Nuts & Volts

Doing these things allowed me to
add five to the ones variable for each
step of voltage measured by the
MC68HC908MR16’s analog-to-digital
converter. The ASCII values 0x30
through 0x39 represent a readable 1
through 9, respectively. So, I initialized
all of the digit variables to 0x30
(readable zero) and looked for an
overflow to 0x3A after each increment
of five operation. The result is an upcounter
that converts the value of
ANALOG_OutV to a human readable
number between 0.000 and 5.120,
which represents the voltage applied
to the MC68HC908MR16’s analog-todigital
converter input pin.
You can observe the caveman
■ PHOTO 3
30 March 2006
counter’s operation by single stepping
through the code in debug mode. Once
the incoming voltage is converted to
human readable form, I used our RS-232
Bean code to send the ASCII voltage
reading to a terminal emulator running
on a PC. To eliminate missing characters
and to make the voltage values a bit
easier to read, the ASCII characters need
to be paced to the terminal emulator
and instead of writing some homebrew
delay code, I enabled the Delay100US in
the Methods area of the Cpu Bean
Inspector.
I recommend you use Tera Term
Pro as the terminal emulator. You
can get a free copy of Tera Term Pro
from the Tera Term Pro home page
at http://hp.vector.co.jp/
authors/VA002416/tera
term.html You can see the
ADC code running and
talking to Tera Term Pro in
Photo 2.
EASY PWM
WITH THE
MC68HC908MR16
There are two ways to
get a usable PWM signal out
■ PHOTO 2. This is a good look at the
debugging tools provided by the P&E
Cyclone Pro. The voltage readings
are shown in the Tera Term Pro window.
Using the 10K pot, I dialed in +2.500
volts as the input voltage to the MC68HC
908MR16’s ADC. Although the results look
really bad in raw form, if you average the
20 readings you see, the average voltage
works out to be +2.57 volts.
of the MC68HC908MR16. The first
method entails reading the 41 pages of
the MC68HC908MR16 datasheet
describing the care and feeding of the
MC68HC908MR16’s multi-channel PWM
subsystem. Then, write the PWM code
you need. The second method does not
require any datasheet study and allows
you to programmatically set the PWM
period and duty cycle in real time units
of microseconds or milliseconds.
Behold Listing 4. Just by looking
at the code snippet in Listing 4, you
already know which of the aforementioned
PWM generation methods I
used. The song remains the same. I
selected the PWM Bean from the
Timer folder within the Bean Selector
and double-clicked it into my new
MR16_PWM project. Configuration of
the PWM Bean included renaming the
Bean to SERVO, setting a PWM period
of 20 mS, and setting up the pulse
polarity to produce a high-going
pulse.
I compiled the project and inserted
the result=SERVO_SetDutyUS
function, which was generated in the
SERVO.c file. Notice the absence of my
omnipresent LED blinker code. There’s
no need for it here, as I attached an
Airtronics 94102 servo to the fourth bit
of the MC68HC908MR16’s Port E. The
source code you are looking at in
Listing 4 continually takes the servo
rotor to and from both the clockwise
and counterclockwise extremes
stopping at the center point with each
traversal.
ROTATING OUT
All of the additional modifications
to the solderless breadboard
can be seen in Photo 3. As you can
see, I’ve added an RS-232 port, a
potentiometer, and a hobby servo. All
of this circuitry could have just as easily
been assembled in a point-to-