Asynchronously accessing the program counter values of a data processing system by applying an independent clock on the latching and scan-chain circuits

摘要

A processing system 2 is described including a processor unit 4 and a memory 6. Diagnostic hardware 10 within the processing system 2 includes a shadow PC register 12 that stores a shadow program counter value following the true program counter value held within a program counter register 8 of the processor unit 4. The diagnostic hardware 10 is responsive to a program counter request signal from an external diagnostic system 14 to transfer a program counter value into a scan chain 16 from where it can be returned to the diagnostic system 14. The operation of the processor unit 4 is independent of any of the diagnostic operations and so true real time operation of the system may be observed and de-bugged.