Download DYNAMIC LOGIC

DYNAMIC
LOGIC
[Adapted from Rabaey’s Digital Integrated Circuits, ©2002, J. Rabaey et al.]
EE415 VLSI Design
Dynamic CMOS

In static circuits the output is connected to
either GND or VDD via a low resistance path.
» fan-in of n requires 2n (n N-type + n P-type)
devices

Dynamic circuits use temporary storage of
signal values on the capacitance of high
impedance nodes.
» requires on n + 2 (n+1 N-type + 1 P-type)
transistors
EE415 VLSI Design
Dynamic Gate
Clk
Clk
Mp
off
Mp on
Out
In1
In2
In3
CL
PDN
Clk
A
C
B
Me
Clk
Two phase operation
Precharge (Clk = 0)
Evaluate (Clk = 1)
EE415 VLSI Design
1
Out
((AB)+C)
off
Me on
Conditions on Output

Once the output of a dynamic gate is
discharged, it cannot be charged again until
the next precharge operation.

Inputs to the gate can make at most one
transition during evaluation.

Output can be in the high impedance state
during and after evaluation (PDN off), state is
stored on CL
EE415 VLSI Design
Properties of Dynamic Gates

Logic function is implemented by the PDN only
» number of transistors is N + 2 (versus 2N for static complementary
CMOS)



Full swing outputs (VOL = GND and VOH = VDD)
Non-ratioed - sizing of the devices does not affect
the logic levels
Faster switching speeds
» reduced load capacitance due to lower input capacitance (Cin)
» reduced load capacitance due to smaller output loading (Cout)
» no Isc, so all the current provided by PDN goes into discharging CL
EE415 VLSI Design
Properties of Dynamic Gates

Overall power dissipation usually higher than static
CMOS
» no static current path ever exists between VDD and GND
(including Psc)
» no glitching
» higher transition probabilities
» extra load on Clk

PDN starts to work as soon as the input signals
exceed VTn, so VM, VIH and VIL equal to VTn
» low noise margin (NML)

Needs a precharge/evaluate clock
EE415 VLSI Design
Issues in Dynamic Design 1:
Charge Leakage
CLK
Clk
Mp
Out
CL
A
Clk
Me
Evaluate
VOut
Precharge
Leakage sources
Leakage is dominated by the subthreshold current
EE415 VLSI Design
Solution to Charge Leakage
Keeper
Clk
Mp
A
Mkp
CL
Out
B
Clk
Me
Same approach as level restorer for pass-transistor logic
EE415 VLSI Design
Issues in Dynamic Design 2:
Charge Sharing
Clk
Mp
Out
A
CL
B=0
Clk
CA
Me
EE415 VLSI Design
CB
Charge stored originally on
CL is redistributed (shared)
over CL and CA leading to
reduced robustness
Charge Sharing Example
Clk
A
A
B
B
B
Cc=15fF
C
C
Ca=15fF
Clk
EE415 VLSI Design
Out
CL=50fF
!B
Cb=15fF
Cd=10fF
Charge Sharing
Assume that initial voltage Vx=0 and Vout=VDD
VDD
case 1) if V out < VTn
VDD
Mp
Clk

