2.2.2. Operation of the SWJ-DP

SWJ-DP enables you to design an Application Specific Integrated Circuit (ASIC) that you can use in systems that require either a JTAG interface or a SWD interface. There is a trade-off between the number of pins used and compatibility with existing hardware and test equipment. There are several scenarios where you must use a JTAG debug interface. These enable:

You can connect an ASIC fitted with SWJ-DP support to legacy JTAG equipment without making any changes. If an SWD tool is available, only two pins are required, instead of the usual four used for JTAG. You can therefore use the other two pins for something else.

You can only use these two pins if there is no conflict with their use in JTAG mode. To support use of SWJ-DP in a scan chain with other JTAG devices, the default state after reset must be to use these pins for their JTAG function. If the direction of the alternative function is compatible with being driven by a JTAG debug device, the transition to a shift state can be used to transition from the alternative function to JTAG mode. You cannot use the other function while the ASIC is in JTAG debug mode.

The switching scheme is arranged so that, provided there is no conflict on the TDI and TDO pins, a JTAG debugger can connect by sending a specific sequence. The connection sequence used for SWD is safe when applied to the JTAG interface, even if hot-plugged, enabling the debugger to continually retry its access sequence. A sequence with TMS=1 ensures that JTAG-DP, SW-DP, and the watcher circuit are in a known reset state. The pattern used to select SWD has no effect on JTAG targets. SWJ-DP is compatible with a free-running TCK, or a gated clock supplied by external tools.

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