Issue 140 March 2002
Spy-Size Event Logger

Event-Driven

Often the data you’ll need to sample and save will be logical (1-bit data). This might be the position of a door or whether the temperature in a room is above or below the thermostat’s set point. Sample time is unnecessary with event data logging because the sampling times are not based on the maximum frequency of the data. Instead sampling is based on the event of interest, which can occur at any point in time.

The drawback to event-driven sampling is that some kind of time reference (time stamp) must be saved to indicate when the event occurred. This time stamp allows each sample to be accurately referenced to the start of data logging series. Although time stamp data will gobble up available logging memory at an increased rate, the rate between samples can be extremely long with the result of actually using less memory.

Although a 1-bit A/D converter (comparator) is an example of a signal that could be sampled and saved in Event Driven mode, the additional time stamping overhead may make it a more reasonable candidate for Periodic mode. Therefore, you’ll need to analyze data for each application to determine the most effective mode from both an accuracy and memory availability standpoint.