PDM library: Update PIO and interrupt use (#496)

The PIO and state machine were hard wired, so this caused problems if they were not free. The approach used by the Servo library has been adopted so a free PIO and SM are searched.

The DMA_IRQ_0 was grabbed exclusively, but this conflicts with SPI DMA use. The interrupt is now shared, but has been allocated the highest possible priority.

Since the PDM PIO use is receive only, the PIO state machine RX FIFO's can be joined to reduce DMA interrupt load.

libraries/PDM/src/PDM.h

@@ -58,6 +58,12 @@ private:
   int _gain;
   int _init;
 
+// Hardware peripherals used
+  uint _dmaChannel;
+  PIO _pio;
+  int _smIdx;
+  int _pgmOffset;
+
   PDMDoubleBuffer _doubleBuffer;
 
   void (*_onReceive)(void);

libraries/PDM/src/rp2040/PDM.cpp

@@ -11,11 +11,7 @@ extern "C" {
 }
 #include "hardware/sync.h" 
 #include "pdm.pio.h"
-
-// Hardware peripherals used
-uint dmaChannel = 0;
-PIO pio = pio0;
-uint sm = 0;
+static PIOProgram _pdmPgm(&pdm_pio_program);
 
 // raw buffers contain PDM data
 #define RAW_BUFFER_SIZE 512 // should be a multiple of (decimation / 8)
@@ -49,7 +45,11 @@ PDMClass::PDMClass(int dinPin, int clkPin, int pwrPin) :
   _gain(-1),
   _channels(-1),
   _samplerate(-1),
-  _init(-1)
+  _init(-1),
+  _dmaChannel(0),
+  _pio(nullptr),
+  _smIdx(-1),
+  _pgmOffset(-1)
 {
 }
 
@@ -89,29 +89,35 @@ int PDMClass::begin(int channels, int sampleRate)
 
   // Configure PIO state machine
   float clkDiv = (float)clock_get_hz(clk_sys) / sampleRate / decimation / 2; 
-  uint offset = pio_add_program(pio, &pdm_pio_program);
-  pdm_pio_program_init(pio, sm, offset, _clkPin, _dinPin, clkDiv);
+
+  if (!_pdmPgm.prepare(&_pio, &_smIdx, &_pgmOffset)) {
+      // ERROR, no free slots
+      return -1;
+  }
+  pdm_pio_program_init(_pio, _smIdx, _pgmOffset, _clkPin, _dinPin, clkDiv);
 
   // Wait for microphone 
   delay(100);
 
   // Configure DMA for transferring PIO rx buffer to raw buffers
-  dma_channel_config c = dma_channel_get_default_config(dmaChannel);
+  _dmaChannel = dma_claim_unused_channel(false);
+  dma_channel_config c = dma_channel_get_default_config(_dmaChannel);
   channel_config_set_read_increment(&c, false);
   channel_config_set_write_increment(&c, true);
-  channel_config_set_dreq(&c, pio_get_dreq(pio, sm, false));
+  channel_config_set_dreq(&c, pio_get_dreq(_pio, _smIdx, false));
   channel_config_set_transfer_data_size(&c, DMA_SIZE_8);
 
   // Clear DMA interrupts
-  dma_hw->ints0 = 1u << dmaChannel; 
+  dma_hw->ints0 = 1u << _dmaChannel; 
   // Enable DMA interrupts
-  dma_channel_set_irq0_enabled(dmaChannel, true);
-  irq_set_exclusive_handler(DMA_IRQ_0, dmaHandler);
+  dma_channel_set_irq0_enabled(_dmaChannel, true);
+  // Share but allocate a high priority to the interrupt
+  irq_add_shared_handler(DMA_IRQ_0, dmaHandler, 0);
   irq_set_enabled(DMA_IRQ_0, true);
 
-  dma_channel_configure(dmaChannel, &c,
+  dma_channel_configure(_dmaChannel, &c,
     rawBuffer[rawBufferIndex],        // Destinatinon pointer
-    &pio->rxf[sm],      // Source pointer
+    &_pio->rxf[_smIdx],      // Source pointer
     RAW_BUFFER_SIZE, // Number of transfers
     true                // Start immediately
   );
@@ -123,7 +129,7 @@ int PDMClass::begin(int channels, int sampleRate)
 
 void PDMClass::end()
 {
-  dma_channel_abort(dmaChannel);
+  dma_channel_abort(_dmaChannel);
   pinMode(_clkPin, INPUT);
 }
 
@@ -171,10 +177,10 @@ void PDMClass::IrqHandler(bool halftranfer)
   static int cutSamples = 100;
 
   // Clear the interrupt request.
-  dma_hw->ints0 = 1u << dmaChannel; 
+  dma_hw->ints0 = 1u << _dmaChannel; 
   // Restart dma pointing to the other buffer 
   int shadowIndex = rawBufferIndex ^ 1;
-  dma_channel_set_write_addr(dmaChannel, rawBuffer[shadowIndex], true);
+  dma_channel_set_write_addr(_dmaChannel, rawBuffer[shadowIndex], true);
 
   if (_doubleBuffer.available()) {
     // buffer overflow, stop

libraries/PDM/src/rp2040/pdm.pio

@@ -26,6 +26,7 @@ static inline void pdm_pio_program_init(PIO pio, uint sm, uint offset, uint clkP
   sm_config_set_in_pins(&c, dataPin);
   sm_config_set_sideset_pins(&c, clkPin);
   sm_config_set_clkdiv(&c, clkDiv);
+  sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_RX);
 
   pio_sm_set_consecutive_pindirs(pio, sm, dataPin, 1, false);
   pio_sm_set_consecutive_pindirs(pio, sm, clkPin, 1, true);

libraries/PDM/src/rp2040/pdm.pio.h

@@ -43,6 +43,7 @@ static inline void pdm_pio_program_init(PIO pio, uint sm, uint offset, uint clkP
   sm_config_set_in_pins(&c, dataPin);
   sm_config_set_sideset_pins(&c, clkPin);
   sm_config_set_clkdiv(&c, clkDiv);
+  sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_RX);
   pio_sm_set_consecutive_pindirs(pio, sm, dataPin, 1, false);
   pio_sm_set_consecutive_pindirs(pio, sm, clkPin, 1, true);
   pio_sm_set_pins_with_mask(pio, sm, 0, (1u << clkPin) );