FPGA pins

FPGAs tend to have lots of pins...

Dedicated pins vs. user pins

FPGA pins fall into 2 categories: "dedicated pins" and "user pins".

About 20% to 30% of the pins of an FPGA are "dedicated pins", which means that they are hard-coded to a specific function.
The dedicated pins fall into the 3 following sub-categories.

• Power pins: ground or power pins (core or IO).
• Configuration pins: used to "download" the FPGA.
• Dedicated inputs, or clock pins: these are able to drive large nets inside the FPGA, suitable for clocks or signals with large fan-outs.

The rest are user pins.

User pins

The bulk of the pins of an FPGA are "user pins" (called "IOs", or "I/Os", or "user I/Os", or "user IOs", or "IO pins", or ... you get the idea). IO stands for "input-output".

Note that:

• You have total control over user IOs. They can be programmed to be inputs, outputs, or bi-directional (tri-statable buffers).
• Each "IO pin" is connected to an "IO cell" inside the FPGA. The "IO cells" are powered by the VCCIO pins (IO power pins).

IO banks

An FPGA has many VCCIO pins (IO power pins), usually all connected to the same voltage. But new generations of FPGAs have a concept of "user IO banks". You split IOs into groups, each having its own VCCIO voltage. That allows using the FPGA as a voltage translator device, useful for example if one part of your board works with 3.3V logic, and another with 2.5V.

FPGA power

FPGAs usually require two voltages to operate: a "core voltage" and an "IO voltage". Each voltage is provided through separate power pins.

• The internal core voltage (called VCCINT here for simplicity), is used to power the logic gates and flipflops inside the FPGA. The voltage can range from 5V for older FPGA generations, to 3.3V, 2.5V, 1.8V, 1.5V and even lower for the latest devices! The core voltage is fixed (set by the model of FPGA that you are using).
• The IO voltage (called VCCIO here for simplicity) is used to power the I/O blocks (= pins) of the FPGA. That voltage should match what the other devices connected to the FPGA expect.

Actually, FPGA devices themselves allow VCCINT and VCCIO to be the same (i.e. the VCCINT and VCCIO pins could be connected together). But since FPGAs tend to use low-voltage cores and higher voltage IOs, the two voltages are usually different.

Naming

The internal voltage is named "VCC" for Xilinx and "VCCINT" for Altera.
The IO voltage is named "VCCO" for Xilinx and "VCCIO" for Altera.

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This page was last updated on December 27 2007.