Merge branch 'master' into cmm_devicename_docs

Author: h2zero

docs/Usage_tips.md

@@ -55,6 +55,16 @@ This characteristic is read <b>after</b> connecting to the device.
 * If no <i>'Advertising "Local name"'</i> is set, OSes, such as iOS, may display the devices name as 'Unnamed' until the device is connected to, at which time the <i>'GATT Device Name'</i> is read and used instead.
 
 It is recommended that both <i>'Advertising "Local name"'</i> <b>and</> <i>'GATT Device Name'</i> be set appropriately, after considering the above described behavior.
+## 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