i2s: support swapped LRCK / BCK pins

src/rp2_common/pico_audio_i2s/audio_i2s.c

@@ -61,7 +61,9 @@ const audio_format_t *audio_i2s_setup(const audio_format_t *intended_audio_forma
     pio_sm_claim(audio_pio, sm);
 
 
-    uint offset = pio_add_program(audio_pio, &audio_i2s_program);
+    const struct pio_program *program =
+        config->clock_pin_swapped ? &audio_i2s_swapped_program : &audio_i2s_program;
+    uint offset = pio_add_program(audio_pio, program);
 
     audio_i2s_program_init(audio_pio, sm, offset, config->data_pin, config->clock_pin_base);
 

src/rp2_common/pico_audio_i2s/audio_i2s.pio

@@ -42,6 +42,24 @@ bitloop0:
 public entry_point:
     set x, 14         side 0b11
 
+.program audio_i2s_swapped
+.side_set 2
+
+                    ;        /--- BCLK
+                    ;        |/-- LRCLK
+bitloop1:           ;        ||
+    out pins, 1       side 0b01
+    jmp x-- bitloop1  side 0b11
+    out pins, 1       side 0b00
+    set x, 14         side 0b10
+                               
+bitloop0:                      
+    out pins, 1       side 0b00
+    jmp x-- bitloop0  side 0b10
+    out pins, 1       side 0b01
+public entry_point:            
+    set x, 14         side 0b11
+
 % c-sdk {
 
 static inline void audio_i2s_program_init(PIO pio, uint sm, uint offset, uint data_pin, uint clock_pin_base) {

src/rp2_common/pico_audio_i2s/include/pico/audio_i2s.h

@@ -115,6 +115,7 @@ typedef struct audio_i2s_config {
     uint8_t clock_pin_base;
     uint8_t dma_channel;
     uint8_t pio_sm;
+    bool clock_pin_swapped;
 } audio_i2s_config_t;
 
 /** \brief Set up system to output I2S audio