Low latency, low power mode for interrupt-driven wearable

RE: Low latency, low power mode for interrupt-driven wearable

Einar Thorsrud — Mon, 20 Nov 2023 15:21:00 GMT

When you need to maintain a BLE connection, the only viable low-power mode (and the one that is by far most used) is system ON low power mode. In this mode, the CPU enters sleep (wait for event), and most peripherals are disabled, but the RTC keeps running for time keeping. In the nRF5 SDK, this mode is entered by calling sd_app_evt_wait(), either directly or often via nrf_pwr_mgmt_run(). This is done by most if not all BLE examples in the SDK.

Note that the SDK examples enable UART logging, and this will prevent low power (keeping the HF clock running etc), so in order to test power consumption you should disable logging.

To enable constant latency mode (increasing power consumption) you can enable that with sd_power_mode_set(NRF_POWER_MODE_CONSTLAT) after enabling the SoftDevice. (or using NRF_POWER->TASKS_CONSTLAT = 1; if testing without SoftDevice).

RE: Low latency, low power mode for interrupt-driven wearable

apdobaj — Fri, 17 Nov 2023 23:50:26 GMT

And one thing in particular, what sort of low power modes are available when you need to maintain a BTLE connection?

RE: Low latency, low power mode for interrupt-driven wearable

apdobaj — Fri, 17 Nov 2023 16:54:13 GMT

OK, great, I can do that but first I need some information about how to programmatically using the 17.1 SDK get the device into the low power low latency mode in the first place. Can you please point me to where this is explained?

RE: Low latency, low power mode for interrupt-driven wearable

Einar Thorsrud — Fri, 17 Nov 2023 11:41:56 GMT

Hi,

You can find most relevant numbers at the end of the Device startup times table. This post is also relevant. A key point is that the interrupt latency is depends on several factors, so it makes sense to test your configuration with a logic analyzer to see that the actual numbers are close to what you would expect (and within your requirements).