1. COMBINATIONAL LOGIC - 3

2. Dynamic Logic

3. Dynamic CMOS
In static circuits at every point in time (except when switching) 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 rely on the temporary storage of signal values on the capacitance of high impedance nodes.
requires on n + 2 (n+1 N-type + 1 P-type) transistors

4. Dynamic Gate Two phase operation Precharge (CLK = 0)
Out
Clk A B C Mp Me
Clk Mp
Out CL
In1
In2
PDN
In3
Clk Me
For class handout
Two phase operation
Precharge (CLK = 0)
Evaluate (CLK = 1)

5. Dynamic Gate Two phase operation Precharge (Clk = 0)
Out
Clk A B C Mp Me
off
Clk Mp on 1
Out CL
((AB)+C)
In1
In2
PDN
In3
Clk Me
off
For lecture
Evaluate transistor, Me, eliminates static power consumption on
Two phase operation
Precharge (Clk = 0)
Evaluate (Clk = 1)

6. Dynamic CMOS 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
This behavior is fundamentally different than the static counterpart that always has a low resistance path between the output and one of the power rails.

7. Properties of Dynamic CMOS 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
CL being lower also contributes to power savings

8. Properties of Dynamic CMOS 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

9. Dynamic 4 Input NAND Gate
VDD
Out
In1
In2
In3
In4 f
GND

10. Issues in Dynamic Design 1: Charge Leakage
CLK
Clk Mp
Out CL
(1) A
(2)
Evaluate
VOut
Clk Me
Precharge
leakage sources are reverse-biased diode and the sub-threshold leakage of the NMOS pulldown device.
Charge stored on CL will leak away with time (input in low state during evaluation)
Requires a minimum clock rate - so not good for low performance products such as watches (or when have conditional clocks)
PMOS precharge device also contributes some leakage due to reverse bias diode and subthreshold conduction that, to some extent, offsets the leakage due to the pull down paths.
Leakage sources
Dominant component is subthreshold current

11. Solution to Charge Leakage
Keeper
Clk Mp
Mkp CL A
Out B
Clk Me
During precharge, Out is VDD and inverter out is GND, so keeper is on
During evaluation if PDN is off, the keeper compensates for drained charge due to leakage.
If PDN is on, there is a fight between the PDN and the PUN - circuit is ratioed so PDN wins, eventually
Note Psc during switching period when PDN and keeper are both on simultaneously
Same approach as level restorer for pass-transistor logic

12. Issues in Dynamic Design 2: Charge Sharing (redistribution)
Charge stored originally on CL is redistributed (shared) over CL and CA leading to reduced robustness
Clk Mp
Out CL A CA
B=0 CB
Clk
CA initially discharged and CL fully charged. Me

13. Charge Sharing Example (redistribution)
Clk
Out = A xor B xor C
Out
CL=50fF A A
Ca=15fF B
Cb=15fF B B B
Cc=15fF
Cd=10fF C C
Out = A xor B xor C
What is the worst case change in voltage on node Out - assume all inputs are low during precharge and all internal capacitances are initially 0V
Worst case is obtained by exposing the maximum amount of internal capacitance to the output node during evaluation. This happens when !A B C or A !B C
30/(30+50) * 2.5 V = 0.94 V so the output drops to = 1.56 V
Clk
DV = 30/(30+50) * 2.5 V = 0.94 V
Output drops to = 1.56 V

14. Charge Sharing (redistribution)V DD
Clk M p
Out C L A M a X C a B = M b C b
Clk M e C
|V | Tp
Must keep Vout < |VTp| a
<
 0.2 C DD V Tn – L

15. Solution to Charge Redistribution
Clk
Clk Mp
Mkp
Out A B
Clk Me
Precharge internal nodes using a clock-driven transistor (at the cost of increased area and power)

16. Issues in Dynamic Design 3: Backgate Coupling
Clk Mp
Out1 =1
Out2 =0
CL1
CL2 In
A=0
B=0
Due to capacitive backgate coupling between the internal and output node of the static gate and the output of the dynamic gate, Out1 voltage reduces
Clk Me
Dynamic NAND
Static NAND

17. Backgate Coupling Effect
Due to clk feedthrough
Out1
Due to backgate
Voltage
Clk
Out2
Does not discharge to GND
Out1 overshoots Vdd (2.5V) due to clock feedthrough
And Out2 never quite makes it to GND In
Time, ns

18. Issues in Dynamic Design 4: Clock Feedthroughp e V DD
CLK
Out A B C L a b X
could potentially forward bias the diode
2.5V
CLK
overshoot
out
Coupling between Out and Clk input of the precharge device due to the gate to drain capacitance.
So voltage of Out can rise above VDD. The fast rising (and falling edges) of the clock couple to Out.
Danger is that signal levels can rise enough above VDD that the normally reverse-biased junction diodes become forward-biased causing electrons to be injected into the substrate.

19. Clock Feedthrough and Charge SharingM p e V DD
CLK
Out A B C L a b X
Clock feedthrough
Clock feedthrough

20. Other Effects Capacitive coupling Substrate coupling
Minority charge injection
Supply noise (ground bounce)

21. Cascading Dynamic GatesV
Clk
Clk
Clk Mp Mp
Out2
Out1 In In
Out1
VTn
Clk
Clk Me Me
Out2 V
Out2 should remain at VDD since Out1 transitions to 0 during evaluation. However, since there is a finite propagation delay for the input to discharge Out1 to GND, the second output also starts to discharge.
The second dynamic inverter turns off (PDN) when Out1 reaches VTn.
Setting all inputs of the second gate to 0 during precharge will fix it.
Correct operation is guaranteed (ignoring charge redistribution and leakage) as long as the inputs can only make a single 0 -> 1 transition during the evaluation period t
Only 0  1 transitions allowed at inputs!

