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I loved Louis Dratwa's article, The
Lost Art of Strip Board Prototyping, in
the June 2013 issue. I also use strip
board extensively for my prototyping
— and even for multiple copies of a
project — but I still learned a lot from
his article. One thing I would like to
add is to avoid using flux remover
spray on the board.
I had a case of unexpected
voltages in a circuit. I started removing
parts one by one, and found that the
supply voltage was leaking through the
board material and appearing in the
strangest places. I even had a resistor
with parts no longer connected to it
that measured three volts on one end
and 0.2 on the other. Apparently, the
board absorbed the fluid and became
San Diego, CA
PIC Better Pick?
In the June 2013 article, An
Electronic Photocell for Lighting
Control, Gerry Shand uses a Maxim
DS1302+ real time clock chip.
Couldn't Gerry have used the
Microchip PIC18F2520-I/SP's Timer 1
counter to implement a software real
time clock and calendar (RTCC)?
Microchip application note AN1303
shows how to implement an RTCC on
a PIC16F1827 (see ww1.microchip.
A.pdf) which also uses a 32.768 kHz
crystal (the schematic in Figure 2 and
Parts List description call out 32.767
kHz but the Parts List part number
calls for 32.768 kHz).
Tim Brown PhD EE, PE
Honea Path, SC
Good catch on the crystal
For my part, I'd say yes, the PIC
would have worked fine. Actually, just
about every project in N&V can be
implemented in some fashion on a
microcontroller. The DS1302 is
relatively inexpensive, assuming you
have one on hand (shipping would
wipe out any savings).
Bryan Bergeron, Editor
Thanks for your feedback, Don.
Here is my answer on your two
1. Yes the crystal frequency for the
DS1302 on the schematic should read
32.768 kHz instead of 32.767 kHz.
2. We chose the DS-1302+ as the
timekeeper chip because we were
already familiar with the IC and we
had already developed code from
previous projects that we could
transplant onto this project.
If you look at the circuit, you'll
find that we have the math coprocessor for doing the sunset and
sunrise math calculations, the
timekeeper chip for keeping time
(and it also has a super capacitor for
keeping the memory fresh), and the
PIC as the "traffic cop" to marshall all
the signals to and from everything
else — plus, an independent LCD. This
was the original design topology that
was agreed upon when the design
first began. This configuration uses a
few more parts but it also made code
testing, debugging, and
troubleshooting much easier.
In my article, I also point out that
the final PIC chosen was the third
choice, so we minimized the change
in the design cycle to keep the
project on track. So, yes we could
have taken advantage of more
features in the PIC18F2520, but this
would have led to more iterations
during the design phase.
My experience with major
changes like this during detailed
design on any project I have
undertaken only creates more design
challenges and increased capital
costs. It's better to stay the course
and manage any changes efficiently in
smaller chunks ... like the old Proverb
goes: If the dog hadn't stopped to lift
his leg, he'd have caught the rabbit.
There are other design options
including your suggestion, as well as
creating a "one chip wonder." There is
no one correct solution or answer to
this design challenge.
August 2013 9