TWO TYPES OF LOGIC CIRCUIS Combinational At any point in time, the output of the circuit is related to its current input signals by some Boolean expression(e.g, multiplexer, ROM, adder, etc) Sequential Its output is not only a function of the current input data, but also of previous values of the input signals(e.g, registers, counters and memory) Combinational Combinational Out Out Circuit (a) Combinatio (b) Sequential
▪Combinational ▪ At any point in time, the output of the circuit is related to its current input signals by some Boolean expression (e.g., multiplexer, ROM, adder, etc). ▪Sequential ▪ Its output is not only a function of the current input data, but also of previous values of the input signals (e.g., registers, counters and memory). 6
SWITCHES Implementing a simple circuit (arrow shows action if wire changes to 1” or is asserted): Open switch(if A is0"or unasserted)and turn off light bub(4 Close switch(if A is"1"or asserted) and turn on light bub( 4 Z= A
A Z ▪ Implementing a simple circuit (arrow shows action if wire changes to “1” or is asserted): 7 Z A A Z Close switch (if A is “1” or asserted) and turn on light bulb (Z) Open switch (if A is “0” or unasserted) and turn off light bulb (Z)
SWITCHES (CONTD Compose switches into more complex ones(Boolean functions): AND AB Z≡ A and b AND B A OR Z≡AorB OR B
AND OR Z A and B Z A or B A B A B ▪Compose switches into more complex ones (Boolean functions): 8 A B AND C OR A B C
HOW TO MAKE ELECTRONIC SWITCHES? Idea: Use voltage to represent 0 and l High voltage (Vdd) represents 1.0 Volt Low voltage(0 Volt or Ground) represents O Pick a midpoint voltage to decide if ao or a l Voltage greater than midpoint =1 Voltage less than midpoint =0 This removes noise as signals propagate-a big advantage of digital systems over analog systems
▪ Idea: Use voltage to represent 0 and 1 ▪ High voltage (Vdd) represents 1 (Vdd ~ 1.0 Volt). ▪ Low voltage (0 Volt or Ground) represents 0. ▪Pick a midpoint voltage to decide if a 0 or a 1 ▪ Voltage greater than midpoint = 1 ▪ Voltage less than midpoint = 0 ▪ This removes noise as signals propagate – a big advantage of digital systems over analog systems 9
CMOS TRANSISTORS Modern digital systems designed in CMOs MOS: Metal-Oxide on semiconductor C for complementary: use pairs of normally-open and normally-closed switches 5V CMOS transistors act as voltage-controlled switches N-type MOSFET Switch is closed when input is 5V NMOS transistors Switch closed(open) for a high(low)voltage PMOS transistors 5V P-type MOSFET Switch closed(open) for a low(high) voltage Switch is closed when input is OV
▪Modern digital systems designed in CMOS ▪ MOS: Metal-Oxide on Semiconductor ▪ C for complementary: use pairs of normally-open and normally-closed switches ▪ CMOS transistors act as voltage-controlled switches. ▪NMOS transistors ▪ Switch closed (open) for a high (low) voltage ▪PMOS transistors ▪ Switch closed (open) for a low (high) voltage 10