PFR - Aerospace Engineering Sciences Senior Design Projects ...

Project Final Report – CUDBF April 30 th , 2009
ASEN 4028: Aerospace Senior Projects
The board features a PIC18F2553 microcontroller, which has one compare, capture, and PWM
(CCP) pin. The CCP pin is used to read in the PWM signal from the receiver. Due to the fact
that the board only had one CCP pin, only one signal could be read from the transmitter, thus the
board was used for proof-of concept. In order to read all of the required incoming signals, a
multiplexer chip was implemented. On the board there are digital input/output pins that can be
set high or low in software and the state of these pins can be read into the multiplexer chip with
the corresponding signal from the transmitter. In other words, each signal (rudder, elevator,
aileron, and switch) will be one of the inputs into the multiplexer and the digital I/O pins will be
the other input. The output of the multiplexing chip is then connected to the CCP pin and
depending what the PWM signal is the desired servo will be controlled. Table 22 illustrates how
the output of the multiplexer is determined and Figure 83 illustrates how everything is wired.
Table 22: Determination of output from multiplexer
A (I/O Pin) B (I/O Pin) Switch Aileron Elevator Rudder Output
L L L X X X L
L L H X X X H
L H X L X X L
L H X H X X H
H L X X L X L
H L X X H X H
H H X X X L L
H H X X X H H
Figure 83: Wiring Diagram
The power distribution board on the diagram illustrates that the PWM signal outputted by the
microcontroller cannot power the servos; a 5 V battery must be used to provide power to the
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