4.3.2. System interface

To connect the Debug APB into the system memory, an APB multiplexer is provided within the DAP. The bus type of the system must be converted to APB, for example through an AXI to APB bridge or an AHB to APB bridge. The address bits used to decode the region within the system memory must not be passed into the interface, only the address bits for that region, these are then decoded to individual components within the Debug APB address decoder. Because only a reduced address range is used for the Debug APB, this must exist on an aligned boundary. It is possible to translate system memory space into an aligned region, but this must be done prior to connection to the system interface on the DAP to ensure that the ROM table and its contents are still correct. Any high address bit that was used to decode the debug address region within the system memory, must be tied LOW on the system input to the DAP. For example, if debug region exists within the range 0x3F50_0000 to 0x3F50_FFFC,only PADDRSYS[15:2] must be connected with PADDRSYS[30:16] = 0x0000.

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