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Usage_tips.md - Detail persisted bonds limitations and considerations relative to CONFIG_BT_NIMBLE_MAX_CCCDS

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Chris Morgan
2025-09-19 21:52:59 -04:00
committed by Ryan Powell
parent 6f0b9ddf5d
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@@ -32,6 +32,17 @@ or nullptr such as when calling `NimBLEClient::getService`. The latter being a
Most of the functions in this library return something that should be checked before proceeding. Most of the functions in this library return something that should be checked before proceeding.
<br/> <br/>
## Persisted bonds can be lost due to low MAX_CCCDS
The symptom: CONFIG_BT_NIMBLE_MAX_BONDS is set to N, but a smaller number of bonds is preserved, perhaps even a single bond. The limitation of persisted bonds can be observed via NimBLEDevice::getNumBonds(). The number may not reach CONFIG_BT_NIMBLE_MAX_BONDS.
Cause: For each bond, NimBLE persists each of the CCCD (client characteristic configuration descriptor) values that have been subscribed
to by the client. If CONFIG_BT_NIMBLE_MAX_CCCDS is too low, the older CCCD values are overwritten by the newer ones. The loss of the older
CCCDs values results in those bonds being lost.
Fix: Increase CONFIG_BT_NIMBLE_MAX_CCCDS. These take approximately 40 bytes in NVS, 2 bytes for the CCCD value and the NVS metadata overhead. The value of
CONFIG_BT_NIMBLE_MAX_CCCDS should conservatively be no less than (CONFIG_BT_NIMBLE_MAX_BONDS * {maximum number of characteristics that can be subscribed to}).
## There will be bugs - please report them ## There will be bugs - please report them
No code is bug free and unit testing will not find them all on it's own. If you encounter a bug, please report it along with any logs and decoded backtrace if applicable. No code is bug free and unit testing will not find them all on it's own. If you encounter a bug, please report it along with any logs and decoded backtrace if applicable.