Answers Database

Operational level of voltage/current on FPGA inputs


Record #1390

Problem Title:
Operational level of voltage/current on FPGA inputs


Problem Description:
Keywords: voltage, input, protection diodes
Urgency: Standard

What is the operational level of voltage on the input of an FPGA?


Solution 1:

Maximum DC overshoot or undershoot above Vcc or below GND must be
limited to either 0.5V or 10mA, whichever is easier to achieve.
Durring transisions, the device pins may undershoot to -2.0V or
overshoot to Vcc+2.0V, provided this over-or undershoot lasts less
than 20ns.

There are very strong ESD-protection diodes (impedence=1 Ohm)
to help insure these voltage levels are not violated internally.
Typical diode currents increase from several microamps @ 0.7V
to 100mA @ 1V.

Negative voltages will be clamped.

Excess positive input voltage may pull Vcc high.

Xilinx suggests to control the input current to stay below 10mA
constant current.  The protection diodes can handle 100mA
without trouble, however, if this amount of current is maintained
for an extended period of time, the diodes will heat up and effect
the surrounding circuitry.

For more information see the Xilinx Programable Logic Data Book
page 13-47 "Overshoot and Undershoot"

For information on specific devices see the latest reliability
data on the WEB at:

http://www.xilinx.com/products/qa_data

For mixed voltage compatibility information see Chapter 6 of the
9/96 or 1/98 Programable Logic data Book.





End of Record #1390