4.4.2. Mixing JTAG devices with SWJ-DP

It is possible to enable direct access to existing TAPs and gain the pin-count advantage of SWD when only debug access is required by disabling access to the TAPs when switching to SWD. The SWJ-DP component of the DAP provides additional outputs that indicate the mode of operation that is in force, JTAG or SWD, to enable a debugger to change between them without causing the TAPs to enter into UNPREDICTABLE states.

Figure 4.5 shows an example JTAG connection that supports multiple TAP controllers that describes how to connect an additional TAP with the DAP enabling either serial access when using JTAG, or access to the DAP when using the reduced pin-count mode of SWD.

Figure 4.5. An example JTAG connection

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Note

You must fit an external tri-state buffer on TMS to enable output of data when operating in SWD mode and a multiplexor on TDO for re-use with SWO. In Figure 4.5, when in SWD mode, the other JTAG TAP state machine is held in reset through the assertion of nTRST and clamping of TMS HIGH. In some devices, this can cause unexpected consequences so it is strongly recommended that you read the documentation for the TAP as to a safe method of disabling the device.

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