I am designing a USB JTAG adapter using FT2232D.
Most JTAG reference designs use a level translator to drive JTAG pins with VREF. I understand VREF is used because different voltage levels at target boards can be programmed this way.
But I am using fixed 3.3V with an LDO from 5V USB power supply, so I don't want to use level translators.(I will only use JTAG adapter to access 3.3v target boards)
Only design, which I found, that doesn't use level translator is in this document(page 23) http://www.risec.aist.go.jp/project/sasebo/download/SASEBO-GII_Spec_Ver1.01_English.pdf
In this design VREF is connected to JTAG adapter's 3.3V supply. Is this necessary? I don't understand why VREF is connected to JTAG adapter. Isn't only GND connection enough to drive JTAG port since target board and JTAG adapter has same voltage level which is 3.3V?