SoC Bus Evolution

A SoC contains a large number of re-usable IP (intellectual property) blocks. Blocks include CPUs, RAMs and standard I/O devices such as USB, Ethernet and UART. The SoC assembly process is basically a decision about which IP blocks to deploy and then a second decision about how they are to be interconnected. The traditional way to connect such blocks is to use a so-called `bus', but the term has evolved in meaning and is now pretty much a misnomer.

The traditional bus, dating from the 1970's, is the tri-state bus. An address bus is controlled by a single bus master and a bi-directional data bus conveys loaded/read data from the device to the master or stored/written data from the master to the device. Today we still use the term `bus', but it denotes both an IP interconnection standard found on the IP blocks and a collection of decoders, multiplexors and arbiters that convey the data between the blocks.