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Basys 2™ FPGA Board Reference Manual
Revised April 8, 2016
This manual applies to the Basys 2 rev. C
Overview
The Basys 2 board is a circuit design and implementation platform that anyone can use to gain experience building
real digital circuits. Built around a Xilinx Spartan-3E Field Programmable Gate Array and a Atmel AT90USB2 USB
controller, the Basys 2 board provides complete, ready-to-use hardware suitable for hosting circuits ranging from
basic logic devices to complex controllers. A large collection of on-board I/O devices and all required FPGA support
circuits are included, so countless designs can be created without the need for any other components.
Full Speed
USB2 Port
(JTAG and data transfers)
Platform
Flash
(config ROM)
Settable Clock
Source
(25 / 50 / 100 MHz)
20
Data
port
JTAG
port
Xilinx Spartan3E-100 CP132
32
2
8 bit
color
4
4
4
4
JA
PS/2
Port
VGA Port
JB
JC
JD
Pmod Connectors
I/O Devices
100,000-gate Xilinx Spartan 3E FPGA
Atmel AT90USB2 Full-speed USB2 port
providing board power and programming/data
transfer interface
Xilinx Platform Flash ROM to store FPGA
configurations
8 LEDs, 4-digit 7-segment display, 4 buttons, 8
slide switches
PS/2 port and 8-bit VGA port
User-settable clock (25/50/100MHz), plus
socket for 2
nd
clock
Four 6-pin header expansion connectors
ESD and short-circuit protection on all I/O
signals.
Figure 1. The Basys 2 board block diagram and features.
Four standard expansion ports allow designs to grow beyond the Basys 2 board using breadboards, user-designed
circuit boards, or Pmods (Pmods are inexpensive analog and digital I/O modules that offer A/D & D/A conversion,
motor drivers, sensor inputs, and many other features). Signals on the 6-pin connectors are protected against ESD
damage and short-circuits, ensuring a long operating life in any environment. The Basys 2 board works seamlessly
with all versions of the Xilinx ISE tools, including the free WebPack. It ships with a USB cable that provides power
and a programming interface, so no other power supplies or programming cables are required.
The Basys 2 board can draw power and be programmed via its on-board USB2 port. Digilent’s freely available PC-
based Adept software automatically detects the Basys 2 board, provides a programming interface for the FPGA and
Platform Flash ROM, and allows user data transfers (see
www.digilentinc.com
for more information).
DOC#: 502-155
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
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Basys2™ FPGA Board Reference Manual
The Basys 2 board is designed to work with the free ISE WebPack CAD software from Xilinx. WebPack can be used
to define circuits using schematics or HDLs, to simulate and synthesize circuits, and to create programming files.
WebPack can be downloaded free of charge from
www.xilinx.com/ise/.
The Basys 2 board ships with a built-in self-test/demo stored in its ROM that can be used to test all board features.
To run the test, set the Mode Jumper (see below) to ROM and apply board power. If the test is erased from the
ROM, it can be downloaded and reinstalled at any time. See
Basys 2
for the test project as well as further
documentation, reference designs, and tutorials.
1
Board Power
The Basys 2 board is typically powered from a USB cable, but a battery
connector is also provided so that external supplies can be used. To use USB
power, simply attach the USB cable. To power the Basys 2 using a battery or
other external source, attach a 3.5V-5.5V battery pack (or other power
source) to the 2-pin, 100-mil spaced battery connector (three AA cells in
series make a good 4.5+/- volt supply). Voltages higher than 5.5V on either
power connector may cause permanent damage.
Input power is routed through the power switch (SW8) to the four 6-pin
expansion connectors and to a Linear Technology LTC3545 voltage
regulator. The LTC3545 produces the main 3.3V supply for the board, and it
also produces 2.5V and 1.2V supply voltages required by the FPGA. Total
Figure 2. Basys 2 power circuits.
board current is dependent 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 100mA of current is drawn from the 1.2V supply, 50mA from the 2.5V supply, and 50mA from
the 3.3V supply. Required current will increase if larger circuits are configured in the FPGA, or if peripheral boards
are attached.
The Basys 2 board uses a four layer PCB, with the inner layers dedicated to VCC and GND planes. The FPGA and the
other ICs on the board have large complements of ceramic bypass capacitors placed as close as possible to each
VCC pin, resulting in a very clean, low-noise power supply.
2
Configuration
After power-on, the FPGA on the Basys 2 board must be configured before it can perform any useful 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.
Digilent’s PC-based program called Adept can be used to configure the FPGA with any suitable bit file stored on the
computer. Adept uses the USB cable to transfer a selected bit file from the PC to the FPGA (via the FPGA’s JTAG
programming port). Adept can also program a bit file into an on-board non-volatile ROM called “Platform Flash”.
Once programmed, the Platform Flash can automatically transfer a stored bit file to the FPGA at a subsequent
power-on or reset event if the Mode Jumper (JP3) is set to ROM. The FPGA will remain configured until it is reset
by a power-cycle event. The Platform Flash ROM will retain a bit file until it is reprogrammed, regardless of power-
cycle events.
Copyright Digilent, Inc. All rights reserved.
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Basys2™ FPGA Board Reference Manual
To program the Basys 2 board, set the mode jumper
to PC and attach the USB cable to the board. 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 “Status
LED” LED (LD_8) will also blink after the FPGA has
been successfully configured. For further
information on using Adept, please see the Adept
documentation available at the Digilent website.
Atmel
AT90USB2
LD8
PC
Mode
Jumper
ROM
USB miniB
connector
JTAG
Spartan 3E
FPGA
XCF02
Platform
Flash
JTAG
port
Slave
serial
port
Figure 4. Basys 2 Programming Circuits.
