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CMPEN 411VLSI Digital CircuitsVLSI Digital Circuits
Digital integrated circuits experience exponentialgrowth in complexity (Moore’s law) and performance
Design in the deep submicron (DSM) era creates new challenges
Devices become somewhat differentDevices become somewhat differentGlobal clocking becomes more challengingInterconnect effects play a more significant rolePower dissipation may be the limiting factor
Our goal in this class will be to design digital integrated i it i 0 5 CMOS t h l d d t dcircuits in 0.5um CMOS technology and understand
digital integrated circuits in advanced technologies, below 180 nanometer.
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Today we look at some basic design metrics
Fundamental Design Metrics
Functionality
CostNRE (fixed) costs - design effortRE (variable) costs - cost of material, parts, assembly, test
Performance/PowerSpeed (delay)Speed (delay)Power consumption; energy
Time-to-market
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Time to market
How a chip is manufactured?
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Cost of Integrated Circuits( )NRE (non-recurring engineering) costs
Fixed cost to produce the design- design effortg- design verification effort- mask generation
Influenced by the design complexity and designer productivityInfluenced by the design complexity and designer productivityMore pronounced for small volume products
Recurring costs – proportional to product volumeRecurring costs proportional to product volumesilicon processing, material
- also proportional to chip areabl ( k i )assembly (packaging)
testfixed cost
cost per IC = variable cost per IC + -----------------
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cost per IC variable cost per IC + volume
NRE Cost is Increasing
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Silicon Wafer
Single die
Wafer
From http://www.amd.com
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Silicon Wafer
300mm wafer and Pentium 4 IC Photos courtesy of Intel
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300mm wafer and Pentium 4 IC. Photos courtesy of Intel.
Recurring Costscost of die + cost of die test + cost of packagingcost of die + cost of die test + cost of packaging
variable cost = ----------------------------------------------------------------final test yield
cost of wafercost of wafercost of die = -----------------------------------
die yield = (1 + (defects per unit area × die area)/α)-α
Defect Density Trends
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www.icknowledge.com
Yield ExampleExampleExample
wafer size of 12 inches, die size of 2.5 cm2, 1 defects/cm2, α = 3 (measure of manufacturing process complexity)252 dies/wafer (remember, wafers round & dies square)die yield of 16%252 x 16% = only 40 dies/wafer die yield !y y
Di t i t f ti f di
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Die cost is strong function of die areaproportional to the third or fourth power of the die area
Final cost: 25.57$ / chip source:www.icknowledge.com
ReliabilityNoise in Digital Integrated Circuits
Noise – unwanted variations of voltages and currents at the logic nodes
v(t)
From two wires placed side by sidecapacitive coupling
voltage change on one wire can
i(t)
- voltage change on one wire can influence signal on the neighboring wire
- cross talkinductive couplinginductive coupling
- current change on one wire can influence signal on the neighboring wire
VDD
From noise on the power and ground supply railscan influence signal levels in the gate
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can influence signal levels in the gate
Example of Capacitive CouplingS % fSignal wire glitches as large as 80% of the supply voltage will be common due to crosstalk between neighboring wires as feature sizes continue to scale
Crosstalk vs. Technology
160nm CMOS120nm CMOS
Pulsed Signal
250 CMOSBlack line quiet
350nm CMOS
250nm CMOSRed lines pulsedGlitches strength vs technology
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From Dunlop, Lucent, 2000
Static Gate BehaviorS fSteady-state parameters of a gate – static behavior – tell how robust a circuit is with respect to both variations in the manufacturing process and to noise disturbances.
