1.2 Return Clock (RTCK) signal
Occasionally, a target device requires the JTAG interface to be externally synchronized to a clock within
the device due to it being slow, non-continuous, or variable. The adaptive clocking feature uses the
Return Clock signal (RTCK) to address this requirement.
The RTCK signal is an output from the target device which is typically fed from the last flip-flop in the
synchronization chain.
If used, the RTCK signal must be pulled LOW on the target.
Warning
RTCK should never be directly linked to TCK on the target board. If it is directly linked, it is likely to
cause false clock-edges to be received by the TCK input of the target device.
Adaptive clocking
When adaptive clocking is enabled, the debug unit issues a TCK signal and waits for the RTCK signal
to return before sampling TDO. The debug unit does not progress to the next TCK transition until
RTCK is received, allowing the target device to control the flow of the JTAG interface, as required.
Note
• If you use the adaptive clocking feature, then the transmission delays, gate delays, and
synchronization requirements might result in a lower clock frequency, compared to using fixed
clocking. Adaptive clocking mode is not recommended unless the target design requires it.
• Adaptive clocking can be enabled using the configuration settings in Arm Development Studio. For
more information, see Debug Hardware configuration in the Arm Development Studio User Guide.
• If adaptive clocking is used, the debug unit cannot detect the clock speed, and therefore cannot scale
its internal timeouts. If the target clock frequency is too low, a JTAG timeout might occur, leaving the
JTAG interface in an unknown state. To recover the connection, you must reset the debug unit. To
disable JTAG timeouts, use the configuration settings in Arm Development Studio. For more
information, see Debug Hardware configuration in the Arm Development Studio User Guide.
1 Debug and trace interface
1.2 Return Clock (RTCK) signal
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