Resolving a resolvable address

As per Bluetooth specification, a private resolvable address is composed of six bytes.

A private resolvable address composed of 6 bytes. — Figure 2. Resolvable address

To resolve an address, IN.PTR must point to a job list describing both the Hash and Prand parts of the private resolvable address (DEVICEADDR) field from the Bluetooth packet, as well as a number of Identity Resolving Keys (IRK). This is illustrated in the examples below. How many IRKs are used is determined by the number of IRKs in the job list. See EasyDMA for an introduction to EasyDMA job lists.

The resolver is started by triggering the START task. A RESOLVED event is generated if AAR manages to resolve the address using one of the Identity Resolving Keys (IRK). AAR will generate a NOTRESOLVED event if it is not able to resolve the address using the specified list of IRKs. If there are no IRKs in the joblist, the NOTRESOLVED event is generated.

Address resolution with packet preloaded into RAM — Figure 3. Address resolution with 16 IRKs and DEVICEADDR preloaded into RAM

Figure 4. Address resolution with packet device address preloaded into multiple RAM locations, and three IRK keys

AAR will go through the list of available IRKs in the job list, and for each IRK try to resolve the address according to the Resolvable Private Address Resolution Procedure described in the Bluetooth Specification¹. The time it takes to resolve an address may vary depending on where in the list the resolvable address is located. The resolution time will also be affected by RAM accesses performed by other peripherals and the CPU. See the Electrical specifications for more information about resolution time.

Note: Maximum number of IRKs supported in a job list is 4096.

AAR only compares the received address to those programmed in the module without checking the address type.

AAR will stop when it has managed to resolve the maximum number allowed, specified in the MAXRESOLVED register. Each time AAR resolves an IRK, the index of the corresponding IRK is written to memory through the output job list in OUT.PTR. For each IRK found, OUT.AMOUNT is updated accordingly.

The output job list must define a memory region large enough to hold list of resolved IRK indices. This is calculated from the MAXRESOLVED register, where each IRK index occupies two bytes in memory. In the example below, the n indicates the number of bytes (size) in the resolved IRK index array. The value of n must be exactly 2 times the value of MAXRESOLVED.

Figure 5. Resolved IRK index structure at RAM

At the end of the operation, AAR will generate the END event.

Triggering the STOP task will stop AAR. If AAR is stopped before the operation has completed, the END, RESOLVED, and NOTRESOLVED events are not generated. However, if STOP is triggered close to the end of the operation the events can be generated.

¹ Bluetooth Specification Version 4.0 [Vol 3] chapter 10.8.2.3.