Page 172 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

Chapter 6: Radio-Controlled Systems and Telemetr y 151

‘frqa := 134_218 ‘ 2500 Hz
frqa := 3_222 ‘ 60 Hz
‘frqa := 672 ‘ ~12.5 Hz
dira[0] := 1 ‘ make p0 output

repeat
term.str(string(HOME, “Freq: “))
f := fc.freq ‘ get frequency
if f > 0 ‘ valid?
term.dec(f/10) ‘ print whole part
term.tx(“.”)
term.dec(f//10) ‘ print fractional part
term.str(string(“ Hz”, CLREOL))
else
term.str(string(“???”, CLREOL))
waitcnt(clkfreq + cnt)
This program makes use of another Spin program named jm_freqin, which actually
measures and displays the pulse frequency on the Propeller Serial Terminal. The program
was created by Jon “JonnyMac” McPhalen (also known as Jon Williams), who is a prolific
contributor to the Parallax forums. Jon also made provision for self-generating pulses to test
the program. Those pulses emit from pin 0. However, I used pin 14 as an input, since that is
one of the servo ports on the BOE. The jm_freqin program is shown below with Jon’s
introductory comments included because I think they are very helpful in understanding
how this program functions:

{{
This object uses ctra and ctrb of its own cog to measure the period
of an input waveform. The period is measured in clock ticks; this
value can be divided into the Propeller clock frequency to
determine the frequency of the input waveform. In application, the
period is divided into 10x the clock frequency to increase the
resolution to 0.1Hz; this is especially helpful for low
frequencies. Estimated range is 0.5Hz to ~40MHz (using 80MHz
clkfreq).

The counters are setup such that ctra measures the high phase of
the input and ctrb measures low phase. Measuring each phase
independently allows the input waveform to be asymmetric. In order
to prevent a loss of signal from causing an erroneous value from
the freq() method the fcCycles value is cleared after a valid
frequency is calculated; this means that you should not call this
method at a rate faster than the expected input frequency.
}}

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