The USB-Blaster™ II download cable has a default clock frequency for TCK (JTAG) or DCLK (Active Serial programming) of 24MHz. The frequency may be changed to lower frequencies where signal integrity and timing does not allow operation at 24MHz.
The following comamnd may be used to change the clock frequency of the USB-Blaster II:
jtagconfig --setparam <cable number> JtagClock <frequency>
<cable number> is the USB-Blaster II cable to be modified.
<frequency> is the desired TCK or DCLK frequency. 24M, 16M and 6M are supported.