Why do 24 MHz operations fail on some hardware systems?
At higher frequencies you might see IDCODE mismatch failures or problems identifying the chain in iMPACT. At 24 MHz, there is a timing window of approximately 30 ns for TDO to propagate from the target device to the cable with sufficient setup margin for sampling. Hardware systems using TDO buffers or logical devices between TDO and the cable can have excess TDO propagation delay causing the data to be incorrectly sampled. Signal integrity issues increased by board noise can also be an issue. JTAG lines are often given low priority routing and, as a result, are prone to noise on the board. Putting the system in Reset or Quiet mode can often help in these cases. In addition, some devices are not specified to operate properly at 24 MHz. Refer to the device data sheet for information about the maximum supported TCK_CCLK frequency. For additional details, refer to the cable data sheets; Platform Cable USB Data Sheet (DS300) and Platform Cable USB II Data Sheet (DS593): http://www.xilinx.co
