Hi,
I'm trying to pass audio data from PDM callback function to a USB device and I'm kind of stuck.
Background:
I'm using the nRF52840-DK board and 2 MP34DT05-A microphones. Since there is no PDM example in the SDK I took the liberty of writing the code from scratch based on the non-legacy PDM driver. It is very similar to I2S in design so didnt take much to make it work. To verify that it works I used fstorage library to save the samples into memory nd then Segger J-Flash to dump contents to file on a windows computer and the WaveSurfer to make sure it is actual audio data. (basically up to date code and applications described here)
IT WORKED
Here is a snippet of the relevant code in my program:
static void nrfx_pdm_event_handler(nrfx_pdm_evt_t const *const p_evt) {
/*NRF_LOG_INFO("nrfx_pdm_event_handler(buffer_released:%d buffer_requested:%d error:%d)",
p_evt->buffer_released == NULL ? 0 : 1,
p_evt->buffer_requested ? 1 : 0,
p_evt->error);*/
// we have an error ?
if (p_evt->error != NRFX_PDM_NO_ERROR) {
NRF_LOG_INFO("PDM error event: %d", p_evt->error);
return;
}
// got a buffer with data ?
if (p_evt->buffer_released != NULL) {
// we have buffer ready
audio_buffer_txed = 0;
// do we have space to write ?
if (flash_addr + NRFX_PDM_BUFFER_SIZE*2 < FSTORAGE_END_ADDR) {
if (fstorage_write_done /*nrf_fstorage_is_busy(&fstorage_instance)*/) {
// write to flash
fstorage_write_done = 0;
ret_code_t rc = nrf_fstorage_write(
&fstorage_instance, /* The instance to use. */
flash_addr, /* The address in flash where to store the data. */
p_evt->buffer_released, /* A pointer to the data. */
NRFX_PDM_BUFFER_SIZE*2, /* Lenght of the data, in bytes. */
NULL /* Optional parameter, backend-dependent. */
);
if (rc != NRF_SUCCESS)
NRF_LOG_ERROR("nrf_fstorage_write() = %d", rc);
} else NRF_LOG_ERROR("fstorage busy -> dumping buffer");
}
}
// expected to set a new buffer ?
if (p_evt->buffer_requested) {
NRF_LOG_INFO("PDM buffer requested");
// update buffer index
pdm_buffer_i = pdm_buffer_i == 0 ? 1 : 0;
// set new buffer
nrfx_pdm_buffer_set(pdm_buffer[pdm_buffer_i], NRFX_PDM_BUFFER_SIZE);
}
}
int main(void)
{
// .
// .
// .
nrfx_pdm_config_t config = NRFX_PDM_DEFAULT_CONFIG(ARDUINO_SCL_PIN, ARDUINO_SDA_PIN);
ret = nrfx_pdm_init(&config, nrfx_pdm_event_handler);
APP_ERROR_CHECK(ret);
// .
// .
// .
}
Now, the problem I'm facing is how to pass that data to a USB audio device.
I started with the USB audio device example in the SDK and stripped all the headphones related code since it is irrelevant.
When I looked at the code I could see that the function app_usbd_audio_class_tx_start() that actually passes data to the USB device is called inside the SOF handler of the headphones.
I needed to find a new home for it ..
My naive first attempt was to place it directly inside the PDM handler (where fstorage_write_done() is right now). To cut a long story short - it didnt work, it returns with code 8 (invalid state). So I went to the documentation and it says there that writing can only occur during an SOF event. (and it has a small example here that I tried to use).
I came up with this code (which doesnt work .. same old return code 8 - invalid state):
static void usbd_user_ev_handler(app_usbd_event_type_t event)
{
switch (event)
{
case APP_USBD_EVT_DRV_SOF:
// make sure we're in proper state (configured)
if (APP_USBD_STATE_Configured != app_usbd_core_state_get()) break;
// are we in the middle of a transfer ?
if (!usb_tx_done) break;
// do we have a buffer ready for transfer ?
if (audio_buffer_txed) break;
// transfer the off-line buffer to the USB device now
usb_tx_done = 0;
ret_code_t ret = app_usbd_audio_class_tx_start(&m_app_audio_microphone.base,
pdm_buffer[pdm_buffer_i == 0 ? 1 : 0], NRFX_PDM_BUFFER_SIZE);
// haven't started transferring -> wont reach TX done
if (ret != NRF_SUCCESS) {
usb_tx_done = 1;
NRF_LOG_INFO("APP_USBD_EVT_DRV_SOF -> app_usbd_audio_class_tx_start = %d", ret);
}
// this buffer is being transferred
else audio_buffer_txed = 1;
break;
// .
// .
// .
}
// .
// .
// .
}
My question is - how can I pass the data from the PDM callback to the USB device reliably with no errors ?
Remark #1:
In the code snippets I attached the "// ." signify mode code there that I didnt bother to attach.
Remark #2:
I didnt change anything in the definition of the USB microphone which might be a problem.
Especially the following line:
#define MIC_INTERFACES_CONFIG() APP_USBD_AUDIO_CONFIG_IN(2, 3)
When I connect the board to a Linux machine and look at dmesg output I get a warning/error that max interface should be 1. I guess the 2 and 3 values here are wrong.
Remark #3:
I read in the documentation that the isochroneous USB device can have a maximum buffer size of 1000 bytes, so I use a double buffer of 500 int16_t for the PDM:
// buffers for the PDM source #define NRFX_PDM_BUFFER_SIZE 500 static volatile int8_t pdm_buffer_i = 0; static int16_t pdm_buffer[2][NRFX_PDM_BUFFER_SIZE];
