Start
• Simple
Video Signal •
Video with the LPC2138 •
Video Algorithm •
Tricks Used
•
Vertical
Sync & Resolution •
Video for other Apps •
Oscilloscope •
Implementation •
Construction & Results
•
Generate Video •
Sources and PDF
IMPLEMENTATION
The
firmware consists of two parts, one for the video
controller and one for the oscilloscope. All of
the oscilloscope routines are in the Oscilloscope.c
file posted on the Circuit Cellar FTP site. The
main thread is in main.c file (see Figure 5).
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(Click
here to enlarge)
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Figure
5—The GetKey procedure doesn’t suspend the
loop; it just returns the code. Code = 0 when
the keys aren’t pressed. |
The
main loop, which starts after initialization,
manages data collection, visualization, and the
device control. The special Menu flag assigns
the current action for the Down and Up buttons.
This flag is toggled with the Store button.
During
normal operation (the Menu flag is 0), the Down
and Up buttons make the timebase (horizontal scale)
one step faster or slower. It’s achieved by processing
several ADC samples (the “N” in Figure 5) to obtain
a single display sample, not by a variation of
the ADC sample rate. This allows the system to
implement Envelope mode in which the highest value
and the lowest value are calculated for each group
of N ADC samples and stored as the data of a single
display sample. On the plot, a vertical line between
the two values is drawn so glitches or other high-frequency
components of the signal will always be visible.
A line between two adjacent samples is also drawn.
The
data collection thread fills the special array
of samples and signals to the main loop that the
data is ready. The main loop then waits in the
TV frame to start the visualization of data. This
is necessary to prevent any problems caused by
interference between the video controller’s refresh
rate and the data collection rate. Four visualization
algorithms are used for this purpose.
The
first algorithm is for high data collection rates
(i.e., horizontal scales). It waits until the
data array is completely filled for one record
view.
The
second and third algorithms are for middle and
low rates. They immediately visualize the current
portion of data, which is collected between two
adjacent TV frames. The difference between them
is in a Trigger mode. At low collection rates,
it isn’t necessary to wait for the trigger event
because there is enough time to consider the graph.
As a result, the trigger system is off here.
The
fourth algorithm (transient recorder) is intended
for super-slow data collection rates. It immediately
draws the new sample and also uses Roll mode.
This means that the each new sample causes a change
in the record view offset and makes the space
for the new sample. For measuring slowly changing
signals, it looks nicer than Scan mode, where
the display area is filled on the left side when
the graph reaches the right side.
The
data collection thread is written as an ADC interrupt
service routine (ISR) that’s rich in functionality.
It serves all of the visualization algorithms
and uses the different conditions to start a data
collection cycle. One condition is a trigger event
that occurs when the signal crosses a threshold
(trigger level). In this version, I detect the
crossings of the rising parts of a curve. Comparing
the current sample value and the trigger level
value does this. The trigger value is calculated
using the voltage from the rotating control located
on the front panel.
The
real signal has a noise component that causes
false triggers near the cross points. The falling
edge can be recognized as rising. To prevent this,
I used a debouncing algorithm like the one commonly
used for mechanical switches (see Figure 6, p.
55). Of course, this isn’t the only solution.
Many devices use variable hysteresis algorithms
or HF filters.
The
voltages from rotating controls also have a noise
component, but another algorithm should be applied
to kill it: math filtering. I implemented a simple
procedure, moving average:
GetVoltages_TrigLevel=(GetVoltages
_TrigLevel+val)>>1;
//New (filtered)
value
of trigger level
Another
function of the data collection thread is to create
a delay between the trigger event and the start
of data collection. This is equivalent to the
beam offset function in analog oscilloscopes.
The value for the delay is obtained like the value
for a trigger level from the rotating control
at the front panel.
The
Store button stops the data collection process,
captures the graph, and calls the Store menu.
You can use the Down and Up buttons to move the
special cursor along the row of numbers. The stored
graphs are shown when you move the cursor to the
required position. Each number corresponds to
a stored graph that can be replaced by the current
waveform when the Store button is repeatedly pressed.
The Exit position causes the return to normal
operation without saving anything.
Ample
flash memory (512 KB in the LPC2138) enables you
to implement a life-prolonging algorithm for a
storage system. Normally, a single flash memory
block has a limited number of erasure cycles.
The data for one graph (1 KB) is considerably
less than a block size (32 KB), so one flash memory
block can contain a set of 32 (32 KB/1 KB) stored
graphs. I use this technique when the flash memory
block isn’t erased every time a new graph is stored.
The new data is written in the free space, and
the flash memory block is completely erased only
when it’s filled by all 32 graphs. For visualization,
a procedure seeks the last stored data and draws
a graph.
