JR.S00 1

Lecture 15: Busses and Networking (1)

Prof. Jan Rabaey

Computer Science 252, Spring 2000

Based on slides from Dave Patterson, John KubiatowiczBill Dally, and Sonics, Inc

JR.S00 2

A Communication-Centric World

• Computation is getting distributed …– Internet, WAN, LAN, BodyLAN, Home Networks,

Microprocessor Peripherals, Processor-Memory Interface, System-on-a-Chip

• Efficient Networking and Communication is Crucial

• The System-on-a-Chip implies the Network-on-a-Chip

• In Next Set of Lectures: – Busses and Networks

– But more importantly, the impact of integration

JR.S00 3

A Bus Is:

• shared communication link

• single set of wires used to connect multiple subsystems

• A Bus is also a fundamental tool for composing large, complex systems

– systematic means of abstraction

Control

Datapath

Memory

ProcessorInput

Output

What is a bus?

JR.S00 4

Busses

JR.S00 5

• Versatility:– New devices can be added easily– Peripherals can be moved between computer

systems that use the same bus standard

• Low Cost:– A single set of wires is shared in multiple ways

MemoryProcesser

I/O Device

I/O Device

I/O Device

Advantages of Buses

JR.S00 6

• It creates a communication bottleneck– The bandwidth of that bus can limit the maximum I/O throughput

• The maximum bus speed is largely limited by:– The length of the bus– The number of devices on the bus– The need to support a range of devices with:

» Widely varying latencies » Widely varying data transfer rates

MemoryProcessor

I/O Device

I/O Device

I/O Device

Disadvantage of Buses

JR.S00 7

• Control lines:– Signal requests and acknowledgments

– Indicate what type of information is on the data lines

• Data lines carry information between the source and the destination:

– Data and Addresses

– Complex commands

Data Lines

Control Lines

General Organization of a Bus

JR.S00 8

• A bus transaction includes two parts:– Issuing the command (and address) – request

– Transferring the data – action

• Master is the one who starts the bus transaction by:– issuing the command (and address)

• Slave is the one who responds to the address by:– Sending data to the master if the master ask for data

– Receiving data from the master if the master wants to send data

BusMaster

BusSlave

Master issues command

Data can go either way

Master versus Slave

JR.S00 9

Types of Busses• Processor-Memory Bus (design specific)

– Short and high speed

– Only need to match the memory system

» Maximize memory-to-processor bandwidth

– Connects directly to the processor

– Optimized for cache block transfers

• I/O Bus (industry standard)– Usually is lengthy and slower

– Need to match a wide range of I/O devices

– Connects to the processor-memory bus or backplane bus

• Backplane Bus (standard or proprietary)– Backplane: an interconnection structure within the chassis

– Allow processors, memory, and I/O devices to coexist

– Cost advantage: one bus for all components

JR.S00 10

Processor/MemoryBus

PCI Bus

I/O Busses

Example: Pentium System Organization

JR.S00 11

A Computer System with One Bus: Backplane Bus

• A single bus (the backplane bus) is used for:– Processor to memory communication

– Communication between I/O devices and memory

• Advantages: Simple and low cost

• Disadvantages: slow and the bus can become a major bottleneck

• Example: IBM PC - AT

Processor Memory

I/O Devices

Backplane Bus

JR.S00 12

A Two-Bus System

• I/O buses tap into the processor-memory bus via bus adaptors:

– Processor-memory bus: mainly for processor-memory traffic– I/O buses: provide expansion slots for I/O devices

• Apple Macintosh-II– NuBus: Processor, memory, and a few selected I/O devices– SCCI Bus: the rest of the I/O devices

Processor Memory

I/OBus

Processor Memory Bus

BusAdaptor

BusAdaptor

BusAdaptor

I/OBus

I/OBus

JR.S00 13

A Three-Bus System

• A small number of backplane buses tap into the processor-memory bus

– Processor-memory bus is only used for processor-memory traffic– I/O buses are connected to the backplane bus

• Advantage: loading on the processor bus is greatly reduced

Processor Memory

Processor Memory Bus

BusAdaptor

BusAdaptor

BusAdaptor

I/O BusBackplane Bus

I/O Bus

JR.S00 14

North/South Bridge architectures: separate busses

• Separate sets of pins for different functions– Memory bus – Caches– Graphics bus (for fast frame buffer)– I/O busses are connected to the backplane bus

• Advantage: – Busses can run at different speeds– Much less overall loading!

