setup.h

void mic_array_resources_configure(pdm_rx_resources_t *pdm_res, int divide)

Configure the hardware resources needed by the mic array.

Several hardware resources are needed to correctly run the mic array, including 3 ports and 1 or 2 clock blocks (depending on whether SDR or DDR mode is used). This function configures these resources for operation with the mic array.

The pdm_rx_resources_t struct is a container for identifying precisely these resources. All three ports are reset by this function; any existing port configuration will be clobbered.

The parameter divide is the ratio of the audio master clock to the desired PDM clock rate. For example, to generate a desired 3.072 MHz PDM clock from an audio master clock with frequency 24.576 MHz, a divide value of 8 is needed. Divide can also be calculated from the master and PDM clock frequencies using mic_array_mclk_divider().

pdm_res->p_mclk is the resource ID for the 1-bit port on which the audio master clock is received. This function will enable this port and configure it as the source port for pdm_res->clock_a and for pdm_res->clock_b if operating in a DDR configuration.

pdm_res->clock_a is the resource ID for the first (in SDR configuration, only) clock block required by the mic array. Clock A divides the audio master clock (by a factor of divide) to generate the PDM clock. This function enables it with the audio master clock as its source.

pdm_res->p_pdm_clk is the resource ID for the 1-bit port from which the PDM clock will be signaled to the microphones. This function enables it and configures Clock A as its source clock.

pdm_res->clock_b is the resource ID for a second clock block, which is only required by the mic array in a DDR configuration. In DDR mode, this function enables Clock B with the audio master clock as its source. The divider for Clock B is half of that for Clock A (so it runs at twice the frequency). In a DDR configuration Clock B is used as the PDM capture clock. In an SDR configuration, this field must be set to 0 (this is how SDR/DDR is determined).

pdm_res->p_pdm_mics is the resource ID for the port on which PDM data is received. This function enables it and configures it as a 32-bit buffered input. If operating in an SDR configuration, Clock A is used as the capture clock. If operating in a DDR configuration, Clock B is used as its capture clock.

This function only configures and does not start either Clock A or Clock B. A call to mic_array_pdm_clock_start() with pdm_res as the argument can be used to start the clock(s).

This function should be called during initialization, before any PDM data can be captured or processed.

Parameters

  • pdm_res – The hardware resources used by the mic array.

  • divide – The divider to generate the PDM clock from the master clock.

void mic_array_pdm_clock_start(pdm_rx_resources_t *pdm_res)

Start the PDM and capture clock(s).

This function starts Clock A, and if using a DDR configuration, Clock B.

mic_array_resources_configure() must have been called already to configure the resources indicated in pdm_res.

Clock A is the PDM clock. Starting Clock A will cause pdm_res->p_pdm_clk to begin strobing the PDM clock to the PDM microphones.

In an SDR configuration, Clock A is also the capture clock. In a DDR configuration, Clock B is the capture clock. In either case, the capture clock is also started, causing pdm_res->p_pdm_mics to begin storing PDM samples received on each period of the capture clock.

In DDR configuration, this function starts Clock B, waits for a rising edge, and then starts Clock A, ensuring that the rising edges of the two clocks are not in phase.

This function must be called prior to launching the decimator or PDM rx threads.

Warning

Once this function has been called, the port receiving PDM data will begin capturing samples. If the mic array unit is not started by the time the port buffer fills ((32/mic_count) sample times) samples will begin to be dropped.

Parameters

pdm_res – The hardware resources used by the mic array.

static inline unsigned mic_array_mclk_divider(const unsigned master_clock_freq, const unsigned pdm_clock_freq)

Compute clock divider for PDM clock.

This is a convenience function which computes the required clock divider to derive a pdm_clock_freq Hz clock from a master_clock_freq Hz clock. This function is simple integer division.

Parameters

  • master_clock_freq – The master audio clock frequency in Hz.

  • pdm_clock_freq – The desired PDM clock frequency in Hz.

Returns

Required clock divider.