Mp
Out
Out
CL
Ma
A
A
CL
Ma
X X
M
B = 0 B = 0 Mb b

Clk
Me
Me
EE415 VLSI Design
C
Caa
Cb
Cb
Vx increases up to VDD-VTn
when Ca is small comparing to CL
C L VDD = C L Vout  t  + Ca  VDD – V Tn  V X  
or
Ca
V out = Vout  t  – V DD = – --------  V DD – V Tn  V X  
CL
case 2) if V out > VTn
Ca 
 ---------------------
Vout = –V DD 
C
+
C
 a
L
as CL VDD=CL VOUT + Ca VOUT
VOUT = VDD CL / (CL+Ca)
Vx increases less up to
VOUT
when Ca is larger
Solution to Charge
Redistribution
Clk
Mp
Mkp
Clk
Out
A
B
Clk
Me
Precharge internal nodes using a clock-driven transistor
(at the cost of increased area and power)
EE415 VLSI Design
Issues in Dynamic Design 3:
Backgate Coupling
Clk
Mp
A=0
Out1 =1
Out2
CL1
CL2
Dynamic NAND
Static NAND
B=0
Clk
Me
When In goes up from 0 to VDD, output of static
NAND gate (Out2) goes down from VDD to 0 and
pulls down Out1 through the capacitive coupling
EE415 VLSI Design
In
Backgate Coupling Effect
3
2
Out1
1
Clk
0
In
Out2
2
Time, ns
-1
0
EE415 VLSI Design
4
6
Issues in Dynamic Design 4:
Clock Feed Through
Clk
Mp
A
CL
So voltage of Out can rise above
VDD.
B
Clk
Out
Coupling between Out and Clk
input of the precharge device due
to the gate to drain capacitance.
Me
The fast rising (and falling edges)
of the clock couple to Out.
EE415 VLSI Design
Clock Feed Through
Clock feed through
Clk
Out
2.5
In1
In2
1.5
In3
In &
Clk
0.5
In4
Clk
Out
-0.5
0
EE415 VLSI Design
0.5
Time, ns
More noise is generated as
a result of clock coupling
1
Clock feed through
Other Effects
Capacitive coupling between output
wires pulls down prestored charges
 Substrate coupling
 Minority charge injection
 Supply noise (negative ground bounce
may discharge the output)

EE415 VLSI Design
Cascading Dynamic Gates
V
Clk
Clk
Mp
Clk
Mp
Out1
In
Clk
Out2
In
Out1
Me
Clk
Output signal loss
VTn
Me
V
Out2
t
Only 0  1 transitions allowed at inputs!
So do not connect these gates directly
EE415 VLSI Design
Domino Logic
Clk
In1
In2
In3
Clk
Mp
11
10
PDN
Me
Out1
Clk
Mp Mkp
Out2
00
01
In4
In5
Clk
PDN
Me
Here we guarantee proper 0 to 1 transitions between gates
EE415 VLSI Design
Why Domino?
Clk
Ini
Inj
Clk
PDN
Ini
Inj
PDN
Ini
Inj
PDN
Like falling dominos!
EE415 VLSI Design
Ini
Inj
PDN
Properties of Domino Logic


Only non-inverting logic can be implemented
Very high speed
» static inverter can be skewed, only L-H transition
so make PMOS of inverter stronger
» Input capacitance reduced
so smaller logical effort
EE415 VLSI Design
Designing with Domino Logic
VDD
VDD
VDD
Clk
Mp
Clk
Mp
Out1
Mr
Out2
In1
In2
In3
PDN
Clk
Me
PDN
In4
Clk
Inputs = 0
during precharge
EE415 VLSI Design
Me
Can be eliminated!
but be aware of the
short circuit path from
delayed clock
Footless Domino
VDD
Clk
VDD
Mp
Clk
Mp
Out1
0
Clk
Mp
Out2
1
0
In1
1
VDD
0
Outn
1
0
In2
1
0
In3
1
0
Inn
1
0
The first gate in the chain needs a foot switch
Precharge is rippling – short-circuit current
A solution is to delay the clock for each stage
EE415 VLSI Design
1
Differential (Dual Rail)
Domino
off
Mp Mkp
Clk
Out = AB
1
on
Mkp
0
Clk
Mp
1
A
!A
0
!B
B
Clk
Me
Solves the problem of non-inverting logic
EE415 VLSI Design
Out = AB
np-CMOS
Clk
In1
In2
In3
Clk
Mp
11
10
PDN
Me
Out1
Clk
Me
In4
In5
PUN
Possible coupling
in longer runs to
dynamic node
00
01
Clk
Mp
Out2
(to PDN)
Only 0  1 transitions allowed at inputs of PDN
Only 1  0 transitions allowed at inputs of PUN
EE415 VLSI Design
NORA Logic
Clk
In1
In2
In3
Clk
Mp
11
10
Out1
PDN
Clk
Me
In4
In5
PUN
use np-CMOS blocks
00
01
Clk
Me
Mp
to other
PDN’s
WARNING: Very sensitive to noise!
EE415 VLSI Design
Out2
(to PDN)
to other
PUN’s