Problem with JTAG configuration over USB-Blaster Rev.C with VCCTRGT=2.5V

Most JTAG programming problems occuring with particularly adapters are caused by a combination of specific adapter behaviour and signal quality issues of the respective board. Distorted TCK, e.g. with slightly ringing edges is the favourite candidate.

Small changes to the board hardware can often fix the issues, e.g. a small parallel capacitor (5 - 10 pF) at TCK near the FPGA, series termination of TCK at the JTAG connector. The signal quality problems should reveal when probing the JTAG signals with active high impedance probes.