Processor MemoryProcessor Memory Bus

BusAdaptor

BusAdaptor

I/O BusBackplane Bus

I/O Bus

“backsidecache”

JR.S00 15

Bunch of Wires

Physical / Mechanical Characteristics – the connectors

Electrical Specification

Timing and Signaling Specification

Transaction Protocol

What defines a bus?

JR.S00 16

• Synchronous Bus:– Includes a clock in the control lines

– A fixed protocol for communication that is relative to the clock

– Advantage: involves very little logic and can run very fast

– Disadvantages:

» Every device on the bus must run at the same clock rate

» To avoid clock skew, they cannot be long if they are fast

• Asynchronous Bus:– It is not clocked

– It can accommodate a wide range of devices

– It can be lengthened without worrying about clock skew

– It requires a handshaking protocol

Synchronous and Asynchronous Bus

JR.S00 17

° ° °Master Slave

Control LinesAddress LinesData Lines

Bus Master: has ability to control the bus, initiates transaction

Bus Slave: module activated by the transaction

Bus Communication Protocol: specification of sequence of events and timing requirements in transferring information.

Asynchronous Bus Transfers: control lines (req, ack) serve to orchestrate sequencing.

Synchronous Bus Transfers: sequence relative to common clock.

Busses so far

JR.S00 18

Bus Transaction

• Arbitration: Who gets the bus

• Request: What do we want to do

• Action: What happens in response

JR.S00 19

• One of the most important issues in bus design:– How is the bus reserved by a device that wishes to use it?

• Chaos is avoided by a master-slave arrangement:– Only the bus master can control access to the bus:

It initiates and controls all bus requests

– A slave responds to read and write requests

• The simplest system:– Processor is the only bus master

– All bus requests must be controlled by the processor

– Major drawback: the processor is involved in every transaction

BusMaster

BusSlave

Control: Master initiates requests

Data can go either way

Arbitration: Obtaining Access to the Bus

JR.S00 20

Multiple Potential Bus Masters: the Need for Arbitration

• Bus arbitration scheme:– A bus master wanting to use the bus asserts the bus request– A bus master cannot use the bus until its request is granted– A bus master must signal to the arbiter the end of the bus utilization

• Bus arbitration schemes usually try to balance two factors:

– Bus priority: the highest priority device should be serviced first– Fairness: Even the lowest priority device should never

be completely locked out from the bus

• Bus arbitration schemes can be divided into four broad classes:

– Daisy chain arbitration– Centralized, parallel arbitration– Distributed arbitration by self-selection: each device wanting the bus places a code

indicating its identity on the bus.– Distributed arbitration by collision detection:

Each device just “goes for it”. Problems found after the fact.

JR.S00 21

The Daisy Chain Bus

Arbitrations Scheme

• Advantage: simple

• Disadvantages:– Cannot assure fairness:

A low-priority device may be locked out indefinitely

– The use of the daisy chain grant signal also limits the bus speed

BusArbiter

Device 1HighestPriority

Device NLowestPriority

Device 2

Grant Grant Grant

Release

Request

wired-OR

JR.S00 22

• Used in essentially all processor-memory busses and in high-speed I/O busses

BusArbiter

Device 1 Device NDevice 2

Grant Req

Centralized Parallel Arbitration

JR.S00 23

• All agents operate synchronously

• All can source / sink data at same rate

• => simple protocol– just manage the source and target

Simplest bus paradigm

JR.S00 24

• Even memory busses are more complex than this– memory (slave) may take time to respond

– it may need to control data rate

BReq

BG

Cmd+AddrR/WAddress

Data1 Data2Data

Simple Synchronous Protocol

JR.S00 25

• Slave indicates when it is prepared for data xfer

• Actual transfer goes at bus rate

BReq

BG

Cmd+AddrR/WAddress

Data1 Data2Data Data1

Wait

Typical Synchronous Protocol

JR.S00 26

• Separate versus multiplexed address and data lines:– Address and data can be transmitted in one bus cycle

if separate address and data lines are available

– Cost: (a) more bus lines, (b) increased complexity

• Data bus width:– By increasing the width of the data bus, transfers of multiple words require fewer

bus cycles

– Example: SPARCstation 20’s memory bus is 128 bit wide

– Cost: more bus lines

• Block transfers:– Allow the bus to transfer multiple words in back-to-back bus cycles

– Only one address needs to be sent at the beginning

– The bus is not released until the last word is transferred

– Cost: (a) increased complexity (b) decreased response time for request

Increasing the Bus Bandwidth

JR.S00 27

• Overlapped arbitration– perform arbitration for next transaction during current transaction

