4.1.1. DAP flow of control

Figure 4.2 shows the flow of control for the DAP when used with an off-chip debugging unit.

The DAP acts as a component to translate data transfers from one external interface format to another internal interface. The external interface is either JTAG or serial wire. This provides a link for an external debug tool to generate accesses into a SoC. The debug port controls the JTAG-AP, AXI-AP, AHB-AP, and APB-AP through a standard bus interface:

Figure 4.2. DAP flow of control

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The external hardware tools directly communicate with the SWJ-DP in the DAP and perform a series of operations to the debug port. Some of these accesses result in operations being performed on the DAP internal bus.

The DAP internal bus implements memory-mapped accesses to the components that are connected using the parallel address buses for read and write data. The debug port, SWJ-DP, is the bus master that initiates transactions on the DAP internal bus in response to some of the transactions that are received over the debug interface. Debug interface transfers are memory-mapped to registers in the DAP, and both the bus master and the slaves contain registers. This DAP memory map is independent of the memory maps that exist in the target system.

Some of the registers in the access ports can translate interactions into transfers on the interconnects to which they are connected. For example, in the JTAG-AP, a number of registers are allocated for reading and writing commands that result in Test Access Port (TAP) instructions on connected devices, for example, processors. The processor is also a bus master on a system memory structure to which the AHB-AP has access, so both the processor and AHB-AP have access to shared memory devices, or other bus slave components.

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