For some devices, a pulse on the PROG pin resets the TAP controller. The TAP controller is used in JTAG and Boundary Scan operations as per the IEEE 1149.1 specification.
The effect that the PROG pin has on the TAP controller is device dependent. Here is a list of devices in which a pulse of the PROG pin does not have any effect on the TAP controller:
The devices that reset the TAP controller when the PROG pin is pulsed are listed below. For these devices, the chain should be re-aligned to TLR after the PROG pin has been pulsed.
If a delay in configuration is required, the INIT pin should be held Low as opposed to resetting the TAP controller.
For the XC4K/XC5K/Spartan/-XL families, the TAP controller is also reset. The following Boundary Scan instructions are still available with the PROG_B pin held Low: