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CC2640: External vs internal oscillator

Part Number: CC2640

If the device is configured to use the external LF oscillator (as below), and the external oscillator is physically removed, will the device automatically switch to use the internal oscillator? That seems to be the case via experimentation:

    #define SET_CCFG_MODE_CONF_SCLK_LF_OPTION               0x2 

We are seeing an issue where we are using the external oscillator as above, but sometimes our devices become unresponsive in our cooking and calibration process of the device. It looks to be UART overrun, perhaps due to the device relying on the internal oscillator for UART system communication. If we remove the external oscillator, the device still advertises and communicates via UART. But it is unable to be connected to as the advertisement/connection interval times become slightly off..suggesting that perhaps that internal oscillator needs to be calibrated for consistent / robust communication in UART, BLE.

The chip revision is 2.3, and we do not have the calibration of the internal oscillator enabled. But we are going to experiment with calibration enabled. Thank you.

  • Hello Andy ,

    Yes, if there is some reason that prevents the 32.768 kHz external crystal from starting (e.g., damaged during PCBA assembly or incorrectly sized load caps are common reasons we see), the MCU will fail-over to the internal RCOSC_LF SCLK source. This will manifest as problems with BLE connection timing, esp. on longer CIs, as the clock sources have different frequencies and the RCOSC_LF requires calibration as called out in app note SWRA499 www.ti.com/.../swra499. ADV will be ok as it is intentionally jittered.

    You an add a check in your firmware to confirm the SCLK is the external xtal. See the code snippet in the CC26xx BLE HW Troubleshooting & Bring Up article on the BLE Wiki.

    I hope this helps.

    Best wishes