Page 175 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

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

NEG_DETECT long %01100 << 26
tmp1 res 1
tmp2 res 1
mask res 1 ‘ mask for frequency input
pin

cyclepntr res 1 ‘ hub address of cycle
count

cycles res 1 ‘ cycles in input period

fit 492

DAT

This program utilizes both the A and B cog counters as noted in Jon’s comments. I will
not step through all the assembly language lines as I have done in several previous programs.
However, I will mention that using either assembly language or C language routines is the
only way to handle very high-speed measurements, such as those made by this program.
I have not done it myself, but Jon estimates that this program could measure frequencies
as high as 40 MHz, which is incredible considering that the Prop clock rate is only 80 MHz.
The other Spin program that is used in this frequency-measuring application is named
jm_txserial, which is Jon’s adaptation of a fairly standard Spin program named Full_Duplex.
It is a common and recommended procedure to use existing open source code and adapt it
to your purposes, as Jon has done in this case. I will not discuss jm_txserial specifically other
than to say it is a highly efficient communications program that communicates to the PSerT
program to display the main program results.

Figure 6.22 Measuring the throttle channel pulse frequency.