Page 161 - The Art of Designing Embedded Systems

148 THE ART OF DESIGNING EMBEDDED SYSTEMS

Given that the scope is a general-purpose tool used by RF engineers,
digital computer designers, and even software gurus, it has to accept a
wide range of inputs. Computer people work mostly with 5-volt levels
(Le., a zero is about 0 volts; a one is 3 to 5 volts). Audio engineers might
need to measure millivolt levels. Your embedded system probably detects
or generates some sort of real-world data, which is probably not in the
0- to 5-volt scale.
Thus, the scope’s Vertical section is born. The run-of-the-mill two-
channel scope has two identical vertical sections.
A BNC connector (like the kind used in thin Ethernet applications)
connects to the scope probe. The signal sensed by the probe runs to the ver-
tical amplifier, which increases the input from perhaps a few volts to sev-
eral hundred, which is ultimately applied to the plates in the CRT.
Like any good amplifier, each vertical channel has an amplitude con-
trol (i.e., the same thing as a volume control in your stereo). Unlike a vol-
ume control, it has an exact calibration associated with each position. Set
the knob to, say, 2 volts/division, and a 4-volt signal will move the beam
up two divisions. Divisions are denoted by a grid of boxes on the CRT so
you can easily measure levels.
Each channel has a “position” control that lets you move the rest po-
sition of the beam up or down to the most convenient point. If you wanted
to measure voltage, with no signal applied, set the beam right on one of the
division marks on the screen. Then, count how many boxes the waveform
occupies. Convert divisions to voltage using the setting of the amplitude
control.
The position control lets you move the beam all the way off the
screen. It can be pretty challenging to find the damn beam at times, so a
“beam find” button brings it into view, giving you an idea which way to
move the position controls.
A channel selector lets you put either channel 1 or channel 2 on
the screen. Most software work involves measuring the relationship be-
tween two inputs, so you’ll select “both.” Two sweeps will pop up. Use
the two sets of amplitude and position knobs to control each channel
independently.
Controlling up and down beam deflection is only half of the problem.
The Horizontal Amplifier sweeps the dot back and forth across the screen.
Note that you only see the left-to-right deflection; the return sweep is very
fast and is never displayed.
In software debugging I hardly ever care about amplitude, since
mostly I’m looking for the input’s shape or duration. If the amplitude is