22. Domino Logic Clk Clk Out1 Out2 In1 In4 PDN In2 PDN In5 In3 Clk Clk Mp
Mkp
Clk Mp
Out1
Out2
1  1
1  0
0  0
0  1
In1
In4
PDN
In2
PDN
In5
In3
Ensures all inputs to the Domino gate are set to 0 at the end of the precharge period. Hence, the only possible transition during evaluation is 0 -> 1
Clk Me
Clk Me

23. Why Domino? Like falling dominos! Ini PDN Inj Ini Inj PDN Ini PDN Inj
Clk
Clk
Like falling dominos!

24. Properties of Domino Logic
Only non-inverting logic can be implemented
Very high speed
static inverter can be skewed, only L H transition
Input capacitance reduced – smaller logical effort
Adding level restorer reduces leakage and Charge redistribution problem
Optimise Inverter for fan-out
First 32 bit micro (BellMAC 32) was designed in Domino logic
Now a rather rare design style due to non-inverting logic only

25. Domino Manchester Carry Chain AdderP C
i,0 1 G 2 3 4 f V DD M
o,4
1.5
2.5
3.5
0.5

26. Designing with Domino LogicV V DD DD V DD
Clk M
Clk p M p M
Out1 r
Out2 In 1 In
PDN In
PDN 2 4 In 3
Can be eliminated!
Clk M M e
Clk e
Inputs = 0
during precharge

27. Footless Domino
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

28. Differential (Dual Rail) Domino
off on
Clk
Clk Mp
Mkp
Mkp Mp
Out = AB
Out = AB A !A !B B
AND/NAND differential logic gate. The inputs and their complements come from other differential DR gates and thus all inputs are low during precharge and make a conditional transition from 0 to 1.
Annotations show state during evaluate cycle (CLK = 1)
Expensive - but can implement any arbitrary function.
Use significant power since they have a guaranteed transition every single clock cycle (regardless of signal statistics, since either Out or !Out will transition from 0 to 1).
Not ratioed (even though have a cross-coupled PMOS pair)
Clk Me
Solves the problem of non-inverting logic

29. np-CMOS
Clk Me
Clk Mp
Out1
1  1
1  0
In4
PUN
In1
In5
In2
PDN
0  0
0  1
In3
Out2
(to PDN)
Clk Mp
Clk Me
Also called zipper logic - In4 and In5 must be from PDN’s
DEC alpha uses np-CMOS logic (Dobberpuhl)
Have to size the PUN’s to equalize the delay to that of the PDN’s
Really dense layouts and very high speed (20% faster than domino with the correct sizing)
Reduced noise margin (as with any dynamic gate)
Have two clock signals to generate and route - CLK and !CLK
Only 0  1 transitions allowed at inputs of PDN Only 1  0 transitions allowed at inputs of PUN

30. NORA Logic WARNING: Very sensitive to noise! Clk Clk Out1 In4 PUN In1Me
Clk Mp
Out1
1  1
1  0
In4
PUN
In1
In5
In2
PDN
0  0
0  1
In3
Out2
(to PDN)
Clk Mp
Clk Me
NORA - no race CMOS
to other
PDN’s
to other
PUN’s
WARNING: Very sensitive to noise!

31. np CMOS Adder V V V V CLK CLK S CLK CLK C A B B A A B C A B CLK CLK1 C i 1 A B B 1 1 1 A 1 A B C 1 1 i 1 A 1 B
CLK 1
CLK
CLK C i 2
CLK V V DD DD V DD
CLK
CLK
CLK B A C i 1 A B C i A A B B C i S
CLK
CLK
CLK C i
Carry Path

32. CMOS Circuit Styles - Summary
Must consider area, performance, power, robustness (noise immunity), ease of design, system clocking requirements, fan-out, functionality, ease of testing

33. SEQUENTIAL LOGIC- I

34. Sequential Logic 2 storage mechanisms • positive feedback
• charge-based

35. Naming Conventions In the text:
a latch is level sensitive
a register is edge-triggered
There are many different naming conventions
For instance, many books call edge-triggered elements flip-flops
This leads to confusion however

36. Latch versus Register Latch Register stores data when clock is low
stores data when clock rises D Q D Q
Clk
Clk
Clk
Clk D D Q Q

37. Latches

38. Latch-Based Design N latch is transparent when CLK = 0
P latch is transparent when CLK = 1
CLK N P
Logic
Latch
Latch
Logic

39. Timing Definitions CLK t Register t t D Q D DATA CLK STABLE t t Q DATAsu
hold D
DATA
CLK
STABLE t t c  q Q
DATA
STABLE t

40. Characterizing TimingD  Q D Q D Q
Clk
Clk t t C  Q C  Q
Register
Latch

41. Maximum Clock Frequency
CLk s ’ F F
LOGIC
Also:
tcdreg + tcdlogic > thold
tcd: contamination delay = minimum delay t
p,comb
T  tclk-Q + tp,comb + tsetup

42. Positive Feedback: Bi-Stability1 V i 1 A C B o 2 = o1
Vi2 o 1 o V V 5 2 i V 1 o V 5 i 2 V

43. Meta-Stability
Gain should be larger than 1 in the transition region