ARM Technical Support Knowledge Articles

When should I use adaptive clocking? Do I need to route RTCK to the JTAG connector?

Applies to: DSTREAM, Multi-ICE, RealView ICE and Trace (RVI / RVT)


RTCK is only used by the JTAG debug hardware (DSTREAM or RealView ICE) when it is configured in adaptive clocking mode. Adaptive clocking ensures that the TCK is slow enough to work with a synthesizable core and that its frequency is near the maximum possible.

In most cases, adaptive clocking is "nice to have" but it is not really necessary. You just need to set up TCK with a frequency slow enough for your target. In this case you don't need to take RTCK to a pin of the SoC. RTCK on the JTAG connector should be pulled-down.

However, adaptive clocking is necessary when debugging a target with variable clock speed. This way the JTAG frequency is always near the maximum possible. Without adaptive clocking you would need to set TCK at very low frequency, so that it works with all the possible clock speeds.

RTCK is not used on the SWD (Serial Wire Debug) protocol.

Article last edited on: 2011-11-04 09:18:36

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