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Addressing the System-on-a-Chip
Interconnect Woes Through
Communication-Based Design
N. Vinay Krishnan
EE249 Class Presentation
System-on-Chips
 Not as easy as Fish’n’Chips
 Formal approach-Platform based Design
 Orthogonalization of concerns


Keep the computation of the IP cores distinct from the
communication between them
Helpful for re-use of the core designs
Communication Design Woes
 Predictability

Reduce design iterations
 Wiring Delay

Signals taking multiple clock cycles to reach
 Increasing Power Dissipation of Interconnect
 Diverse Interconnect Architectures

More blocks to connect. More worries
Addressing the Woes
 Design has to begin at a higher layer of
abstraction than the RTL level
 Paper introduces the idea –
Adopt the OSI Standard!
 Called Network-On-Chip methodology
OSI Model – An overview
 Physical

Signal voltages, bus widths, pulse shape
 Data Link

Arbitration, MAC
 Network

Packet routing
 Transport

Segmentation, flow control
OSI Model-An Overview-2
 Session

End-to-End connections
 Presentation

Data format conversion
 Application

Application
NOC E.g.-Pleiades Platform
Pleiades Platform-Maia
Processor
 Heterogeneous collection of logic units, like
ALU’s, memories, processors, called
satellites
 Connected to each other and main controller
using a reconfigurable interconnect
 Asynchronous communication
 Reduced swing signaling (LVDS)
Metropolis Approach
 Communication design as the declaration of
a set of constraints
 Communication and computation
independently formulated constraints
 Adapters introduced to overcome constraints
mismatches
Metropolis Adapters
 Behavioural Adapters
for Segmenting and Bit
Rate matching
 Channel Adapters for
matching delay,
throughput, reliability,
etc…
Metropolis e.g.-Intercom
 Embedded Microprocessor and custom logic
connected through a chip-wide silicon
backplane
 Cadence VCC Design environment used

Models system components as a network of
asynchronously communicating finite state
machines
Intercom-2
 Behavioral adapters used :

Packet segmenting to break voice stream
MAC TDMA-Round Robin token passing
 Channel Adapters used :

Memory mapped addressing scheme.
Buffer to queue data sending
MESCAL
 Seeks to provide tools to formally specify
protocol stacks for on-chip communication
architectures
 Ptolemy modeling environment used to
provide high level description language of
the Stack model
Family of Architectures
 A MESCAL communication architecture can be
described with a graph


Vertices are Communicators (PEs, memories, I/Os)
Edges are Communication Channels
External
I/O
External
Memory
PE
PE
PE
PE
PE
PE
On-Chip
Memory
Communicators
 A Communicator is a system component paired with a
Communication Assist (CA) Co-Processor
 The CA can range in complexity from a simple FSM to a
fully programmable processor
Local
Memory
Cache
Communication
Assist
PE
Comm
Channels
Communication Assists
 A Communicator may also be a CA by itself
 This is useful for building:


Bridges between communications channels
Programmable switch nodes
Data Table
Communication
Assist
Multiple Channel
Implementations
Communication Assists
 The CA provides an interface between the system
component and one or more communication
channels
Session
 OSI
stack model:
Programmer’s model interface
Transport
SW Network protocols
Network
Data Link
Physical
HW
Queues,
buffers, arbitration
Channel electrical interfaces
 The CA provides the minimum set of features necessary to utilize
the communication channels
Communication Assists
 Lower levels designed in hardware to suit
architecture of application
 Programmable higher levels can be changed
to suit later modifications to application
(programmable platforms, here we
come…)