Pt.1: exploring the I²C bus

Basically, this datalogger consists of four input channels that can be sampled and stored (logged) at user-defined intervals. One channel is dedicated for use with a digital temperature sensor, whereas the remaining three can be used as analog or digital inputs. Logging can be carried out at regularly spaced intervals (typically one minute to several hours), or an optional real-time clock (RTC) chip can be added to ensure accurate logging intervals over longer periods (once a week, once a month, etc). Use of the RTC will be covered in next month's article.

Data is saved in an onboard EEPROM memory chip. If desired, this memory chip can be upgraded for increased memory capacity. An optional memory expansion board can also be used to greatly increase memory capacity.

Once the "mission" is complete, data can be uploaded for analysis on a computer. Data can also be displayed (at the time of logging) on an optional liquid crystal display (LCD) if desired.

The PICAXE-18X datalogger makes extensive use of the I²C bus for communication between ICs. Therefore, before proceeding any further let's take a detailed look at the principles of the I²C bus.

Main Features Low-cost design Four logging channels One dedicated digital temperature sensor channel EEPROM data storage (easily expandable) PICAXE micro means simple programming 3 x AA battery operation, low power consumption Small footprint (approx. same size as 3 x AA cells) Optional real-time clock with lithium battery backup