4.9.4. DAP transfers

This section describes DAP transfers.

Effects of DAPABORT

The APB-AP does not cancel the system-facing operation, and returns dapready HIGH one cycle after dapabort is asserted by the debug port. The externally driving APB master port does not violate the APB protocol. After a transfer is aborted, the Control and Status Register can be read to determine the state of the transfer in progress bit, TrInProg. When TrInProg returns to zero, after completing the external transfer or on a reset, the APB-AP returns to normal operation. All other writes to the APB-AP are ignored until the TrInProg bit is returned LOW after a transfer Abort.

APB-AP error response generation

APB-AP error response generation is described in:

System initiated error response

An error response received on the APB master interface propagates onto the DAP bus when the transfer is completed. This is received by the debug ports.

AP-initiated error response

After a DP-initiated abort operation is carried out, and an external transfer is still pending, that is, the TrInProg bit in the CSW Register remains HIGH:

  • Reads of all registers return a normal response except for reads of the Data Read/Write Register and banked registers. Reads of the Data Read/Write Register and banked registers return an error response because they cannot initiate a new system read transfer until CSW.TrInProg is set to 0 either by completing the system transfer or by a reset

  • Writes to the access port return an error response, because they are ignored until the TrInProg bit has been set to 0.

Differentiation between System-initiated and AP-initiated error responses

If dapslverr is HIGH and TrInProg is LOW, then the error is from a system error response.

If dapslverr is HIGH and TrInProg is HIGH, then the error is from an access port error response. The transfer has not been accepted by the access port. This case can occur after an abort has been initiated and while the system transfer has not completed.

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