Digilent Basys Board Rev.E - Manual (Page 4)

Digilent
Basys Reference Manual
www.digilentinc.com
Copyright Digilent, Inc. Page 4/12 Doc: 502-107
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
Figure 6. Basys I/O circuits