Issue 99, October 1998
X-Y Graphing Data Logger

MAIN CODE

The software copes with communication protocols over a serial line, combining sleep mode and watchdog techniques to achieve minimal power consumption, low-power ADC driving, and data conversion to the Casio format.

Complete packet templates are stored in data tables in program memory. At this point, I found RETLW invaluable.

Only segments with variable values are replaced by real data read from the ADC in real time. Most parts of the input packet are ignored, and only the variable name is stored to select the right ADC input. When issuing Receive(X), you can specify any other variable name to select a different input.

Even the simpler communication protocol must deal with errors and interruptions. To keep program overhead low, I set up the watchdog timer to reset the device if it waits for an answer from the calculator for more than 2 s.

Once reset (and for most of the time), the device is left in sleep mode. It’s awakened by another WDT timeout or by receiving a character.

In the former case, the device goes back into sleep mode. In the latter case, the LED flashes, the ADC awakens, and the communication flow restarts.

When the micro sleeps, the ADC is left in full power-down mode. The shutdown pin is a three-level selection input, and the MAX186 sleeps with that pin at 0.

It is awakened by putting the pin in a high impedance state, which is done long before the conversion starts to let the voltage reference capacitor charge completely.

The MAX186 input can be reconfigured to bipolar input mode (with input spanning from –Vref/2 to +Vref/2) or pseudodifferential input mode by changing the command word that is ORed in the ReadADC code segment. Figure 3 shows the signals involved and how the control word is made.

The Casio needs variables in BCD format with separated exponent digits. The most significant digits come first. The BCD conversion and the bit-banging serial-port routines are derived from public-domain Microchip libraries.