Setting the clock speed

It is important to select the best clock speed for your system. Higher clock speeds enable faster downloads, but setting the clock speed too high can result in intermittent faults and reliability problems. If you are experiencing such problems, try manually reducing the clock speed. If you are not sure which clock speed to use, try setting the default speed, 10MHz.


For RVI, for reliable operation at high clock speeds you must use the Low Voltage Differential Signaling (LVDS) cable.

Show/hidePredefined clock speed

To select a predefined clock speed:

  1. Select the Devices node in the tree diagram.

  2. Select the clocking that you want to use from the Clock Speed drop-down list. The following figure shows an example:

    Figure 29. The scan chain speed controls

    The scan chain speed controls


    Although the debug hardware can support JTAG clock speeds down to 13Hz, your debugging environment might become unstable at speeds lower than 1kHz.

Show/hideCustom clock speed

If the clock speed you want to use is not available as a preset value, enter the required clock speed in the Clock Speed field with the Hz, kHz, or MHz suffix as required. For example, 40.0 kHz.

Show/hideSee also

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