Honored Contributor
Honored ContributorPin 4 of the JTAG header is used to power circuitry in the USB-Blaster. This voltage determines the voltage at which the JTAG signals are driven to the target.
The USB-Blaster will happily operate at 3.3V. however, JTAG in Cyclone IV devices is designed to operate at 2.5V. So, doing this will result in you over driving the device's JTAG signals. It'll probably work for a while but is clearly not recommended and may well kill your device. You have 3.3V available. Can you regulate this down to 2.5V externally? You clearly must have some external breakout board/cable to which you connect your USB-Blaster. It would only take a tiny 3-pin LDO regulator. The current consumed by the USB-Blaster is very low - datasheet states 10mA max. In practice it's nearer 1mA. So, I suspect you may even be able to drop the voltage accurately enough with a couple of resistors...
