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# Storing BLE Bonding Information/Keys on External Flash.

Associate III

Hello,

I am building an application using STM32WB55 ( 1 MB Flash) series microcontroller and planning to use BLE 5.0 to make secure connection with a device.

In our use case, I am trying to compute how many keys/Bonding Info can be stored to speed up pairing.

I was wondering if you someone can confirm my calculations below as something doesn't feel right.

From what I read in AN5289 Rev 6, on Page 97, there is a formula to compute maximum bonded device info, which is

N = (total_size_of_nvm – 1) / [(size_of_sec_record + 1) + (size_of_gatt_record + 1)]

In my case, after plugging in the values for

total_size_of_nvm : 253 Bytes ( 507 words of 4 Byte each)

size_of_sec_record : 80 Bytes

size_of_gatt_record : 12 Bytes

N comes to be : 252/94 ~= 2.

Is this correct? that we can only store maximum of 2 bonded devices?

Help!!!

Thanks

ST Employee

Hello,

The formula to compute maximum bonded device info is the following, you have right:

N = (total_size_of_nvm – 1) / [(size_of_sec_record + 1) + (size_of_gatt_record + 1)]

The values to be used are:

size_of_sec_record = 20 words (of 4 bytes) = 80 bytes

total_size_of_nvm = 507 words (of 4 bytes) = 2028 bytes

size_of_gatt_record = 12 words (of 4 bytes) = 48 bytes Default GATT configuration without adding services

N = (2027) / (81 + 49) = 15

So, you can store maximum of 15 bonded devices.

To take into account your services and characteristics to compute size_of_gatt_record value, see part 7.6.6 of AN5289:

The GATT record in NVM is composed:

· Per service:

– 2 bytes for handle

– 3 bytes if 16-bit UUID or 17 bytes if 128-bit UUID

· Per characteristic attribute:

– 2 bytes for handle

– 3 bytes if 16-bit UUID or 17 bytes if 128-bit UUID

– 2 bytes for CCCD value (only if the attribute is a CCCD)

Best Regards