MOS logic 1
Properties of MOS Gates Snapshot High noise margins: V OH and V OL are at V DD and GND, respectively. No static power consumption: There never exists a direct path between V DD and V SS (GND) in steady-state mode. omparable rise and fall times: (under appropriate sizing conditions) Extremely high input resistance: nearly zero steady-state input current. lways a path to Vdd or Gnd in steady state: low output impedance. 2
Static MOS ircuit - t every point in time (except during the switching transients) each gate output is connected to either V DD or V ss via a low-resistive path. - The outputs of the gates assume at all times the value of the oolean function, implemented by the circuit (ignoring, once again, the transient effects during switching periods). - This is in contrast to the dynamic circuit class, which relies on temporary storage of signal values on the capacitance of high impedance circuit nodes. 3
Static omplementary MOS V DD In1 In2 InN In1 In2 InN PUN PDN PMOS only NMOS only F(In1,In2, InN) PUN and PDN are dual logic networks 4
Threshold Drops PUN V DD V DD S D V DD D 0 V DD V GS S 0 V DD -V Tn L L PDN V DD 0 V DD V Tp V DD D L V GS S L S D 5
NMOS Transistors in Series/Parallel onnection Transistors can be thought as a switch controlled by its gate signal NMOS switch closes when switch control input is high X Y Y = X if and X Y Y = X if OR NMOS Transistors pass a strong 0 but a weak 1 6
PMOS Transistors in Series/Parallel onnection PMOS switch closes when switch control input is low X Y Y = X if ND = + X Y Y = X if OR = PMOS Transistors pass a strong 1 but a weak 0 7
omplementary MOS Logic Style 8
Example Gate: NND 9
Example Gate: NOR 10
Two-input MOS NOR gate and reference inverter 11
MOS NOR Gate Truth Table and Transistor States 12
Three-input MOS NOR gate and reference inverter 13
Two-input MOS NND gate and reference inverter 14
Switch Delay Model R eq R p R p R p R p R n L R n L R p int NND2 R n int INV R n R n L NOR2 15
Input Pattern Effects on Delay R p R n R n R p L int Delay is dependent on the pattern of inputs Low to high transition both inputs go low delay is 0.69 R p /2 L one input goes low delay is 0.69 R p L High to low transition both inputs go high delay is 0.69 2R n L 16
Delay Dependence on Input Patterns Voltage [V] 3 2,5 2 1,5 1 0,5 0-0,5 ==1 0 =1, =1 0 =1 0, =1 0 100 200 300 400 time [ps] Input Data Pattern ==0 1 =1, =0 1 = 0 1, =1 ==1 0 =1, =1 0 = 1 0, =1 Delay (psec) 17 69 50 62 35 57 76 NMOS = 0.5μm/0.25 μm PMOS = 0.75μm/0.25 μm L = 100 ff
Fan-In onsiderations D M4 M3 3 L Distributed R model (Elmore delay) D M2 M1 2 1 t phl = 0.69 R eqn ( 1 +2 2 +3 3 +4 L ) Propagation delay deteriorates rapidly as a function of fan-in quadratically in the worst case. 18
t p of MOS NND as a function of Fan-In t p (psec) 1250 1000 750 500 250 0 t phl 2 4 6 8 10 12 14 16 fan-in t p t plh quadratic linear Gates with a fan-in greater than 4 should be avoided. 19
t p as a Function of Fan-Out t p (psec) t p NOR2 t p NND2 t p INV 2 4 6 8 10 12 14 16 eff. fan-out ll gates have the same drive current. Slope is a function of driving strength 20
t p as a Function of Fan-In and Fan- Out Fan-in: quadratic due to increasing resistance and capacitance Fan-out: each additional fan-out gate adds two gate capacitances to L t p = a 1 FI + a 2 FI 2 + a 3 FO 21