• Bus parking– master holds onto bus and performs multiple transactions as long as no

other master makes request

• Overlapped address / data phases– requires one of the above techniques

• Split-phase (or packet switched) bus– completely separate address and data phases

– arbitrate separately for each

– address phase yield a tag which is matched with data phase

• ”All of the above” in most modern memory buses

Increasing Transaction Rate on Multimaster Bus

JR.S00 28

Bus MBus Summit Challenge XDBus

Originator Sun HP SGI Sun

Clock Rate (MHz) 40 60 48 66

Address lines 36 48 40 muxed

Data lines 64 128 256 144 (parity)

Data Sizes (bits) 256 512 1024 512

Clocks/transfer 4 5 4?

Peak (MB/s) 320(80) 960 1200 1056

Master Multi Multi Multi Multi

Arbitration Central Central Central Central

Slots 16 9 10

Busses/system 1 1 1 2

Length 13 inches 12? inches 17 inches

1993 CPU- Memory Bus Survey

JR.S00 29

Address

Data

Read

Req

Ack

Master Asserts Address

Master Asserts Data

Next Address

Write Transaction

t0 t1 t2 t3 t4 t5

• t0 : Master has obtained control and asserts address, direction, data

• Waits a specified amount of time for slaves to decode target

• t1: Master asserts request line

• t2: Slave asserts ack, indicating data received

• t3: Master releases req

• t4: Slave releases ack

Asynchronous Handshake (4-phase)

JR.S00 30

Address

Data

Read

Req

Ack

Master Asserts Address Next Address

t0 t1 t2 t3 t4 t5

• t0 : Master has obtained control and asserts address, direction, data

• Waits a specified amount of time for slaves to decode target\

• t1: Master asserts request line

• t2: Slave asserts ack, indicating ready to transmit data

• t3: Master releases req, data received

• t4: Slave releases ack

Read Transaction

Slave Data

JR.S00 31

Bus SBus TurboChannel MicroChannel PCI

Originator Sun DEC IBM Intel

Clock Rate (MHz) 16-25 12.5-25 async 33

Addressing Virtual Physical Physical Physical

Data Sizes (bits) 8,16,32 8,16,24,32 8,16,24,32,648,16,24,32,64

Master Multi Single Multi Multi

Arbitration Central Central Central Central

32 bit read (MB/s) 33 25 20 33

Peak (MB/s) 89 84 75 111 (222)

Max Power (W) 16 26 13 25

1993 Backplane/IO Bus Survey

JR.S00 32

• Examples– graphics

– fast networks

• Limited number of devices

• Data transfer bursts at full rate

• DMA transfers important– small controller spools stream of bytes to or from memory

• Either side may need to squelch transfer– buffers fill up

High Speed I/O Bus

JR.S00 33

• All signals sampled on rising edge

• Centralized Parallel Arbitration– overlapped with previous transaction

• All transfers are (unlimited) bursts

• Address phase starts by asserting FRAME#

• Next cycle “initiator” asserts cmd and address

• Data transfers happen on when– IRDY# asserted by master when ready to transfer data

– TRDY# asserted by target when ready to transfer data

– transfer when both asserted on rising edge

• FRAME# deasserted when master intends to complete only one more data transfer

PCI Read/Write Transactions

JR.S00 34

– Turn-around cycle on any signal driven by more than one agent

PCI Read Transaction

JR.S00 35

PCI Write Transaction

JR.S00 36

The System-on-a-Chip Nightmare

Bridge

DMA CPU DSP

MemCtrl.

MPEG

CI O O

System Bus

PeripheralBus

Control Wires

Custom Interfaces

The “Board-on-a-Chip”Approach

JR.S00 37

Sonics SOC Integration Architecture

SiliconBackplaneAgent™

Open Core Protocol™

SiliconBackplane™

(patented)

MultiChipBackplane™{

DSP MPEGCPUDMA

C MEM I O

JR.S00 38

Open Core Protocol Goals• Bus Independent

• Scalable

• Configurable

• Synthesis/Timing Analysis Friendly

• Encompass entire core/system interface needs (data, control, and test flows)

JR.S00 39

Data, Control, and Test Flows• Data Flow

– Signals and protocols associated with moving data

– Includes address, data, handshaking, etc.

– Similar to services provided by traditional computer buses

• Control Flow– Signals and protocols associated with non-data communication

– Sideband - not synchronized to data flow (out of band)

– Examples include interrupts, high-level flow control, etc.

