Multidrop serial wire debug allows simultaneous access to an unlimited number of devices through a single connection. This is useful for connectivity-constrained products that contain multiple chips with multidrop support.
In order to select a target in a multidrop capable product, the debugger must write the correct TINSTANCE, TPARTNO, and TDESIGNER fields into the SW-DP TARGETSEL register. The values for these fields are located in and fetched from two registers, TARGETID and DLPIDR.
For more information about multidrop SWD, see Arm Debug Interface Architecture Specification, ADIv5.0 to ADIv5.2.