Digital Signal Processing Reference
In-Depth Information
configuration. This active LOW output is actually the arrangement used on all
seven segment LED displays on all of the FPGA development boards.
The Problem Definition
To illustrate the capabilities of the software in the simplest terms, we will
build a circuit that turns off the LED when one OR the other pushbutton is
pushed. In a simple logic equation, one could write:
LED_OFF = PB1_HIT + PB2_HIT
At first, this may seem too simple; however, the active low inputs and outputs
add just enough complication to illustrate some of the more common errors,
and it provides an opportunity to compare some of the different syntax features
of VHDL and Verilog. (Students needing an exercise in DeMorgan's Law will
also find these exercises particularly enlightening.)
We will first build this circuit with the graphical editor and then implement it
in VHDL and Verilog. As you work through the tutorial, note how the design
entry method is relatively independent of the compile, timing, and simulation
steps, and which FPGA board is used for the hardware implementation.
Resolving the Active Low Signals
Since the pushbuttons generate inverted signals and the LED requires an
inverted or low level logic signal to turn off, we could build an OR logic
circuit using the layout in Figure 1.7a. Recalling that a bubble on a gate input
or output indicates inversion, careful examination shows that the two circuits
in Figure 1.7 are functionally equivalent; however, the circuit in Figure 1.7a
uses more gates and would take a bit longer to enter in the schematic editor.
We will therefore use the single gate circuit illustrated in Figure 1.7b.
(a) (b)
Figure 1.7a and 1.7b.
Equivalent circuits for ORing active low inputs and outputs.
This form of the OR function is known as a "negative-logic OR." If you are
confused, try writing a truth table to show this Boolean equality. (In Exercise 1
at the end of the chapter, this circuit will be compared with its DeMorgan's
equivalent, the "positive-logic AND.").
O
N THE
UP2
AND
UP1
BOARDS
,
THE
LED
S OUTPUT STATE WILL APPEAR INVERTED SINCE ITS
LED
OUTPUT CIRCUIT IS INVERTED
,
SO PUSHING ONE OF THE PUSHBUTTONS WILL TURN ON
THE
LED.
