Solutions for Midterm - EECS 145M Spring 1997
1a
Moon
µComputer*
Counter/
Timer*
Parallel
output
port*
Parallel
input
port*
Mirror
Photodetector*
Laser*
Trigger*
Detect
light*
Laser beam*
[showing moon or mirror was good but not required for full credit- they are strictly speaking not a
part of your system]
[1.5 points off for each starred (*) element missing; 0.5 points off for each starred element without
a label] [0.5 points off if one of the laser beam legs is missing]
Note: “Connection” is not limited to wires- consider radio and fiber-optic communications.
1b
long i1, i2,;
double time, dist;
outport(1,0); /* zero counter */
outport(2,0); /* make trigger low */
outport(2,1); /* make trigger high to fire laser */
while(inport(3,1) == 1); /* make sure detection signal starts low
(ignore any prompt laser light) */
while(inport(3,1) == 0); /* wait for reflected laser beam */
outportb(1,1); /* latch timer*/
i1 = inport(1); /* read least significant word */
i2 = inport(1); /* most least significant word */
time = i2 << 16 | i1; /* pack time value in µs */
dist = time * 0.15; /* convert to km */
For full credit, it was essential to do the following:
1Assert the trigger output first low, then high
2Wait for the reflected laser light to go high (ignoring any prompt light is good design but not
required for full credit)
3Latch the timer before reading
4Convert time to distance in km
[2 points off if the outport(2,0); command is missing]
February 26, 1997 page 1 S. Derenzo
