CSS Lecture 2 Chapter 3 – Connecting Computer Components with Buses Bus Structures Synchronous, Asynchronous Typical Bus Signals Two level, Tri-state, Wired Or Hierarchical Bus Organizations PCI Bus Example

What is a Bus? A communication pathway connecting two or more devices (Computers, Components, I/O, …) Usually broadcast Often grouped –A number of channels in one bus –e.g. 32 bit data bus is 32 separate single bit channels Power lines may not be shown

What do Buses look like? –Parallel lines on circuit boards –Ribbon cables –Strip connectors on mother boards –Sets of wires

Types of Buses Synchronous Asynchronous (Hand Shaking) Serial (Twisted pair, Coaxial Cable,..) Parallel (Ribbon Cable,

Types of Buses Dedicated –Separate data & address lines Multiplexed –Shared lines –Address valid or data valid control line –Advantage - fewer lines –Disadvantages More complex control Ultimate performance

Physical Considerations for Buses Media (voltage, optic) Signal levels – the higher, the more immune to noise Noise Absorption – wires can pick up noise from neighboring wires Noise Generation – wires can be antennas Length Creates Delay ( reduces Bandwidth) Consumes Power Creates reflections – (Terminations become more critical)

Logic Threshold Voltage Levels

Signal Scheme Alternatives Totempole - High or Low output level Line always at a 1 level or 0 level Open collector, open drain, wired-or Line is nominally at a 1 level or 0 level – line is “pulled” to non-nominal level Tristate Has third state – open Differential Uses a pair of lines – the level is the difference of signals on the two lines.

Bus Challenges Lots of devices on one bus leads to: –Propagation delays Long data paths mean that co-ordination of bus use can adversely affect performance –Traffic congestion Too many devices communicating reduces bandwidth Alternative - Systems use multiple buses

Simple Computer Bus + S + clock(s), power(s), and ground (S) Notes: 1) Bus lines need to be properly terminated 2) Power lines are to furnish reference voltage, not power

Adding an Expansion Bus

Hierarchical Bus Structure

Bus Arbitration More than one module may need to control the bus e.g. CPUs and DMA controller Only one module may control the bus at one time Arbitration may be centralised or distributed

Centralised or Distributed Arbitration Centralised –Single hardware device controlling bus access Bus Controller Arbiter –May be part of CPU or separate Distributed –More than one module may claim the bus Need control logic on all these modules

Timing Co-ordination of events on bus Synchronous –Events determined by clock cycles –Control Bus includes clock line(s) –A single 1-0 is a bus cycle (or phase) –All devices can read clock line –Likely sync on leading edge –Likely a single cycle for an event (may be multiple clock cycles or phases)

Timing Diagram Conventions

Synchronous Timing Diagram

Asynchronous Timing – Read Diagram

Asynchronous Timing – Write Diagram

Example - PCI Bus Peripheral Component Interconnection Intel released to public domain 32 or 64 bit 50 lines

Typical PCI Bus Usage

Multiple PCI Bus Configuration

PCI Commands Transaction between initiator (master) and target Master claims bus Determine type of transaction –e.g. I/O read/write Address phase One or more data phases

PCI Read Timing Diagram

PCI Bus Arbiter

PCI Bus Arbitration Timing