3
Oscillators
The Basys 2 board includes a primary, user-settable silicon
oscillator that produces 25MHz, 50MHz, or 100MHz based
on the position of the clock select jumper at JP4. Initially,
this jumper is not loaded and must be soldered in place. A
socket for a second oscillator is provided at IC6 (the IC6
socket can accommodate any 3.3V CMOS oscillator in a half-
size DIP package). The primary and secondary oscillators are
connected to global clock input pins at pin B8 and pin M6
respectively.
Both clock inputs can drive the clock synthesizer DLL on the
Spartan 3E, allowing for a wide range if internal frequencies,
from 4 times the input frequency to any integer divisor of
the input frequency.
Spartan-3E
FPGA
The primary silicon oscillator is flexible and inexpensive, but
it lacks the frequency stability of a crystal oscillator. Some
B8 CLK_OUT
circuits that drive a VGA monitor may realize a slight
improvement in image stability by using a crystal oscillator
Figure 5. Basys 2 oscillator circuits.
installed in the IC6 socket. For these applications, a 25MHz
(or 50MHz) crystal oscillator, available from any catalog
distributor, is recommended (see for example part number SG-8002JF-PCC at
www.digikey.com
).
Linear Tech.
LTC6905
Oscillator
Frequency
Select
Jumper
25MHz
50MHz
100MHz
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Basys2™ FPGA Board Reference Manual
4
User I/O
Four pushbuttons and eight slide switches are provided for circuit inputs. Pushbutton inputs are normally low and
driven high only when the pushbutton is pressed. Slide switches generate constant high or low inputs depending
on position. Pushbuttons and slide switches all have series resistors for protection against short circuits (a short
circuit would occur if an FPGA pin assigned to a pushbutton or slide switch was inadvertently defined as an
output).
Eight LEDs and a four-digit seven-segment LED display are provided for circuit outputs. LED anodes are driven from
the FPGA via current-limiting resistors, so they will illuminate when a logic ‘1’ is written to the corresponding FPGA
pin. A ninth LED is provided as a power-indicator LED, and a tenth LED (LD-D) illuminates any time the FPGA has
been successfully programmed.
4.1
Seven-segment display
Each of the four digits of the seven-segment LED display is composed of seven LED segments arranged in a “figure
8” pattern. Segment LEDs can be individually illuminated, so any one of 128 patterns can be displayed on a digit by
illuminating certain LED segments and leaving the others dark. Of these 128 possible patterns, the ten
corresponding to the decimal digits are the most useful.
The anodes of the seven LEDs forming each digit are tied together into one common anode circuit node, but the
LED cathodes remain separate. The common anode signals are available as four “digit enable” input signals to the
4-digit display. The cathodes of similar segments on all four displays are connected into seven circuit nodes labeled
CA through CG (so, for example, the four “D” cathodes from the four digits are grouped together into a single
circuit node called “CD”). These seven cathode signals are available as inputs to the 4-digit display. This signal
connection scheme creates a multiplexed display, where the cathode signals are common to all digits but they can
only illuminate the segments of the digit whose corresponding anode signal is asserted.
A scanning display controller circuit can be used to show a four-digit number on this display. This circuit drives the
anode signals and corresponding cathode patterns of each digit in a repeating, continuous succession, at an update
rate that is faster than the human eye response. Each digit is illuminated just one-quarter of the time, but because
the eye cannot perceive the darkening of a digit before it is illuminated again, the digit appears continuously
illuminated. If the update or “refresh” rate is slowed to a given point (around 45 hertz), then most people will
begin to see the display flicker.
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Basys2™ FPGA Board Reference Manual
Common anode
AN1
AN2
AN3
AN4
F
A
B
CA CB CC CD CE CF CG DP
G
E
D
C
DP
An un-illuminated seven-segment display, and nine
illumination patterns corresponding to decimal digits
Four-digit Seven
Segment Display
Individual cathodes
Figure 7. Seven-segment display.
For each of the four digits to appear bright and continuously illuminated, all four digits should be driven once every
1 to 16ms (for a refresh frequency of 1KHz to
Refresh period = 1ms to 16ms
60Hz). For example, in a 60Hz refresh scheme,
the entire display would be refreshed once every
Digit period = Refresh / 4
16ms, and each digit would be illuminated for ¼
AN0
of the refresh cycle, or 4ms. The controller must
AN1
assure that the correct cathode pattern is
AN2
present when the corresponding anode signal is
driven. To illustrate the process, if AN1 is
AN3
asserted while CB and CC are asserted, then a
Cathodes
Digit 0
Digit 1
Digit 2
Digit 3
“1” will be displayed in digit position 1. Then, if
AN2 is asserted while CA, CB and CC are
Figure 8. Multiplexed 7seg display timing.
asserted, then a “7” will be displayed in digit
position 2. If A1 and CB, CC are driven for 4ms, and then A2 and CA, CB, CC are driven for 4ms in an endless
succession, the display will show “17” in the first two digits. Figure 8 shows an example timing diagram for a four-
digit seven-segment controller.
5
PS/2 Port
The 6-pin mini-DIN connector can accommodate a PS/2 mouse or keyboard. The PS/2 connector is supplied with
5VDC.
Both the mouse and keyboard use a two-wire serial bus (clock and data) to communicate with a host device. Both
use 11-bit words that include a start, stop and odd parity bit, but the data packets are organized differently, and
the keyboard interface allows bi-directional data transfers (so the host device can illuminate state LEDs on the
keyboard). Bus timings are shown in the figure.
The clock and data signals are only driven when data transfers occur, and otherwise they are held in the “idle”
state at logic ‘1’. The timings define signal requirements for mouse-to-host communications and bi-directional
keyboard communications. A PS/2 interface circuit can be implemented in the FPGA to create a keyboard or
mouse interface.
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
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