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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?

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What you already know:

VREF (Voltage Reference) is usually only used as an indicator.

  • Your JTAG adapter knows nothing about the characteristics of your target (it may be 1.8, 3.3 or 5V driven)
  • If you want to cater for all possible variations, you will need to know which level the microcontroller requests and configure your outputs accordingly (usually you will use level shifters and supply them with the power domain of your target (VREF) for the secondary side)
  • Since you only need one level (and you are sure that this level will always match your target) you can ignore VREF. The device is not powered or anything similar by VREF, it is just intended as a reference voltage.

Back to your main question:

Yet, some JTAG devices or software detect the presence of a target by monitoring VREF and will do nothing if it's low (no device connected). I don't know if this is relevant for you since I know nothing about the backing software, so just to be be safe connect VREF to any 3V3 supply (and add a 0R resistor somewhere so you can try 0V as well, just to be sure :-) ).

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