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122 Bu il d Y o ur O w n Q u a d c o p t e r

I found the meaning of this magic number in the website http://propeller.wikispaces
.com/Full+Duplex+Serial. The 0 in the argument list represents the operating mode for the
full duplex serial object. Below is the wiki documentation regarding the mode:

.start(rxpin, txpin, mode, baudrate)

Start serial driver—starts a cog
mode bit 0 = invert rx
mode bit 1 = invert tx
mode bit 2 = open-drain/source tx
mode bit 3 = ignore tx echo on rx

I guess it is just the teacher in me, but I would prefer to eliminate all magic numbers and
instead modify this code to include a constant definition as follows:

In the CON section add

mode = 0
and in the PUB section modify

fdx.start(rxPin, txPin, mode, baud) ‘start serial driver to PC
You see how much clearer it all becomes, although it is true that you will have to go back
into the documentation to discover exactly how the mode works. Please refer to my rant in
the previous chapter regarding magic numbers. Now back to the analysis.
There is a “repeat – until” loop checking for a character from the PC. This code snippet
is shown below:

repeat
ch := fdx.rx
until ch := “ ”

All you need to do is press the space bar in order to enable you to enter the desired
power level. The motor always starts at the 0 power level meaning, no rotation. The Spin
program will then launch a new cog after the initial position value (80,000 in this case) is
stored. The following code snippet creates this new cog object:

cognew(@SingleServo,@position)

This statement creates a new cog that runs code beginning at the symbolic address
“SingleServo,” which is also the start of an assembly language program. The statement
also instructs the cog to load, at boot time, the value stored in the hub RAM location named
“position” into its “PAR” special purpose register (SPR). I introduced the SPRs at the end of
Chapter 4, but I did not really elaborate on them. The PAR SPR is the so-called boot register,
which means that a value will be stored in it when the cog is created and/or rebooted when
it is executing assembly code. You should note that creating a cog that is designated to
execute assembly language is substantially different from creating a cog to run a method
in another Spin object, which was the situation in all of the Chapter 4 examples. The