FIFO_MOD=0 and disable DMA (FIFO only)
+  if (_i2s_bits == 24) {
+    I2SFC |= (2 << I2STXFM) | (2 << I2SRXFM);
+  }
   I2SFC |= I2SDE; // Enable DMA
 
   // I2STXCMM, I2SRXCMM=0 => Dual channel mode

cores/esp8266/i2s.h

@@ -27,6 +27,7 @@
 How does this work? Basically, to get sound, you need to:
 - Connect an I2S codec to the I2S pins on the ESP.
 - Start up a thread that's going to do the sound output
+- Call i2s_set_bits() if you want to enable 24-bit mode
 - Call i2s_begin()
 - Call i2s_set_rate() with the sample rate you want.
 - Generate sound and call i2s_write_sample() with 32-bit samples.
@@ -42,6 +43,10 @@ speed.
 extern "C" {
 #endif
 
+bool i2s_set_bits(int bits); // Set bits per sample, only 16 or 24 supported.  Call before begin.
+// Note that in 24 bit mode each sample must be left-aligned (i.e. 0x00000000 .. 0xffffff00) as the
+// hardware shifts starting at bit 31, not bit 23.
+
 void i2s_begin(); // Enable TX only, for compatibility
 bool i2s_rxtx_begin(bool enableRx, bool enableTx); // Allow TX and/or RX, returns false on OOM error
 bool i2s_rxtxdrive_begin(bool enableRx, bool enableTx, bool driveRxClocks, bool driveTxClocks);