CMPEN275 – Digital Design Lab
Lab11 – Stopwatch
Penn State Erie, The Behrend College
1 Discussion
The purpose of this lab is to implement the control unit for a simple stopwatch and integrate it with the
datapath developed in lab 9 to form a fully functional stop watch. The entity description for the resulting
stopwatch is given in Listing 1.
entity StopWatch is
port( reset, clk, but: in std_logic;
TenDigit, UnitDigit: out std_logic_vector(6 downto 0));
end StopWatch;
Listing 1: The entity description of the StopWatch component.
The internal organization of the stopwatch will illustrate the datapath and control design philosophy, by
separating the design into two distinct halves, a datapath which performs data manipulation and a control
unit which orchestrates the manipulation of the data into a sequence. The internal organization of the
stopwatch is given in Figure 1.
Figure 1: The datapath and control of the stopwatch.
The clock signal will be generated by the PLDT-3 boards 4Mhz crystal oscillator. Its important to note that
the crystal frequency of 4Mhz is 4,000,000 cycles per second. That is Megahertz are decimal, not binary,
qualtities. The reset and button signals will be generated by push buttons on the PLDT-3 board. Finally,
the tens digit and unit digit will be displayed on the seven segment displays. In Lab 9 you constructed the
datapath and in Lab 10 a control unit. Not all that remains is to modify the control unit so that it operates
according to that shown Figure 1. The entity description of this control unit is given in Listing 1.
1
entity control is
Port ( clk, reset : in STD_LOGIC;
sw, button : in STD_LOGIC;
cw : out STD_LOGIC_VECTOR (3 downto 0));
end control;
Listing 2: The entity description of the StopWatch control unit.
The stopwatch control unit will issue a different control word in state. The value of the control word will
configure the datapath to perform whatever action is required in that state. The stopwatch starts off in the
Reset state (thats why it has a “*“ in it) and stays there while the button is not pressed. In the reset state,
the control unit instructs the datapath to reset the Mod10Counter and sec_cnt counter. When the button is
pressed, the control unit goes to the transistion state R2C, which stands for Reset-to-Count. It waits there
until the the button is released at which point the control unit goes to the Tenth state. The control unit
waits in the Tenth state until the clock divider count reaches a count value representing 1/10 of a second.
When the count values reaches this value then the E output of the comparator will go to one causing the
control unit to transition to the Inc state. The control unit spends a single clock cycle in the Inc state,
where it tells the datapath to increment the Mod10Counters and reset the clock divider counter. If the
user presses the button while the control unit is in the Tenth state the control unit goes to the transistion
state C2H, which stands for Count-to-Hold. It waits there until the the button is released at which point
the control unit goes to the Hold state. In the Hold state, the control unit tells the clock divider to hold
and the Mod10Counters to hold. When the user presses the button the control unit goes to the transistion
state H2R, which stands for Hold-to-Reset. When the button is released the control unit transitions back
to the Reset state to start the process all over again.
2 In-lab week 1
In the lab you will modify and combine existing files to build your stopwatch.
1. Complete the control word table for the stopwatch control unit in Table 1.
State
cw(3)
cw(2)
cw(1 downto 0)
Synch
Enb
Clock divider
0 time unchanged 0 time unchanged 00 hold
1 time=00
1 counting up
01 load
10 count up
11 synch reset
Reset
R2C
Tenth
Inc
C2H
Hold
H2R
Table 1: The control word table for the stopwatch.
2. Modify the lab 10 control unit for proper operation in the stop watch.
o Modify the state_process making it identical to the one depicted in Figure 1.
o Modify the output_process creating a control word identical to Table 1.
o Run the testbench posted on the class web page. Verify the correct operation of the control
unit. Include the following traces in your timing diagram: clk, reset, button, state, and
control word bits relabeled to reflect their titles in Table 1 (e.g. relabel cw(3) as Synch).
o Run the simulation for at least 8uS.
2
3. Combine the files for the datapath (lab 9) and the control unit from the previous step into a single
component for the stopwatch.
o Create a stopwatch component which integrates the datapath and control.
o Change the “tenth” constant to 5 for the simulation.
o Create a testbench for the combined datapath and control. Use the button press statement
from the control unit testbench posted on the class web page.
o Include the signals from the control unit in your simulation as well as the tenths count
(decimal not 7-segment) and the unit count.
4. Use the UCF file from lab9 to create a synthesized stopwatch
o Change the “tenth” constant in Figure 1 to a value to generate a 0.1 second delay at 4Mhz.
o Synthesize the design
o Download and have the instructor verify proper operation of the circuit.
3 In-lab week 2
A formal lab report is due on the last Friday of classes.
3