Problems can occur when attempting to configure via JTAG when a PROM is loaded and the device is in Master Mode. The cause of the problem is that the JTAG interface has caused INIT to go Low and the device will start clocking data in from the PROM. The PROM begins to configure the device before iMPACT starts to program the device. When iMPACT starts to configure the device, there has already been data loaded into the device. This can cause problems where JTAG configuration will fail.
The problems exist in the Spartan-3 devices because the configuration frames need to be initialized. When the first configuration starts, the frames become un-initialized. After the JTAG configuration starts, the frames will not properly configure because the frames have been un-initialized.
1. Set the FPGA's MODE pins for JTAG mode.
2. Erase the contents of the PROM while JTAG testing is being run.
3. Hold INIT low during JTAG configuration to hold off PROM configuration.