Nuts and Volts - August 2008

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