January 2006, Issue 186

Electronic Scarecrow

MAIN ROUTINE

The flowchart posted on the Circuit Cellar FTP site shows the remote unit’s main routine. After initialization and sensor tests, the unit remains in the main loop, which reads the sensors, checks for a switch press, checks for incoming reply packets, and then sends a request packet if necessary.

In order to minimize power consumption, sensors are read only if they are installed and working properly. During initialization, the sensor_init routine executes a presence detection algorithm for each sensor. For the accelerometers, the algorithm uses their built-in hardware self-test functions. The pressure sensor’s external amplifier has a similar self-test function even though it doesn’t actually test the sensor. Because it uses the same input as the MMA1260D Z-axis accelerometer, the self-test voltage level is sufficiently different so the software can tell which one is connected. The MC33794 e-field sensor’s test looks at the reference input levels, while the switch expander test reads six of the input bits that are hard-wired to an alternating high/low pattern.

After all the tests, pull-ups are enabled on all of the MC9S08GT16 microcontroller’s analog input pins that didn’t have valid sensor signals. I didn’t want to set them to outputs in case a sensor was present but failed its test. The next step is to read and store the zero offsets for all the sensors. Lastly, a final test on each sensor ensures that its value is within a valid input range. Intended to check for broken wires, these final tests are repeated each time a ping packet is sent.

The main loop begins by reading all the connected sensors. The accelerometers should be checked every 50 ms to avoid missing an event. The others can be read every 500 ms. The switch expander doesn’t need to be read because it interrupts the MC9S08GT16 microcontroller whenever its status changes. I wanted to be able to change the sensor thresholds without recompiling the code, so I put them in a structure that’s loaded at a fixed address at the beginning of the code. (Refer to struct THRESHOLDS in the Periph.h file posted on the Circuit Cellar FTP site.) This made them easy to patch.