ARM Technical Support Knowledge Articles

Is RTCK needed on the JTAG connection to a CoreSight DAP?

Applies to: CoreSight


The CoreSight DAP (SWJ-DP or JTAG-DP) has no need for an RTCK.

RTCK is not a standard IEEE 1149 JTAG signal. It was created to allow the use of synchronized TAP controllers in ARM9 and ARM11 class of cores, where the JTAG inputs (TMS and TDI) have to cross an asynchronous boundary into the processor clock domain. RTCK has the effect of controlling the TCK from the emulator box (RVI, Multi-ICE, etc) to ensure correct JTAG-like behaviour across that clock domain boundary.

The CoreSight implementation of the DAP uses a fully asynchronous TAP controller which does not require any local synchronization (hence no need for RTCK). It manages the asynchronous relationship between JTAG (TCK) and the DAP bus (DAPCLK/PCLKDBG) internally to the JTAG-DP or SWJ-DP.

TCK can be run at any frequency up to the maximum operating frequency of your ICE unit + interconnect (wires, connectors, board traces) + TAP logic in the TCK domain. TCK frequency is not dependent upon the core clock frequency in a CoreSight system.

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