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AR# 5398

FPGA - Can I drive an I/O when there is no power to the device?


Keywords: 4000, 3000, Virtex, XL, 4000XL, 4000EX, 3100, drive, IO, power

Urgency: Standard

General Description:
What are the effects of driving an I/O on an FPGA when there is no power to the device?



All 5-V Families (3000, 4000E/EX, 5200, Spartan) and all old 3-V familes (3000L)

Let's assume that ground is always connected, so we do NOT describe hot plug-in with conventional connectors (that do not guarantee VCC and ground mating before signal mating).

Each device input has an ESD protection diode pointing from the pin towards the VCC pin(s). Driving any pin with a positive signal while VCC is held rigidly to 0V would result in a lot of current through this diode (50 to a 100 mA, depending on the driving source), and is a reliability hazard.

If VCC is not rigidly held to ground by external loads, then the signal can pull VCC High, which reduces the input current, but VCC might be pulled High enough to start a configuration process, using the signal pin as an unreliable VCC source. This is an undesirable situation.

Do not drive inputs on 5-V devices while VCC is Low or 0V.


All 3.3-V families ( 5-V tolerant ) 4000XL/XV/XLA/XLT, Spartan-XL, and Virtex

The ESD protection is designed differently for these families -- it is a 6-V Zener diode directed towards ground, not VCC. Thus, if a pin is driven when the FPGA is not powered up, there is no current until the Zener diode starts conducting, at about 6 V.

This means the pins are 5-V tolerant, even when their own VCC is as low as 0V. It also means that there is no requirement for a specific power-up sequence of the different supply voltages.

Note that hot plug-in into standard connectors is a more complicated issue, as signal pins can mate before the common ground is established.
AR# 5398
Date 01/16/2002
Status Active
Type General Article