ASN Lab Exercise: Frequently Asked Questions

This page holds a collection of hints and frequently asked questions for the lab exercise of the Ad Hoc and Sensor Networks lecture.

Exercise 1

• Unsigned vs. signed integers

Sensor readings from the light sensors are return as unsigned 16-bit integers:

event void Sensor.readDone(error_t result, uint16_t value) {
(...)
}

Thus, the value variable can only be between 0 and 65535(=0xFFFF). Please be ware that arithmetic operations with unsigned integers (e.g. uint16_t) will always return an unsigned integer.

So, in the following comparison an integer underflow will occur and the result will be true:

uint16_t a = 1000;
uint16_t b = 1001;

uint16_t c = a - b;	// c = 65535

if (c>0) {
  // do something
}

Exercise 2

• Limited beam angle of infrared LED/photodiode

The infrared LED on the sender node and the photodiode on the receiver have only a limited angle of emission/detection. Please make sure that the LED and the photodiode point towards each other.