ARM Technical Support Knowledge Articles

How do I select JTAG or SWD (Serial Wire Debug) debug protocol?

Applies to: Debug Access Port (DAP)


The SWJ-DP component supports both protocols - Serial Wire and JTAG. The selector is part of the SWJ-DP.

The default protocol at power-up is JTAG, so if you want to use Serial Wire, you need to use the TCK and TMS to scan in a special 'key', which is a sequence of TMS values which has no effect for a pure JTAG TAP but is recognised by a SWJ-DP TAP as the signal to switch into Serial Wire mode.

The key sequences for switching to Serial Wire mode and back again are described in section 6.2 of the

ARM Debug Interface v5 Architecture Specification Supplement

ADI v5.1 Supplement

which can be found at:

-> CoreSight on-chip trace and debug

  -> Architecture specifications

    -> ARM Debug Interface v5 Architecture Specification Supplement

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