Test Your EQ
— Issue #154
Each month, Test Your EQ presents
some basic engineering problems for you to test
your Engineering Quotient. What's your EQ?
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Problem
1—The
output stage of a generic '1488 RS-232 driver is shown
below. The input stage has an output impedance of roughly
10 Kilohms and functions so that Q1 is basically either
cut off or saturated. One of the functions of an RS-232
driver is to limit the drive current to approximately
10 mA. Does this circuit accomplish that? If so, show
how it does it for both output states.
Answer
Problem
2—What is the function of D3 in the circuit in
question 1?
Answer
Problem 3—Are the two circuits below equivalent?
Explain.
Answer
Problem
4— Some filters have flat passbands and roll
off smoothly in their transition regions. Other filters
accept a small amount of ripple in their passbands in
order to achieve steeper transitions. What is the implication
in the time domain of these ripples?
Answer
Problem
5— The following circuit is built around a single-chip
microcontroller that has an on-board analog comparator
and a hardware timer that provides a periodic interrupt.
a, b and c are 8-bit variables in
the microcontroller's memory.
The ISR functions as follows: During each interrupt (2560
per second), it reads the output of the comparator and
copies the value to an output pin. If that value is high,
the value of a is incremented. The value of b
is incremented on every interrupt unconditionally. Every
time b overflows, the value of a is copied
to c and then a is cleared.
What does the value of c represent? How does the
circuit work?
Answer
Problem
6—The following circuit is similar to the circuit
in question 5. VREF is a fixed reference voltage (nominally
VCC/2) that can be generated either internally or externally
to the microcontroller. Note that the ISR now inverts
the output of the comparator before sending it to the
digital output pin. The software counters operate as before.
What are the advantages of this circuit over the circuit
in problem 5? What are the disadvantages?
Answer
Problem
7—You're wrapping up a microcontroller-based
project and you just need to connect four push buttons
for user input. Alas, there's only one I/O pin left on
the chip, but you discover that you can rearrange things
so that the available pin is one of the inputs to the
chip's A/D converter! Realizing that you can arrange each
switch to connect to a different voltage, you come up
with the circuit shown here:
Each
switch connects a different voltage from the resistor
divider to the A/D input, and the 1-M pull-down makes
sure the input voltage goes to zero when no buttons are
pressed.
However, you soon realize that there are a number of problems
with this circuit:
•
If two or more buttons are pressed, parts of the divider
are shorted out, creating ambiguous readings for the firmware
•
Neither side of any switch is grounded, creating potential
noise and ESD problems
•
Too many resistors are required
Can
you come up with an alternate solution that fixes these
problems?
