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Motorola Sensor Device Data
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ADDITIONAL NOTES
This type of A/D conversion method (one type of A/D
conversion) inherently takes a finite period of time to digitize
the signal (incrementing the pulse train’s high time while
polling the general I/O pin); however, for most sensor
applications the physical phenomenon being measured does
not change quickly (<1 ms) enough to warrant an ultra–fast
A/D conversion process.
An additional advantage of this design is that the
measurement process may be performed only as necessary,
keeping the CPU processing time and overhead minimal.
If an input capture timer channel (TCAP) is available, it may
be configured to detect the logic–level one to logic–level zero
transition of the comparator’s output. When the edge
transition occurs, an interrupt service routine is executed that
stores the pulse train’s high times, calculates the current
pressure, etc. This is typically more convenient and eliminates
the need to poll a general I/O pin every time the pulse train’s
high time is incremented (interrupt subroutine is executed only
when the edge transition occurs).
SUMMARY
Shown above is a minimal component design that can
convert an analog sensor’s output into a digital output. Each
major subsystem (sensor, amplifier, integrator, comparator,
and microcontroller) is explained in detail simultaneously with
a design example. Next the system operation is discussed
including how it works and how to design a desired system
resolution. Finally a flow chart for measuring and calibrating
the sensor’s output is presented.
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Freescale Semiconductor, Inc.
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