using ADC and SD Streaming

[Scott Buchanan]

Hi,

I've set up a streaming solution that is easily streaming 4+kBytes/s to an iPhone. I've also set up an ADC service that is sampling at 2,083 Hz 10 bit mode, using a PPI channel from timer2->compare[0] event to ADC->TASKS_START event. I've kept the ADC ISR very tight. The problem is when I combine the two and try to stream RT data. I get good reproduction of the input waveform except that every 0.027 sec, or so, it drops 6-7 samples. I've kept the ISR short enough that I am able to run the ADC interrupt in _HIGH mode without impacting the BLE (although if the SD is suppressing the PPI channel during the transmission then I'm not really testing that assertion).

I checked that the ADC ISR routine is NOT being called for the dropped samples.

Questions

1. Has anyone run into this - and if so is there a solution? (I searched the data base to no avail)
2. Does the SD suppress the PPI channels during radio use? I thought it would not.

I'm using 5.2 and SD 6.0 as I haven't had time to accommodate the changes in the new release yet.

Any help appreciated.

Scott

[Scott Buchanan]

A follow up question as well.

Despite the warnings in the documentation I tried setting the priority to the ADC handling routine to _HIGH. The ADC start task is tied to the timer2 cpm[0] through ppi channel 0 as shown below. With this arrangement the BLE works fine but I'm still dropping the point during the transmission.

My expectation would have been either to not drop points as the interrupt is HIGH or to have the BLE not work as it was interfering with the radio.

I had thought the PPI channels and peripherals were independent of the CPU and therefore timer2 would trigger the ADC even during the radio usage AND given that the interrupt priority of the ADC was high the interrupt would get serviced resulting in one of the two above results.

My question is why did neither of the two possible outcome occur? It seems like the SD is suppressing PPI channel 0? Is that correct?

void ars_audio_init() {
NRF_ADC->INTENSET   = ADC_INTENSET_END_Msk;
NRF_ADC->CONFIG =   (ADC_CONFIG_RES_10bit               << ADC_CONFIG_RES_Pos)     |
                    (MICROPHONE_INPUT_PRESCALING        << ADC_CONFIG_INPSEL_Pos)  |
                    (MICROPHONE_REFERENCE_PRESCALING    << ADC_CONFIG_REFSEL_Pos)  |
                    (MICROPHONE_ANALOG_INPUT            << ADC_CONFIG_PSEL_Pos)    |
                    (ADC_CONFIG_EXTREFSEL_None          << ADC_CONFIG_EXTREFSEL_Pos);
NRF_ADC->EVENTS_END = 0;
NRF_ADC->ENABLE     = ADC_ENABLE_ENABLE_Enabled;

// Enable ADC interrupt
NVIC_ClearPendingIRQ(ADC_IRQn);
NVIC_SetPriority(ADC_IRQn, NRF_APP_PRIORITY_HIGH);
NVIC_EnableIRQ(ADC_IRQn);

//Setup the power control for the microphone

//Also set up microphone pin with high drive

NRF_GPIO->PIN_CNF[MICROPHONE_POWER_PIN_NUMBER] = 0x301;  //output and high drive
nrf_gpio_pin_clear(MICROPHONE_POWER_PIN_NUMBER);

sd_ppi_channel_assign(0, &(NRF_TIMER2->EVENTS_COMPARE[0]), &(NRF_ADC->TASKS_START));
sd_ppi_channel_enable_clr(0x1);

}

[Stefan Birnir Sverrisson]

I have added to my former reply in order to answer your questions.