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1Combining the strengths of UMIST andThe Victoria University of Manchester
Asynchronous Signal
Processing Systems
Linda Brackenbury
APT GROUP, Computer Science
University of Manchester
2Combining the strengths of UMIST andThe Victoria University of Manchester
AgendaAgenda• Why asynchronous?
• Applications suited to asynchronous
• Design examples– DSP design– Viterbi decoder
• Future work
• What have we learnt?
3Combining the strengths of UMIST andThe Victoria University of Manchester
System TimingSystem Timing
• Synchronous– uses global clock – any state changes occur on clock edge– system states predictable so good tools
• Asynchronous– uses events to control timing– timing is more unpredictable– tool support not as good
4Combining the strengths of UMIST andThe Victoria University of Manchester
Why Asynchronous?Why Asynchronous?
• No clock generation or distribution– timing uses local handshake signals
• Power only consumed when doing
useful work
• No overhead between idle and active
• Low EMI – switching is spread
5Combining the strengths of UMIST andThe Victoria University of Manchester
ApplicationsApplications• Async - no help to some applications
– power/performance at full activity similar for synchronous and asynchronous!
• Async good for portable systems– battery size and lifetime is important– workload is highly variable– lots of idle time– low EMI requirement
6Combining the strengths of UMIST andThe Victoria University of Manchester
Low Power DSPLow Power DSP• GSM chipsets are typically based on
microprocessor + DSP
• DSP performs intensive calculations
• Challenge is to meet required
throughput without excessive power– throughput met with parallelism– area traded for increased speed
7Combining the strengths of UMIST andThe Victoria University of Manchester
Asynchronous ContributionAsynchronous Contribution
• Design Philosophy– optimize design for typical operation– support design for rarer conditions
• usually at expense of increased operation time
• Simpler logic within processing units– energy and area reduction
8Combining the strengths of UMIST andThe Victoria University of Manchester
DSP Design Examples 1DSP Design Examples 1• Data dependent adder on critical path
– detect completion of carry path– average carry path only half word length
• Address wrap around in circular buffer– synchronous calculates new and possible
corrected value in parallel - two adders– asynchronous new value only – one adder
• if correction required (rare) this done after
9Combining the strengths of UMIST andThe Victoria University of Manchester
DSP Design Examples 2DSP Design Examples 2• Register File has eight single-read
single-write ported 32-word banks– efficient parallel access to sequential
registers from 4 Functional Units (typical)
• Request conflicts to same bank rare– broadcast mechanism available– genuine conflicts take 1 read cycle per
request rather than 1 clock cycle each
10Combining the strengths of UMIST andThe Victoria University of Manchester
Viterbi DecoderViterbi Decoder
• Two data streams transmitted depends on current and previous data
• State transitions of encoder with time can be drawn as a trellis
• Decoder reconstructs trellis
11Combining the strengths of UMIST andThe Victoria University of Manchester
Asynchronous DecoderAsynchronous Decoder
– Clock only used to input and output data•all internal operation is asynchronous•FIFOs buffer data to meet clock demand
12Combining the strengths of UMIST andThe Victoria University of Manchester
Branch Metric UnitBranch Metric Unit
• Calculates gap between input symbol and four ideal symbols
13Combining the strengths of UMIST andThe Victoria University of Manchester
Path Metric UnitPath Metric Unit
j+32
j 2j
2j+1BMa
BMa
BMb
BMb
•add-compare-select operation
previous node metric
next node metric
node node
14Combining the strengths of UMIST andThe Victoria University of Manchester
Node ArithmeticNode Arithmetic
• Serial arithmetic• Counts events• Unary numbers
– Change of state equals count
– one=1111 two=0001 three=1101 etc.
When smaller count empties merge stops
15Combining the strengths of UMIST andThe Victoria University of Manchester
History UnitHistory Unit
• Records PMU node winners and global winner over many timeslots
• No error -global winner is child of last winner
• Error – need to reconstruct good path– compute parent of global winner and repeat
ONLY until it agrees with good path– can have many backtraces in parallel– backtraces decoupled from placing data into HU
16Combining the strengths of UMIST andThe Victoria University of Manchester
History UnitHistory Unit
17Combining the strengths of UMIST andThe Victoria University of Manchester
Low Power ContributionLow Power Contribution• History Unit
– much smaller– highly concurrent independent operation– computation performed minimised
• Path Metric Unit – most of power– smaller, simple, fast +/- units replace
add-compare-select– idea simple but a lot of control complexity
so dissipated a lot of power!
18Combining the strengths of UMIST andThe Victoria University of Manchester
What Have We Learnt?What Have We Learnt?• Asynchronous is advantageous to
some applications• Asynchronous design looks very
different from synchronous design• Get very poor results if just translate
from a synchronous design• Design of asynchronous is harder
– timing and control more complex to design