Zener diodes for JTAG interface

Question

For my new PCB, I want to use a 7-series XILINX FPGA. I was a little bit unsure of the JTAG interface so I looked up some schematics of XILINX evaluation boards.

I found the Arty S7 50 schematic.

The JTAG interface of the Arty S7 50 is depicted below (can be found on page 6 in the schematic).

I think the Zener diodes have the purpose to protect the I/Os of a voltage that would be too big. Zener diodes have different breakdown voltages. I guess the depicted Zener diodes have a breakdown voltage that is bigger than 3.3V. Is this correct? So when I use Zener diodes with a breakdown voltage of around 5V I should be good.

Answer 1

These zener diodes are called a TVS or transient voltage suppressor.

They have various "trigger" voltages where they start to conduct, and can handle large currents for a very short time.

Their trigger voltage is an estimation. Each device will vary slightly.

When they do trigger, the voltage may still rise somewhat. The more power behind the triggering event, the higher this voltage may be.

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For example, a 3.3v "stand-off" device is designed to operate at 3.3v, so should not affect the circuit at all.

The trigger voltage for this hypothetical device may be 4.5v. At 4.5v, 1mA could flow through the TVS. This increases (non-linearly) with current:

• 5v might cause 10mA to flow through that TVS
• 10v might cause 1A to flow
• 15v might cause 10A to flow, etc.

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Research TVS's and find a device with an adequate stand-off voltage. Understand that the voltage downstream of the TVS will exceed the trigger voltage when activated, and may grossly exceed this for a very short time (under worst possible scenarios.)

A spark from a finger is a very high voltage, but at a very low current. A TVS is able to quench such a spark in a very short time, but the voltage on that pin will rise to above the trigger voltage.