Introduction to CMOS VLSI Design Lecture 14 CAMs

Introduction to CMOS VLSI Design Lecture 14: CAMs, ROMs, and PLAs David Harris 14: CAMs, ROMs, and PLAs Harvey Mudd College Spring 2004 1

Outline q Content-Addressable Memories q Read-Only Memories q Programmable Logic Arrays 14: CAMs, ROMs, and PLAs CMOS VLSI Design 2

CAMs q Extension of ordinary memory (e. g. SRAM) – Read and write memory as usual – Also match to see which words contain a key 14: CAMs, ROMs, and PLAs CMOS VLSI Design 3

10 T CAM Cell q Add four match transistors to 6 T SRAM – 56 x 43 l unit cell 14: CAMs, ROMs, and PLAs CMOS VLSI Design 4

CAM Cell Operation q Read and write like ordinary SRAM q For matching: – Leave wordline low – Precharge matchlines – Place key on bitlines – Matchlines evaluate q Miss line – Pseudo-n. MOS NOR of match lines – Goes high if no words match 14: CAMs, ROMs, and PLAs CMOS VLSI Design 5

Read-Only Memories q Read-Only Memories are nonvolatile – Retain their contents when power is removed q Mask-programmed ROMs use one transistor per bit – Presence or absence determines 1 or 0 14: CAMs, ROMs, and PLAs CMOS VLSI Design 6

ROM Example q 4 -word x 6 -bit ROM – Represented with dot diagram – Dots indicate 1’s in ROM Word 0: 010101 Word 1: 011001 Word 2: 100101 Word 3: 101010 Looks like 6 4 -input pseudo-n. MOS NORs 14: CAMs, ROMs, and PLAs CMOS VLSI Design 7

ROM Array Layout q Unit cell is 12 x 8 l (about 1/10 size of SRAM) 14: CAMs, ROMs, and PLAs CMOS VLSI Design 8

Row Decoders q ROM row decoders must pitch-match with ROM – Only a single track per word! 14: CAMs, ROMs, and PLAs CMOS VLSI Design 9

Complete ROM Layout 14: CAMs, ROMs, and PLAs CMOS VLSI Design 10

PROMs and EPROMs q Programmable ROMs – Build array with transistors at every site – Burn out fuses to disable unwanted transistors q Electrically Programmable ROMs – Use floating gate to turn off unwanted transistors – EPROM, EEPROM, Flash 14: CAMs, ROMs, and PLAs CMOS VLSI Design 11

Building Logic with ROMs q Use ROM as lookup table containing truth table – n inputs, k outputs requires __ words x __ bits – Changing function is easy – reprogram ROM q Finite State Machine – n inputs, k outputs, s bits of state – Build with ____ bit ROM and ____ bit reg 14: CAMs, ROMs, and PLAs CMOS VLSI Design 12

Building Logic with ROMs q Use ROM as lookup table containing truth table – n inputs, k outputs requires 2 n words x k bits – Changing function is easy – reprogram ROM q Finite State Machine – n inputs, k outputs, s bits of state – Build with 2 n+s x (k+s) bit ROM and (k+s) bit reg 14: CAMs, ROMs, and PLAs CMOS VLSI Design 13

Example: Robo. Ant Let’s build an Ant Sensors: Antennae (L, R) – 1 when in contact Actuators: Legs Forward step F Ten degree turns TL, TR Goal: L R make our ant smart enough to get out of a maze Strategy: keep right antenna on wall (Robo. Ant adapted from MIT 6. 004 2002 Open. Course. Ware by Ward and Terman) 14: CAMs, ROMs, and PLAs CMOS VLSI Design 14

Lost in space q Action: go forward until we hit something – Initial state 14: CAMs, ROMs, and PLAs CMOS VLSI Design 15

Bonk!!! q Action: turn left (rotate counterclockwise) – Until we don’t touch anymore 14: CAMs, ROMs, and PLAs CMOS VLSI Design 16

A little to the right q Action: step forward and turn right a little – Looking for wall 14: CAMs, ROMs, and PLAs CMOS VLSI Design 17

Then a little to the right q Action: step and turn left a little, until not touching 14: CAMs, ROMs, and PLAs CMOS VLSI Design 18

Whoops – a corner! q Action: step and turn right until hitting next wall 14: CAMs, ROMs, and PLAs CMOS VLSI Design 19

Simplification q Merge equivalent states where possible 14: CAMs, ROMs, and PLAs CMOS VLSI Design 20

State Transition Table Lost RCCW Wall 1 Wall 2 S 1: 0 00 01 01 01 10 10 11 11 11 L 0 1 0 0 X X 1 0 0 14: CAMs, ROMs, and PLAs R 0 X 1 0 0 1 X 0 1 S 1: 0’ 00 01 01 10 10 11 01 10 11 TR 0 0 0 1 1 0 0 0 CMOS VLSI Design TL 0 0 0 1 1 1 F 1 1 1 0 0 0 1 1 1 21

ROM Implementation q 16 -word x 5 bit ROM 14: CAMs, ROMs, and PLAs CMOS VLSI Design 22

ROM Implementation q 16 -word x 5 bit ROM 14: CAMs, ROMs, and PLAs CMOS VLSI Design 23

PLAs q A Programmable Logic Array performs any function in sum-of-products form. q Literals: inputs & complements q Products / Minterms: AND of literals q Outputs: OR of Minterms q Example: Full Adder 14: CAMs, ROMs, and PLAs CMOS VLSI Design 24

NOR-NOR PLAs q ANDs and ORs are not very efficient in CMOS q Dynamic or Pseudo-n. MOS NORs are very efficient q Use De. Morgan’s Law to convert to all NORs 14: CAMs, ROMs, and PLAs CMOS VLSI Design 25

PLA Schematic & Layout 14: CAMs, ROMs, and PLAs CMOS VLSI Design 26

PLAs vs. ROMs q The OR plane of the PLA is like the ROM array q The AND plane of the PLA is like the ROM decoder q PLAs are more flexible than ROMs – No need to have 2 n rows for n inputs – Only generate the minterms that are needed – Take advantage of logic simplification 14: CAMs, ROMs, and PLAs CMOS VLSI Design 27

Example: Robo. Ant PLA q Convert state transition table to logic equations S 1: 0 00 01 01 01 10 10 11 11 11 L 0 1 0 0 X X 1 0 0 R 0 X 1 0 0 1 X 0 1 S 1: 0’ 00 01 01 10 10 11 01 10 11 14: CAMs, ROMs, and PLAs TR 0 0 0 1 1 0 0 0 TL 0 0 0 1 1 1 F 1 1 1 0 0 0 1 1 1 CMOS VLSI Design 28

Robo. Ant Dot Diagram 14: CAMs, ROMs, and PLAs CMOS VLSI Design 29

Robo. Ant Dot Diagram 14: CAMs, ROMs, and PLAs CMOS VLSI Design 30