Digilent Nexys Board User Manual

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Revision: February 19, 2007 215 E Main Suite D | Pullman, WA 99163

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Overview

Digilent’s Nexys circuit board is an integrated
circuit development platform based on a Xilinx
Spartan 3 FPGA. The Nexys board provides
large external memory arrays, a collection of
useful I/O devices, and numerous ports,
making it an ideal platform for experiments
with FPGA-based digital systems, including
embedded cores like Xilinx’s MicroBlaze.

The Nexys board is suitable for designs
ranging from simple logic circuits to complex
digital systems, without needing any other
components. All external signals are ESD and
short-circuit protected, ensuring a long
operating life in any environment.

The Nexys is fully compatible with all versions
of the Xilinx ISE tools, including the free
WebPack. Nexys features include:

• 200K-gate Xilinx XC3S200 FPGA with
500+MHz operation (400K and 1M gate
versions available)

Switching

Power Supplies

USB2

Port

4Mbyte

Flash

16Mbyte

Cellular RAM

• USB2 port for FPGA configuration and
high-speed data transfers (using the free

3V3 1V2 2V5 1V8

32

32 32

Adept Suite Software)

• USB-powered (batteries and/or wall-plug
can also be used)

• 16MB of fast Micron PSDRAM and 4MB
of Intel StrataFlash Flash ROM

• Xilinx Platform Flash ROM that stores
FPGA configurations indefinitely

• High efficiency switching power supplies
(good for battery powered applications)

• 50MHz oscillator

• Connector for 1/8 VGA hi-res graphics

LCD panel or 16x2 character LCD display

JTAG

port

Platform

Flash

(config

ROM)

Xilinx Spartan3-200 FT256

-400 and -1000 FPGAs available

4 4 4 4

32

Clock

(100, 50,

or 25

MHz)

I/O devices

Figure 1: Nexys block diagram

JA

JB

JC

JD

43

High speed

100-pin

connector

• 60 FPGA I/O’s routed to expansion
connectors (one high-speed Hirose FX2 connector and four 6-pin headers)

• 8 LEDs, 4-digit seven-segment display, 4 pushbuttons, 8 slide switches

• Ships in a convenient plastic carry case (together with USB cable)

®

Copyright Digilent, Inc. All rights reserved 12 pages Doc: 502-107

Nexys Reference Manual

Digilent

www.digilentinc.com

Functional Description

The Nexys board provides an inexpensive, robust, and easy-to-use platform that anyone can use to
gain experience with FPGA devices and modern design methods. It is centered on the Spartan 3
FPGA, and it contains all needed support circuits so designs can get up and running quickly. The
large collection of on-board devices allow many designs to be completed without the need for any
other hardware, making the Nexys an ideal platform for experimenting with new designs or learning
about FPGAs and CAD tools. The 100-pin high-speed connector and four 6-pin expansion connectors
allow designs to grow beyond the Nexys board, either with user-designed boards or breadboards
and/or peripheral module (Pmod) boards offered by Digilent. (Pmods are inexpensive analog and
digital I/O modules that offer A/D conversion, D/A conversion, motor drive, sensor input, and a host of
other features). Signals on each 6-pin expansion connector are protected against damage from ESD
and short-circuit connections, ensuring a long operating life in any environment. The Nexys board
works seamlessly with all versions of the Xilinx ISE tools, including the free WebPack tools. The
Nexys recieves power from the USB interface and it ships with a USB cable, so designs can be
implemented immediately without the need for any additional hardware.

FPGA and Platform Flash Configuration

The FPGA on the Nexys board must be configured (or programmed) by the user before it can perform
any functions. Design software, like the free WebPack from Xilinx, can be used to define any number
of circuits that can be programmed into the FPGA. Once programmed, the FPGA will retain its
configuration only as long is power is applied. The FPGA can be programmed in two ways: directly
from a PC, and from an on-board Platform Flash ROM that is also user-programmable. A jumper on
the Nexys board determines which source (PC or ROM) the FPGA will use to load its configuration.
The FPGA can automatically load a configuration at power-on from the Platform Flash ROM by setting
the Mode Select Jumper JP3 to “FLASH”.

