PICAXE PRIMER
to the 5.0-volt supply, the 3.3-volt
supply consistently resulted in lower
supply current at each speed; and,
disabling BOD further reduced the
supply current at every speed. So,
when we get to our battery-powered
data-logger project, we’ll definitely
use the slowest possible speed (but
not 31K, unless we can resolve the
problem described later in this
installment).
If you would like to make your
own supply current measurements,
a schematic of the circuit I used is
presented in Figure 4. A close-up
photo of the breadboard is shown
in Figure 5 and Figure 6 is a photo
of the entire setup including the
multimeter I used for the current
measurements. In the schematic,
I am assuming you are using a
separate programming adapter
board, so I have not included that
portion of the circuitry. Also, note
the pull-down resistors on all of the
unused input pins. Without them, the
inputs would float and possibly
change from a high state to a low
state due to random static charges.
Whenever a CMOS input
changes state, it consumes power,
so to avoid any unnecessary power
consumption I grounded all the
unused inputs. This is a good idea in
any circuit (even if you don’t care
about the power consumption)
because it can also reduce
unnecessary noise in the supply lines.
The push-button switch on input 3
allowed me to keep the 28X1 at its
current speed while I wrote down
the supply current measurement;
pressing the button advanced the
28X1 to its next speed setting and
the next supply current measurement
(except at 31K — we’ll get to that
problem shortly). The software I used
is too long to include in the Primer,
but it’s available for download at the
N&V website ( www.nutsvolts.com);
look for “Current Test.bas.”
The 28X1’s powerful and flexible
resonator features can sometimes
result in a minor inconvenience that
you should keep in mind. Whenever
you initiate a program download to a
PICAXE chip, the Programming Editor
software requires that the target
■ FIGURE 4. 28X1 supply
current circuit schematic.
■ FIGURE 5. Close-up
of breadboard layout.
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