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68 lines
2.6 KiB
ReStructuredText
68 lines
2.6 KiB
ReStructuredText
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Configure clock for controller
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------------------------------
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The NimBLE stack uses OS cputime for scheduling various events inside
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controller. Since the code of controller is optimized to work with 32768
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Hz clock, the OS cputime has to be configured accordingly.
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To make things easier, controller package (``net/nimble/controller``)
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defines new system configuration setting ``BLE_LP_CLOCK`` as sets it to
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``1`` so other packages can be configured if necessary. The next section
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describes configuration required for controller to work properly.
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System configuration
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~~~~~~~~~~~~~~~~~~~~
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**Note:** All BSPs based on nRF5x have below settings automatically
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applied when ``BLE_LP_CLOCK`` is set, there is no need to configure this
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in application.
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The following things need to be configured for NimBLE controller to work
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properly:
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- OS cputime frequency shall be set to ``32768``
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- OS cputime timer source shall be set to 32768 Hz clock source
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- Default 1 MHz clock source can be disabled if not used by application
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- 32768 Hz clock source shall be enabled
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- Crystal settling time shall be set to non-zero value (see below)
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For example, on nRF52 platform timer 5 can be used as source for 32768
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Hz clock. Also, timer 0 can be disabled since this is the default source
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for OS cputime clock and is no longer used. The configuration will look
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as below:
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::
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syscfg.vals:
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OS_CPUTIME_FREQ: 32768
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OS_CPUTIME_TIMER_NUM: 5
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TIMER_0: 0
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TIMER_5: 1
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BLE_XTAL_SETTLE_TIME: 1500
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On nRF51 platform the only difference is to use timer 3 instead of timer
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5.
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On platforms without 32768 Hz crystal available it usually can be
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synthesized by setting ``XTAL_32768_SYNTH`` to ``1`` - this is also
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already configured in existing BSPs.
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Crystal settle time configuration
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The configuration variable ``BLE_XTAL_SETTLE_TIME`` is used by the
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controller to turn on the necessary clock source(s) for the radio and
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associated peripherals prior to Bluetooth events (advertising, scanning,
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connections, etc). For the nRF5x platforms, the HFXO needs to be turned
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on prior to using the radio and the ``BLE_XTAL_SETTLE_TIME`` must be set
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to accommodate this time. The amount of time required is board
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dependent, so users must characterize their hardware and set
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``BLE_XTAL_SETTLE_TIME`` accordingly. The current value of 1500
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microseconds is a fairly long time and was intended to work for most, if
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not all, platforms.
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Note that changing this time will impact battery life with the amount
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depending on the application. The HFXO draws a fairly large amount of
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current when running so keeping this time as small as possible will
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reduce overall current drain.
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