Digilent 410-155P-KIT User Manual

Page 4

Basys2 Reference Manual

Digilent

www.digilentinc.com

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.

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.

3.3V

Push

buttons

Slide

switches

Spartan 3E

FPGA

C11

G12

BTN0

BTN1

BTN2

BTN3

SW0

SW1

SW2

SW3

SW4

SW5

SW6

SW7

3.3V

LD0

LD1

LD2

LD3

LD4

LD5

LD6

LD7

3.3V

LEDs

7seg
Display

AN0

AN1

AN2

AN3

P11

M11

P7
P6

N5
N4

F12

J12

M13

K14

L14

H12
N14
N11

P12

L13

M12

N13

Figure 6. Basys2 I/O circuits

Doc: 502-138

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