NRF51822 Audio Application & Compression

[Lucas]

Hello,

I am working on an application that involves sampling audio data ~8KHz. I have a few questions

1. The real time clock module has a maximum interrupt frequency of 32768/5 = 6554 Hz. Is there another mechanism to achieve a slightly greater interrupt frequency ~8KHz using the RTC? Or will I need to move to the Timer/Counter. Does nordic provide support for the Timer peripheral like it does for the RTC?

2. Considering the BLE throughput limitation of the Iphone 5s = 4KB/s I will need to compress the audio data before transmission. Is nordic aware of any codec that works well on the Cortex M0 core of the NRF51822?

Lucas

[Stefan Birnir Sverrisson]

Hi Lucas

Is the RTC interrupt handler limited to 32768/5 Hz? I haven't heard of that before. The interrupt handler is executed perhaps 3-5 microseconds after the RTC event is triggered and will perhaps take a few microseconds to execute, given your code in the handler is relatively short, so there should be plenty of CPU time available for the RTC interrupt handler to execute with 8kHz frequency or even up to 32kHz frequency.

Is perhaps something else in your code that is using CPU time, thereby limiting the CPU time left for the RTC interrupt handler. Is your code execution in the RTC interrupt handler perhaps taking a long time to execute, thereby limiting the sampling frequency. Also, if you have softdevice enabled it will limit your sampling rate since the CPU is blocked for 1ms-6ms during a BLE radio event.

[Stefan Birnir Sverrisson]

You can see the current consumption of a TIMER peripheral in the nRF51822_PS v2.0, sections 8.13, which is 30uA. However, the TIMER needs 1.2V regulator and 16MHz clock source (either RC or external crystal (XOSC)) and that will consume extra 880uA or 810uA respectively, see table 27.