Integrating a sensor to arduino? Answered

Ahhh. So you want to start learning how to implement a sensor from scratch without having to rely on libraries.

Well. The big thing is that your Arduino is not simply a Arduino. it is in fact an AVR device with an entire "arduino library" ontop of it. So if you want to start learning how to implement things like ADC(analog reading), SPI-sensors or TWI(aka I2C) sensors you should be looking for tutorials on "AVR TWI / I2C" and such. Atleast this is what you should look for if wanting to go completely from scratch. normally if using an arduino i would suggest to keep to the "Wire" library that gives you the simple TWI/I2C functions to use or the SPI library that gives...well..SPI functions.

Of the 2 protocols SPI is the most straightforward and easiest to learn and the ADXL345 should have the option of both. so go with SPI

For a good beginners guide Sparkfun has a tutorial on using the sensor with the SPI library. not as in depth as starting from scratch, but this would still be a good starting point to begin with:

https://www.sparkfun.com/tutorials/240

If you cross-reference how the Code accesses registers and look them up on the datasheet you should get an idea at how the arduino requests and receives data.

http://www.picaxe.com/docs/picaxe_manual3.pdf

Is not for the Arduino specifically BUT the information applies to most PIC type microprocessors.