Configure clock for controller¶
The NimBLE stack uses OS cputime for scheduling various events inside controller. Since the code of controller is optimized to work with 32768 Hz clock, the OS cputime has to be configured accordingly.
To make things easier, controller package (
defines new system configuration setting
BLE_LP_CLOCK as sets it to
1 so other packages can be configured if necessary. The next section
describes configuration required for controller to work properly.
Note: All BSPs based on nRF5x have below settings automatically
BLE_LP_CLOCK is set, there is no need to configure this
The following things need to be configured for NimBLE controller to work properly:
OS cputime frequency shall be set to
OS cputime timer source shall be set to 32768 Hz clock source
Default 1 MHz clock source can be disabled if not used by application
32768 Hz clock source shall be enabled
Crystal settling time shall be set to non-zero value (see below)
For example, on nRF52 platform timer 5 can be used as source for 32768 Hz clock. Also, timer 0 can be disabled since this is the default source for OS cputime clock and is no longer used. The configuration will look as below:
syscfg.vals: OS_CPUTIME_FREQ: 32768 OS_CPUTIME_TIMER_NUM: 5 TIMER_0: 0 TIMER_5: 1 BLE_XTAL_SETTLE_TIME: 1500
On nRF51 platform the only difference is to use timer 3 instead of timer 5.
On platforms without 32768 Hz crystal available it usually can be
synthesized by setting
1 - this is also
already configured in existing BSPs.
Crystal settle time configuration¶
The configuration variable
BLE_XTAL_SETTLE_TIME is used by the
controller to turn on the necessary clock source(s) for the radio and
associated peripherals prior to Bluetooth events (advertising, scanning,
connections, etc). For the nRF5x platforms, the HFXO needs to be turned
on prior to using the radio and the
BLE_XTAL_SETTLE_TIME must be set
to accommodate this time. The amount of time required is board
dependent, so users must characterize their hardware and set
BLE_XTAL_SETTLE_TIME accordingly. The current value of 1500
microseconds is a fairly long time and was intended to work for most, if
not all, platforms.
Note that changing this time will impact battery life with the amount depending on the application. The HFXO draws a fairly large amount of current when running so keeping this time as small as possible will reduce overall current drain.