Circuit Analysis

Before you layout a circuit, it's generally a good idea to understand what is happening in your schematic.  And before you lay out a schematic, it's generally a good idea to understand what is happening in your block diagram.

Hard Way Hughes"This book is a bit technical.  If you are a beginner, it's okay that you don't understand everything that follows -- your first project will still be successful.  Try to pick up as much as you can, and ask questions in the forums!"

3. Data Bus Overview

Communication Protocols

Serial data moves through a circuit from source to destination as a series of changing voltages.  The sender(s) initiates the change on the signal line and the receiver detects that the potential has changed.

There are a variety of protocols that are in common use in the industry: ATA, SCSI, CAN, I²C, SPI, UART,  RS232, RS485, USB, Ethernet, Microwire, 1-wire, etc...  But commercial sensors predominantly use I²C and SPI, so those are the protocols we chose for this project.


Synchronous communication requires a minimum of two signal lines: one line transfers data, the other transfers the clock signal.  This type of communication is used in the SPI and I²C data buses in our circuit.


Asynchronous communication requires a minimum of one signal line.  However, the clock frequency must be agreed to by both the sender and receiver before communication can be successful.  Our circuit's UART bus that connects the SAMD11 and the ATMEGA328P is an example of an asynchronous signal line.

The next several chapters will discuss the I2C, SPI, UART, and USB busses in greater detail.