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Nexys2 Reference Manual

more than 100mA will be drawn through the USB cable (as required by the USB specification). A USB

host can supply only 500mA of current at 5VDC. When using USB power, care must be taken to

ensure the Nexys2 board and any attached peripheral boards do not draw more than 500mA, or

damage to the host may result. The Nexys2 board typically consumes about 300mA of USB current,

leaving about 200mA for peripheral boards. If peripheral boards require more current than the USB

cable can supply, an external power supply should be used.

The Nexys2 board uses a six layer PCB, with the inner layers dedicated to VCC and GND planes.

The FPGA and the other ICs on the board all have a large complement of bypass capacitors placed

as close as possible to each VCC pin. The power supply routing and bypass capacitors result in a

very clean, stable, and low-noise power supply.

FPGA and Platform Flash Configuration

The FPGA on the Nexys2 board must be configured (or programmed) by the user before it can

perform any functions. During configuration, a "bit" file is transferred into memory cells within the

FPGA to define the logical functions and circuit interconnects. The free ISE/WebPack CAD software

from Xilinx can be used to create bit files from VHDL, Verilog, or schematic-based source files.

The FPGA can be programmed in two ways:

directly from a PC using the on-board USB port,

and from an on-board Platform Flash ROM (the

Flash ROM is also user-programmable via the

USB port). A jumper on the Nexys2 board

determines which source (PC or ROM) the

FPGA will use to load its configuration. The

FPGA will automatically load a configuration

from the Platform Flash ROM at power-on if the

configuration Mode jumper is set to "Master

serial". If the Mode jumper is set to "JTAG", the

FPGA will await programming from the PC (via

the USB cable).

Digilent's freely available PC-based Adept

software can be used to configure the FPGA

and Platform Flash with any suitable file stored

on the computer. Adept uses the USB cable to

transfer a selected bit file from the PC to the FPGA or Platform Flash ROM. After the FPGA is

configured, it will remain so until it is reset by a power-cycle event or by the FPGA reset button

(BTNR) being pressed. The Platform Flash ROM will retain a bit file until it is reprogrammed,

regardless of power-cycle events.

To program the Nexys2 board using Adept, attach the USB cable to the board (if USB power will not

be used, attach a suitable power supply to the power jack or battery connector on the board, and set

the power switch to "wall" or "bat"). Start the Adept software, and wait for the FPGA and the Platform

Flash ROM to be recognized. Use the browse function to associate the desired .bit file with the FPGA,

and/or the desired .mcs file with the Platform Flash ROM. 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. The configuration

Cypress

EZ-USB

USB miniB

JTAG

connector

XCF02

Platform

Flash

Figure 4: Nexys2 programming circuits

Page 3/17

www.digilentinc.com

JTAG

Mode

header

Jumper

Spartan 3E

FPGA

JTAG

PROG

port

Slave

serial

DONE

port

Done

LED

Doc: 502-134

Digilent

Vdd

FPGA

Reset

Button

(BTNR)