Cmos

Digital Cells

1 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

Outline
The CMOS inverter Logic Gates Digital circuits
Note: some slides are taken from Dr. Paulo Moreira’s presentation in the course “MICROPROCESSOR LABORATORY SECOND CENTRAL AMERICAN REGIONAL COURSE ON ADVANCED VLSI DESIGN TECHNIQUES”, Puebla – Mexico, 29 November - 17 December 2004. Anotherfigures are taken from R. J. Baker et al., CMOS: Circuit Design, Layout, and Simulation, Wiley Interscience, 1998
2 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

part 1: CMOS Inverter

3 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

A masterpiece
• Logic levels • MOSFET – a simple switch • The CMOS inverter:
– – – – – DC operation Dynamicoperation Propagation delay Power consumption Layout

4 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

CMOS logic: “0” and “1”
• Logic circuits process Boolean variables • Logic values are associated with voltage levels:
– VIN > VIH ⇒ “1” – VIN < VIL ⇒ “0”
Output +V "1" VOH
Noise Margin High

Input +V

VIH
Undefined region

• Noise margin:
– NMH=VOH-VIH –NML=VIL-VOL
VOL "0" 0

VIL
Noise Margin Low

0

5 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

The CMOS inverter
DC Characteristics

6 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

The MOSFET - a simple switch
p-switch S B A p-switch G A 0 0 1 1 B 0 1 0 1 Y bad 0 (source follower) good 1 ? (high Z) ? (high Z)

D n-switch DY Y

A n-switch G A 0 0 1 1

B 0 1 0 1

Y ? (high Z) ? (high Z) good 0 bad 1 (source follower)

S

B

7 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

The CMOS inverter
VDD VDD p-switch

A Y 0 1

Y good 1 good 0

A

Y

A n-switch

VSS

VSS
8

E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

The CMOS inverterRegions of operation (balanced inverter):
Vin 0 VTNVdd/2 Vdd n-MOS cut-off saturation saturation linear linear p-MOS linear linear saturation saturation cut-off Vout Vdd ~Vdd Vdd/2 ~0 0

9 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

Switching Characteristics
Inverter transient response
3 2.5

Vout, Vin (V)

2 1.5 1 0.5 0 -0.5

Vout

Vin 0 2 4 6 Time(ns) 8

CL=250fF
10 12

0.6

(mA)

0.4 0.2 0 -0.2 -0.4 -0.6

I D(nmos)

CL=250fF

ID

I D(pmos) 0 2 4 6 Time (ns) 8 10 12

10 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

Sizing the CMOS inverter

β = µCox ⎜

E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

⎛W ⎝L

⎞ ⎛W ⎞ = kn, p ⎜ ⎟ ⎟ ⎠ ⎝L⎠

11

Design parametersof CMOS inverter
• Propagation delay
– Main origin: load capacitance
t pLH = t pHL = k p (Vdd − VTP ) k n (Vdd − VTN C L ⋅ Vdd C L ⋅ Vdd
2

≈ ≈

CL k p ⋅ Vdd

)2

CL k n ⋅ Vdd ⎛ 1 1⎞ ⎜ + ⎟ ⎝ kn k p ⎠

CL 1 t p ≈ t pLH + t pLH = 2 2 ⋅ Vdd

(

)

– To reduce the delay:
• Reduce CL • Increase kn and kp. That is, increase W/L
12 E. Martinez-Guerrero /Taller de DiseñoFísico/MDE_DESI_ITESO/Otoño_2006

CMOS power budget
– Dynamic power consumption:
• Charging and discharging of capacitors

– Short circuit currents:
• Short circuit path between power rails during switching

– Leakage
• Leaking diodes. • Leaking transistors:
– Sub-threshold currents – In the future devices gate leakage current!?

13 E. Martinez-Guerrero /Taller de DiseñoFísico/MDE_DESI_ITESO/Otoño_2006

Dynamic power dissipation

14 E. Martinez-Guerrero /Taller de Diseño Físico/MDE_DESI_ITESO/Otoño_2006

Design parameters of CMOS inverter
• The dynamic power dissipation is a function of:
– Frequency – Capacitive loading – Voltage swing

•

To reduce dynamic power dissipation
– Reduce: CL – Reduce: f – Reduce: Vdd ⇐ The most effective action
Dynamic power VDD

CMOS...