32768Hz crystal for ble app

[Dan]

Hi 1)In a nrf51822 ble app project, The board must have a external 32768Hz crystal,is that right? or internal 32768Hz RC oscillator can instead of it? 2)when nrf51822 goes into sleeping,the 16MHz crystal will be stopped,is that right? Can 32768Hz RC oscillator or external 32768Hz crystal still work for rtc?

[Stefan Birnir Sverrisson]

Hi

For a BLE project, you need 250ppm accuracy for the low frequency 32kHz clock. This can be obtained with an external 32kHz crystal which has accuracy of 250ppm or better. This can also be obtained by choosing the internal RC oscillator and choose to calibrate it every 4 seconds. When enabling the BLE softdevice with call to ble_stack_init(), you should choose the NRF_CLOCK_LFCLKSRC_RC_250_PPM_4000MS_CALIBRATION option to enable the RC oscillator and the calibration.

Note that when you choose to use the RC oscillator, it will add around 10uA average current consumption compared to a 20ppm external crystal.

The RTC timers need low frequency 32kHz clock in order to operate. You can see what resources different peripherals need in table 28 in the nRF51822 PS v2.0.

When the chip is in low power mode, i.e. either in System On or System Off mode, the 16MHz clock source is disabled. In System ON, 32kHz clock source and an RTC timer can be enabled. In System Off, all peripherals are disabled. For BLE operation, the softdevice will put the chip into System On low power mode between BLE radio events (connection events).

When the softdevice is enabled, 32kHz low frequency clock source and the RTC0 peripheral will be enabled to keep track of time. Between BLE radio events the chip will stay in System On low power mode, which should consume 3.0uA - 3.5uA current. You can see what currents the chip will consume for BLE operation by looking at figures 5 and 6 in the S110 Softdevice Specification v1.1. You should be able to find values for all currents mentioned in figure 5 and 6 in the nRF51822 product specification (nRF51822 PS)