$5000, HP Mixed Signal Oscilloscope,$500 for using PIC16F84
XY LCD Graphing Data Logger—Alberto Ricci Bitti, Lugo, Italy
Do you need sophisticated graphical display of data immediately regardless of whether you’re in the lab, on the production line, or in the field? If so, you need to check out the XY LCD Graphing Data Logger.
Alberto brings us a hand-held data logger capable of displaying data graphically on a 64
´ 128 LCD screen and of performing analytical (e.g., integration and derivation) and statistical (e.g., linear, exponential, polynomial regressions, mean, and deviation) math processing. Its voltage at up to eight inputs is read with 1-mV resolution in the 0–4096-mV range through a 12-bit AD converter.How does he do it? By using a Casio graphing calculator that sells for about the same amount as an LCD display and interfaces through a miniature jack, a PIC16F84, and a MAX186 ADC. The software manages communication protocol over a serial line while also combining sleep mode and watchdog techniques to achieve minimal power consumption, low-power ADC operation, and data conversion to the Casio format.
But first, let’s talk about why he chose the PIC16F84. The chip supports data tables as long as the program memory thanks to its RETLW instruction. It is cheap and powerful enough to handle serial communications entirely in software.
The next most important chip was the MAX186 A/D converter. He realized that to get stable readings of the least significant bits, he had to keep the digital ground and analog ground separate, joined only at one point near the regulator. The MAX186’s good pin layout helped divide input lines from data lines, making a stable reading easily obtainable.
To maximize battery life, Alberto used a low-voltage power regulator. By putting the micro and ADC in sleep mode, he found that the unit only requires 50 ľA. During serial communication with the calculator, power needs jump to 1.5 mA, ADC conversion requires 1.5 mA for a few milliseconds and when the LED flashes.
The entire circuit is assembled using a prototyping board that fits inside a small plastic box as large as the calculator. While operating, the box is affixed to the bottom of the Casio calculator via removable biadhesive strips. This arrangement keeps the calculator stable and on a comfortable slope.
On the software end of things, he discovered that the Casio used a 9600-bps, half-duplex serial stream with single-character ACK/NACK handshaking terminated by checksums. The logger collects the data and passes tables of variable information to the Casio which then performs whatever mathematical analyses are necessary and graphically illustrates the functions.
Is the XY LCD Graphing Data Logger a success? You bet it is. It rings in as 20 times cheaper than any comparable PC-based solution, its battery life is about 20 times longer (50 ľA while not sampling), and the unit is extremely compact and lightweight (290 g including batteries), making it suitable for mobile use.