• Test Flow– Signals and protocols related to debug and manufacturing test

JR.S00 40

OCP Overview• Point-to-point, uni-directional, synchronous

– easy physical implementation

• Master/Slave, request/response– well-defined, simple roles

• Extensions– added functionality to support cores with more complex interface

requirements

• Configurability– pay only for the features needed for a given core

JR.S00 41

Master vs. Slave

IP CoreIP CoreIP Core

On-Chip Bus

Slave

Master SlaveSlave

Slave

Master

Master MasterInitiator Target

Open CoreProtocol Request

Response

JR.S00 42

Basic OCPM

aste

r MCmd [3]

MAddr [N]

MData [N]

SResp [3]

SData [N]

Clk

SCmdAccept

Read:Command, AddressCommand AcceptResponse, Data

Write (posted):Command, Address, DataCommand Accept

MCmd, MAddr

SCmdAccept

SResp, SData

MCmd, Maddr, MData

SCmdAccept

Sla

ve

JR.S00 43

Protocol Phases• Request Phase (begins Transfer)

– Master presents request (command, address, etc.) to Slave

• Response Phase (ends Transfer)– Slave presents response (success/fail, read data) to Master

– Only available for read transfers (posted write model)

• Datahandshake Phase (Optional)– Allows pipelining request ahead of write data

– Only available for write transfers

• Phase ordering– Request -> Datahandshake -> Response

JR.S00 44

OCP Extensions• Simple Extensions

– Byte Enables

– Bursts

– Flow Control

– Data Handshake

• Complex Extensions– Threads and Connections

• Sideband Signals

JR.S00 45

The Backplane: Why Not Use a Computer Bus?

IPCore

IPCore

IPCore

IPCore

ComputerBus

Transmit FIFO Receive FIFO

Time

Data

Arbiter Address

• Expensive to decouple• Not designed for real-time

JR.S00 46

Communication Buses Decouple and Guarantee Real Time

IPCore

IPCore

IPCore

IPCore

CommunicationsBus

Transmit FIFO Receive FIFO

Time

Data

TDMA TDMA

• Connections are expensive• Poor read latency

JR.S00 47

From Communications• Efficient BW decoupling• Guaranteed BW & latency• Side-band signaling

SiliconBackplane™ Employs Best of BothFrom Computing• Address-based selection• Write and read transfers• Pipelining

DSP MPEGCPUDMA

C MEM I O

JR.S00 48

Guaranteed Bandwidth Arbitration

• Independent arbitration for every cycle includes two phases:

- Distributed TDMA- Round robin

• Provides fine control over system bandwidth

CurrentSlot

Arbitration

Command

JR.S00 49

Guaranteed Latency

• Fixed latency between command/address and data/response phases

• Matches pipelined CPU model ensuring high performance access to on-chip resources

• Pipelined data routed through SiliconBackplane™

• Latency re-programmable in software

• Variable-latency blocks do not tie up the SiliconBackplane

JR.S00 51

Integrated Signaling Mechanism

• Dedicated SiliconBackplane™ wires (Flags) support:

– Bus-style out-of-band signaling (interrupts)

– Point-to-point communications (flow control)

– Dynamic point-to-point (retry mechanism)

• Same design flow, timing, flexibility as address/data portion of SonicsIA™

JR.S00 52

MultiChip Backplane

SiliconBackplane

MultiChip Backplane™ ExtendsSonicsIA™ Between Chips

CPU-Based ASSP

ASSP

FPGA

Seamless integration of protocols

JR.S00 53

Validation / Test

• SiliconBackplane™ highly visible for test

– All subsystems communicate through SiliconBackplane

• Test Interfaces:– MultiChip Backplane: 100’s MB/sec.

– ServiceAgent: Scan-based

• Each subsystem can be tested/validated stand-alone

TestVectors

TestVectors

MultiChipBackplane™

JR.S00 54

Summary

• Busses are an important technique for building large-scale systems

– Their speed is critically dependent on factors such as length, number of devices, etc.

– Critically limited by capacitance– Tricks: esoteric drive technology such as GTL

• Important terminology:– Master: The device that can initiate new transactions– Slaves: Devices that respond to the master

• Two types of bus timing:– Synchronous: bus includes clock– Asynchronous: no clock, just REQ/ACK strobing

• System-on-a-Chip approach invites new solutions

– Well-defined and clear communication protocols– Physical layer hidden to designer