Note that a demonstration configuration is loaded into the Nexys board during manufacturing. If that
configuration has not been overwritten, it can be automatically loaded into the FPGA by setting the
Mode Select Jumper JP3 to “FLASH”, and cycling power or pressing the reset button.

JTAG ROM

Configure from on-board ROM

Configure from PC via JTAG

JTAG3
header

XCF02

Platform

Flash

JTAG

Slave
Serial

Spartan 3

FPGA

Mode Select
Jumper

PROG

DONE

DONE

LED

Vdd

PROG
(reset)
button

Mode Select

Mode Select

Jumper

Jumper

JTAG3
header

DONE

LED

RESET

button

Copyright Digilent, Inc. Page 2/10 Doc: 502-107

Nexys Reference Manual

Digilent

www.digilentinc.com

The FPGA and the Platform Flash ROM can be programmed from a PC using Digilent’s Adept
software or Xilinx’s iMPACT software (both are available for free download). Digilent’s Adept Software
works with the USB circuit, and Digilent’s JTAG3 parallel cable is compatible with iMPACT. When
using the USB circuit, a “cable bypass” jumper must be loaded on the JTAG header at J6 to connect
the TDI and TDO signals.

To program the Nexys board, connect the programming cable to the board and to a PC, and apply
power to the board. Start the programming software, and wait for the FPGA and the Platform Flash
ROM to be automatically identified. To program the FPGA, select the desired .bit file; to program the
Platform Flash, select the desired .mcs file. Right-click on the device to be programmed, and select
the “program” function. The configuration file will be sent to the FPGA or Platform Flash, and the
software will indicate whether programming was successful. For more information on device
programming, refer to the Adept or iMPACT reference manual.

Both the FPGA and Platform Flash ROM will always appear in the scan chain. After the Platform
Flash ROM has been loaded with a configuration file, the FPGA can automatically load that file at
power-on if programming mode control jumper is loaded in the ROM position.

A reset button is provided (labeled “reset”) that can erase the configuration in the FPGA, and start a
new programming cycle. An LED labeled “done” will illuminate whenever the FPGA has been
successfully configured.

Power Supply

The Nexys board can be powered from the USB port or
any DC supply that produces a voltage in the 5VDC­9VDC range. The power jack on the Nexys board
requires a center-positive, 2.1mm power supply
connector as is commonly found on wall-plug power
supplies. Voltages higher than 10V may permanently
damage Nexys.

The “raw” voltage from the power jack is routed to the
four 6-pin expansion connectors, the 16-pin expansion
connector, and to a 3.3V voltage regulator. The 2.5V
and 1.2V supplies required by the FPGA are generated
from the 3.3V supply. Total board current is dependant
on FPGA configuration, clock frequency, and external
connections. In test circuits with roughly 20K gates routed, a 50MHz clock source, and all LEDs
illuminated, about 200mA of current is drawn from the 1.2V supply, 50mA from the 2.5V supply, and
100mA from the 3.3V supply. Required current will increase if larger circuits are configured in the
FPGA, and if peripheral boards are attached. The table below summarizes the power supply
parameters.

Vendor

Linear Technology Switcher Main power 3.3V (IC6) LTC1765 3A/100mA
Linear Technology Switcher (dual) FPGA Vaux 2.5V (IC7) LTC3417 1.4A/50mA
Linear Technology Switcher (dual) FPGA Vcore 1.2V (IC7) LTC3417 1.4A/200mA
Linear Technology Linear Vsram 1.8V (IC8) LTC1844 150mA/90mA
Linear Technology Linear Vusb 3.3V (IC4) LTC1844 150mA/60mA

Technology Use Supply PN Current (Max/typ)

Copyright Digilent, Inc. Page 3/10 Doc: 502-107