Design Techniques for large fan-in Transistor sizing as long as fan-out capacitance dominates Progressive sizing In N MN L Distributed R line M1 > M2 > M3 > > MN (the fet closest to the output is the smallest) In 3 M3 3 In 2 In 1 M2 M1 2 1 an reduce delay by more than 20%; decreasing gains as technology shrinks 22
Transistor ordering critical path critical path In 3 1 M3 charged L 0 1 In 1 M3 L charged In 2 1 M2 2 charged In 2 1 M2 2 discharged In 1 0 1 M1 1 charged In 3 1 M1 1 discharged delay determined by time to discharge L, 1 and 2 delay determined by time to discharge L 23
lternative logic structures F = DEFGH 24
Isolating fan-in from fan-out using buffer insertion L L 25
Power consumption in MOS logic gates 26
X Z X Z Example illustrating the effect of signal correlations 27
Glitching in Static MOS 28
Example: hain of NND Gates 29
How to ope with Glitching? D D,,, 30
Input ordering X Z P(=1) = 0.5 P(=1) = 0.2 Y Z P(=1) = 0.1 Reordering of inputs affects the circuit activity 31
Time multiplexing resources 32
Ratioed Logic V DD V DD V DD Resistive Load R L Depletion Load V T < 0 PMOS Load F F V SS F In 1 In 2 In 3 PDN In 1 In 2 In 3 PDN In 1 In 2 In 3 PDN V SS V SS V SS (a) resistive load (b) depletion load NMOS (c) pseudo-nmos Goal: to reduce the number of devices over complementary MOS 33
Resistive Load V DD Resistive Load R L N transistors + Load V OH = V DD F V OL = R PN R PN + R L In 1 In 2 In 3 PDN ssymetrical response Static power consumption V SS t pl = 0.69 R L L 34
ctive Loads V DD V DD Depletion Load V T < 0 PMOS Load F V SS F In 1 In 2 In 3 PDN In 1 In 2 In 3 PDN V SS V SS depletion load NMOS pseudo-nmos 35
Pseudo-NMOS Inverter VT 3.0 2.5 2.0 W/L p = 4 V ou t [V] 1.5 1.0 W/L p = 2 0.5 W/L p = 0.5 W/L p = 0.25 W/L p = 1 0.0 0.0 0.5 1.0 1.5 2.0 2.5 V in [V] 36
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Four-input pseudo-nmos NOR 38
Improved Loads V DD Enable M1 M2 M1 >> M2 F D L daptive Load 39
Improved Loads (2) V DD V DD M1 M2 Out Out PDN1 PDN2 V SS V SS Differential ascode Voltage Switch Logic (DVSL) 40
DVSL ND/NND Transient Response 2.5 V ol ta ge [V] 1.5 0.5,, -0.5 0 0.2 0.4 0.6 0.8 1.0 Time [ns] 41
Pass-Transistor Logic Inputs Switch Network Out Out N transistors No static consumption 42
Example: ND Gate F = 0 43
NMOS-only Switch = 2.5 V = 2.5 V = 2.5 V = 2.5 V X M 2 X M n L M 1 V does not pull up to 2.5V, but 2.5V -V TN Threshold voltage loss causes static power consumption NMOS has higher threshold than PMOS (body effect) 44
NMOS-Only Only Logic V DD In x 0.5μm/0.25μm 1.5μm/ 0.25μm 0.5μm/ 0.25μm Out 3.0 In Voltage [V] 2.0 1.0 Out x 0.0 0 0.5 1 1.5 2 Time [ns] 45
Pass-transistor output (drain-source) terminal should not drive other terminals to avoid multiple threshold drops 46
Pass-Transistor ND Gate VT 47
NMOS Only Logic: Level Restoring Transistor V DD Level Restorer V DD M r M 2 M n X Out M 1 dvantage: Full Swing Restorer adds capacitance, takes away pull down current at X Ratio problem 48
Restorer Sizing 3.0 W/L n = 0.5/0.25 V olta ge [V] 2.0 1.0 W /L r =1.75/0.25 W /L r =1.50/0.25 W / L r =1.0/0.25 W /L r =1.25/0.25 0.0 0 100 200 300 400 500 Time [ps] Upper limit on restorer size 49
Transmission Gate = 2.5 V = 2.5 V L = 0 V 50
Transmission Gate XOR M2 M1 F M3/M4 51
Resistance of Transmission Gate 30 R n 2. 5 V Rn Resistance, Kohms 20 10 R p R n R p 2.5 V 0 V V ou t R p 0 0.0 1.0 2.0 V ou t, V (W/L)p = (W/L)n = 0.5/0.25 52
Delay in Transmission Gate Networks 2.5 2.5 2.5 2.5 In V 1 V i-1 V i V i+1 V n-1 V n 0 0 0 0 (a) In R eq R V eq R eq R 1 V i V i+1 V eq n-1 V n m (b) R eq R eq R eq R eq R eq R eq In (c) 53
Delay Optimization 54