CSS 372 - Lecture 2CSS 372 - 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

++ clock(s), power(s), and ground(SS)

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 ConventionsTiming 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