codal_port: Don't call microbit_pin_audio_free when audio/music is done.

Instead leave any connected pin (pin0 by default) in "music" mode.  This
simplifies the behaviour and can help to avoid clicks going from one sound
to another.

See related issue #50.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

src/codal_port/modaudio.c

@@ -50,7 +50,6 @@ microbit_audio_frame_obj_t *microbit_audio_frame_make_new(void);
 void microbit_audio_stop(void) {
     audio_source_iter = NULL;
     audio_running = false;
-    microbit_pin_audio_free();
 }
 
 STATIC void audio_buffer_ready(void) {

src/codal_port/modmusic.c

@@ -105,7 +105,6 @@ void microbit_music_tick(void) {
                 music_data->async_notes_index = 0;
             } else {
                 music_data->async_state = ASYNC_MUSIC_STATE_IDLE;
-                microbit_pin_audio_free();
                 return;
             }
         }
@@ -289,9 +288,8 @@ STATIC mp_obj_t microbit_music_stop(mp_uint_t n_args, const mp_obj_t *args) {
     // Stop any ongoing background music
     music_data->async_state = ASYNC_MUSIC_STATE_IDLE;
 
-    // Turn off the output and free the audio pin
+    // Turn off the output
     music_output_amplitude(MUSIC_OUTPUT_AMPLITUDE_OFF);
-    microbit_pin_audio_free();
 
     return mp_const_none;
 }
@@ -326,9 +324,6 @@ STATIC mp_obj_t microbit_music_play(mp_uint_t n_args, const mp_obj_t *pos_args,
     // Stop any ongoing background music
     music_data->async_state = ASYNC_MUSIC_STATE_IDLE;
 
-    // Release the previous pin
-    microbit_pin_audio_free();
-
     // get the pin to play on
     microbit_pin_audio_select(args[1].u_obj);
 
@@ -376,15 +371,11 @@ STATIC mp_obj_t microbit_music_pitch(mp_uint_t n_args, const mp_obj_t *pos_args,
     music_data->async_state = ASYNC_MUSIC_STATE_IDLE;
 
     // Update pin modes
-    microbit_pin_audio_free();
     microbit_pin_audio_select(args[2].u_obj);
 
     bool wait = args[3].u_bool;
     music_output_amplitude(MUSIC_OUTPUT_AMPLITUDE_ON);
-    if (frequency == 0) {
-        microbit_pin_audio_free();
-    } else if (music_output_period_us(1000000 / frequency) == -1) {
-        microbit_pin_audio_free();
+    if (frequency != 0 && music_output_period_us(1000000 / frequency) == -1) {
         mp_raise_ValueError(MP_ERROR_TEXT("invalid pitch"));
     }
     if (duration >= 0) {