Digital circuits perform operations on Boolean variables x ∈{0,1}
A logical variable is associated with a nominal voltage level for each logic state
1 V d 0 V1 ⇔ VOH and 0 ⇔ VOL
V(y)V(x)VOH = ! (VOL)
Difference between VOH and VOL is the logic or signal
V(y)V(x)VOL = ! (VOH)
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swing Vsw
DC Operation Voltage Transfer Characteristics (VTC)
Plot of output voltage as a function of the input voltage
V(y)
f
V(y)V(x)
fVOH = f (VIL)
V(y)=V(x)
Switching ThresholdVM
V(x)VOL VOH
VOL = f (VIH)
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( )OL OH
Mapping Logic Levels to the Voltage Domain
The regions of acceptable high and low voltages are delimited by VIH and VIL that represent the points on the VTC curve where the gain = -1
V(y)
VTC curve where the gain 1
Slope = -1VOH
"1" VOH
VIH
Slope = -1
UndefinedRegion
V
V(x)
pVOL
VIL VIH
"0" VOL
VIL
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Noise MarginsFor robust circuits want the “0” and “1” intervals to be a s
VDD VDD
For robust circuits, want the 0 and 1 intervals to be a s large as possible
"1"VOH
DD DD
UndefinedRegion
VIL
VIHNoise Margin High
Noise Margin Low
"0"VOL
G d G dGate Output Gate Input
Gnd Gnd
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Noise MarginsFor robust circuits want the “0” and “1” intervals to be a s
VDD VDD
For robust circuits, want the 0 and 1 intervals to be a s large as possible
"1"VOHNMH = VOH - VIH
DD DD
UndefinedRegion
VIL
VIHNoise Margin High
Noise Margin Low
H OH IH
"0"VOL
NML = VIL - VOL
G d G dGate Output Gate Input
Gnd Gnd
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Large noise margins are desirable, but not sufficient …
The Regenerative PropertyA gate with regenerative property ensure that a disturbed signal converges back to a nominal voltage level
Absolute noise margin values are deceptivea floating node is more easily disturbed than a node driven by a low impedance (in terms of voltage)
Noise immunity expresses the ability of the system to
low impedance (in terms of voltage)
process and transmit information correctly in the presence of noise
For good noise immunity, the signal swing (i.e., the difference between VOH and VOL) and the noise margin h b l h h i f fi d
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have to be large enough to overpower the impact of fixed sources of noise
Fan-In and Fan-Out
Fan-out – number of load gates connected to the output of the d i i tdriving gate
gates with large fan-out are slowerN
Fan-in – the number of inputs to Mthe gate
gates with large fan-in are bigger and slower
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Directivity
A gate must be undirectional: changes in an output level should not appear at any unchanging input of the same pp y g g pcircuit
In real circuits full directivity is an illusion (e.g., due to capacitive coupling between inputs and outputs)p g p p )
Key metrics: output impedance of the driver and inputKey metrics: output impedance of the driver and input impedance of the receiver
ideally, the output impedance of the driver should be zero andinput impedance of the receiver should be infinity
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The Ideal InverterThe ideal gate should haveThe ideal gate should have
infinite gain in the transition regiona gate threshold located in the middle of the logic swinghigh and low noise margins equal to half the swinginput and output impedances of infinity and zero, resp.
Vout
R = ∞Ri = ∞
Ro = 0
Fanout = ∞
NMH = NML = VDD/2
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Vin
The Ideal InverterThe ideal gate should haveThe ideal gate should have
infinite gain in the transition regiona gate threshold located in the middle of the logic swinghigh and low noise margins equal to half the swing
Vout
R = ∞Ri = ∞
Ro = 0
g = - ∞ Fanout = ∞
NMH = NML = VDD/2
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Vin
Delay Definitions
Vin Vout
Vin
inputPropagation delay?
t
waveform
Vout
t toutputwaveform
t
signal slopes?
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t
Delay Definitions
Vin Vout
Vin
input t = (t + t )/2
Propagation delay
50%
t
waveformtp = (tpHL + tpLH)/2
t
50%
tVout
t t
tpHL tpLH
90%
outputwaveform
t
50%
10%
signal slopes
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ttf tr
Modeling Propagation Delayf CModel circuit as first-order RC network
Rv
vout (t) = (1 – e–t/τ)V
where τ = RC
C
vin
vout
Time to reach 50% point ist = ln(2) τ = 0 69 τt = ln(2) τ = 0.69 τ
Time to reach 90% point ispt = ln(9) τ = 2.2 τ
Matches the delay of an inverter gate
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Matches the delay of an inverter gate
Power and Energy DissipationPower consumption: how much energy is consumedPower consumption: how much energy is consumed per operation and how much heat the circuit dissipates
supply line sizing (determined by peak power)P V iPpeak = Vddipeak
battery lifetime (determined by average power dissipation)p(t) = v(t)i(t) = Vddi(t) Pavg= 1/T ∫ p(t) dt = Vdd/T ∫ idd(t) dt
Power and Energy DissipationPropagation delay and the power consumption of a gatePropagation delay and the power consumption of a gate are related
Propagation delay is (mostly) determined by the speed atPropagation delay is (mostly) determined by the speed at which a given amount of energy can be stored on the gate capacitors
the faster the energ transfer (higher po er dissipation) thethe faster the energy transfer (higher power dissipation) the faster the gate
For a given technology and gate topology, the product of th ti d th ti d l ithe power consumption and the propagation delay is a constant
Power-delay product (PDP) – energy consumed by the gate per switching event
An ideal gate is one that is fast and consumes little energy so the ultimate quality metric is
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energy, so the ultimate quality metric isEnergy-delay product (EDP) = power-delay 2
Summary
Digital integrated circuits have come a long way and still have quite some potential left for the coming d ddecades
Some interesting challenges aheadGetting a clear perspective on the challenges and potential solutions is the purpose of this course
Understanding the design metrics that govern digitalUnderstanding the design metrics that govern digital design is crucial
Cost, reliability, speed, power and energy dissipation
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Next Lecture and RemindersNext lectureNext lecture
MOS transistor- Reading assignment – Rabaey et al, 3.1-3.3.2- I will not be covering 3.2 in class (EE 310 material)