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concentrates on the trees and forgets about the forest. The process of verifjmg
the design at this level varies with the product, the company, and the customer
and is beyond the scope of this book. It may include system testing that is several
levels removed from the embedded control system, and it may involve testing in
the field.

A Real- World Example

This example illustrates some of the issues involved in debugging embedded appli-
cations. It is based on the document feeding example used in Chapters 4 and 6.
As described earlier, this system had two requirements: the documents had to be
fed at a certain rate (the throughput) and the interdocument spacing had to be
greater than a certain threshold to meet the requirements of downstream sorting
mechanisms.
The original problem, described in Chapter 4, involved coupling between the
throughput and spacing correction code; a feedback loop was added to eliminate
the spacing errors. Once this code was implemented, the error rate was reduced
significantly, but there were still too many errors.
Figure 7.1 shows the mechanical and electrical arrangement of the system. Each
document was fed into the transport by a feed wheel, driven by a DC motor, which
was in turn controlled by the microcontroller. The motor had a shaft-mounted
encoder to provide speed feedback to the motor driver and the microcontroller.
The microcontroller commanded the motor driver to operate the motor at a par-
ticular speed and the motor driver hardware had a feedback loop to maintain that
speed. There were actually three motors in the feed mechanism, but only one was
involved in the problem.
Documents leaving the feeder were passed through an intermediate accelerator.
This accelerator consisted of a spring-loaded wheel, turning at a constant speed,
that would slip on the document as long as the document was held by the feed
wheel. As soon as the trailing edge of the document left “pinch” of the feeder, it
would be accelerated. A sensor was located after the accelerator. This sensor could
detect the presence or absence of the document and was located so that a docu-
ment of any width could be detected.
After the first sensor, another accelerator brought the document up to the final
speed. After this accelerator, there was another sensor. This sensor was the first
sensor after the document was at full transport speed and was used by the system
to detect spacing errors. There was about 23 inches of track between the feeder
and the second sensor. As mentioned in Chapter 6, a document (or a given point
on a document) took about 150ms to travel between the feeder and the second
sensor.

198 Embedded Microprocessor Systm