How does the SoftDevice Controller decide the connInstant when sending LL_CONNECTION_UPDATE_IND

[Charles Changhun Han]

Hello Nordic Semiconductor tech support team,

HW : nRF52840 ( Build code : nRF52840-CKAA-F )

SW : ncs 2.6.0

Sample Application :  zephyr/samples/bluetooth/hci_spi sample with SoftDevice Controller

We’re using two connection parameter sets with one sensor device.

Mode	CI(ms)	Peripheral Latency	SupervisionTimeout (seconds)
Slow	500	3	30
Medium	30/50	0	1

But the transition from slow (500ms) to medium (50ms) takes a significant amount of time (9 delta instants) due to the senseless connInstant value. Could you explain how the SoftDevice Controller decides the connInstant value, and how can I adjust our connection parameters to achieve a better (or faster) transition time?

The nRF52840 operates in the central role (Device A). Device B is a peripheral sensor device. Please refer to my sequence diagram.

Thanks,

Charles

[Vidar Berg]

Hello Charles,

The Softdevice controller will set the instant to 6 connection events plus the peripheral latency in the future. Excerpt from Core spec v5.4, Vol. 6, section 5.5:

Best regards,

Vidar

[Charles Changhun Han]

Hello, Vidar. Your explanation makes sense. I agree that your implementation follows the spec as well. Thanks, Charles.

[Vidar Berg]

Hello Charles,

Thanks. It looks like the "Connection Subrating" feature introduced in Bluetooth version v5.3 will become available for testing in SDK v2.8.0: https://github.com/nrfconnect/sdk-nrf/pull/15500.