Logic Families
Characterizing Digital ICs Digital ICs characterized several ways Circuit Complexity Gives measure of number of transistors or gates Within single package Four general categories SSI - Small Scale IC < 12 gates or so MSI - Medium Scale IC < 100 gates or so LSI - Large Scale IC < 1000 gates or so VLSI - Small Scale IC > 1000 gates or so
Main Characteristics Voltage Levels Input Output Low High Supply Noise Margins Based upon voltage levels Propagation delay Low to High High to Low Setup and hold times Storage devices Fan In and Fan Out Based upon Input sink capability Output drive capability Power dissipation
Circuit Technologies and Topologies Describes Process used to implement devices Input and output structure of the device
Four general categories
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TTL Transistor Transistor Logic Bipolar transistors on input and output NPN, PNP, diodes Input Diode Transistor BE junction Output Typically two transistors Configured in totem pole arrangement
ECL Emitter Coupled Logic Bipolar Input Configured in non-saturating differential pair Output Emitter follower type of configuration -V Logic done in emitter circuitry rather than collector High speed
MOS Metal Oxide Semiconductor Field Effect Transistors MOS transistors on input and output
N MOS N Channel transistors Operating in enhancement mode Less common as single technology in new designs Input Gate Output Typically two transistors Configured in totem pole arrangement Upper configured as depletion mode load
P MOS P Channel transistors Operating in enhancement mode Rarely used alone in new designs Input Gate
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Output Typically two transistors Configured in totem pole arrangement Upper configured as depletion mode load
C MOS N - P Channel transistor pairs Operating in enhancement mode Input Gate Output Two transistors Configured in totem pole arrangement Upper P channel Lower N channel
BICMOS Both CMOS and Bipolar transistors Bipolar mainly on output Used for speed Works well for combining Analog and digital circuitry on same substrate
Circuit Topology Output Stages Three different types Normal Active drive logic both high and low states
TTL CMOS
Open Collector / Drain Active drive logic low state passive pull up to high state Used in applications where
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Wire OR bus Sink large amounts of current Lamp LED Relay TTL CMOS
Tristate Two states Behave as Normal Active drive logic low and high states Third state Both devices turned OFF Output floats Tristate mode Selected by control line Typically Low enables drive High disables
Used in applications where Bus configurations Bus usually pulled high passively Size of pullup determined by Logic family CMOS - higher value TTL - lower value Desired bus speed
Input Stages TTL Two forms Multiple emitter Diode
Multiple Emitter
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Input transistor behaves in interesting way Q1 When any input low Q1 Q2 Emitters act as emitters Operates in forward direction Turns on to ground Q2 Turns off
When all inputs high Q1 Emitters act as collectors Operates in reverse direction Supplies current/voltage to Q2 Q2 Turns on
Diode Input
Diode inputs work like traditional DTL Both inputs High Q2 turns on Any input low Q2 turns off
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CMOS
Input structure 1 N channel and 1 P channel For each input Circuit above shows "AND" configuration Reverse configuration for "OR" function
Logic Families Continually changing as technologies improve Common Bipolar LS Low Power Schottky AS Advanced Schottky ALS Advanced Low Power Schottky 54 / 74 Military / Commercial
CMOS HC / HCT High Speed CMOS TTL Compatible 4000 Typical numbering
A Look at the Real World Logic gates and circuits always work perfectly on paper Let’s now look at some of the things we encounter When working with real parts
Voltage Levels TTL or CMOS we have supply voltage of +5 VDC ECL will use a negative supply -5.2 VDC
Denoted Vcc - Bipolar Vdd - MOS Vee - ECL
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Logic Levels On paper we deal with logical 0 or logical 1 In physical parts Represented by different voltage levels Most typically Logical 0 - 0 volts Logical 1 - +5 volts
Newer logics Logical 0 - 0 volts Logical 1 - +1.5 or 3.0 volts
We refer to Logic 0 as low Logic 1 as high
Problem When dealing with real parts Logic high never exactly 5.0 v Logic low never exactly 0.0v
If logic High gets too low Interpreted as logic 0 Low gets too high Interpreted as logic 1
Max and Min Values For TTL family we specify Min logic high voltage 2.0 volts Lower either interpret as logic 0 or Don’t know This can be a problem Max logic low voltage 0.7 volts Higher either interpret as logic 1 or Don’t know This can be a problem
Nominal Values We rarely work with parts at max voltages specify Typical Nominal values Must consider when doing worst case design
Typical logic high voltage 3.5 volts Typical logic low voltage 0.2 volts
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Noise immunity Difference between Min (Max) values and nominal values Gives noise immunity Consider if VOH from a gate is 3.5 v VIHmin is 2.0 v
VOH - VIHmin = 1.5v
What this says is we can have 1.5 v of low going noise spike Before it’s interpreted as a logic 0
Consider if VOL from a gate is 0.2 v VILmax is 0.7 v
VOL - VILmin = 0.5v
What this says is we can have 0.5 v of high going noise spike Before it’s interpreted as a logic 1
Observe Based upon typical values have 1.5 v of high noise immunity 0.5 v of low noise immunity
TTL CMOS Noise Margins Fan 4.5 VOHtyp 5.0 5.0 In Noise Margins and 3.5 VOHtyp Fan Out
2.7 VOHmin 2.4 VOHmin
2.0 VIHmin 2.0 VIHmin
0.5 VOLmax 0.7 VILmax 0.4 VOLmax 0.8 VILmax
0 0
0.2 VOLtyp 0.0 VOLtyp
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Limited Such limits determine how many other devices Can be driven Fan Out Measure of output drive capability Specifies how current device can Supply to other devices in logic high state Sink from other devices in logic low state
Fan In Measure of device input requirements Specifies how current device Supplies to other devices in logic low state Sink from other devices in logic high state
Analyzing Let’s specify drive capability for typical gate IOL 8 mA @ VOL = 0.5VDC IOH -400 µA @ VOH = 3.4 VDC IIL -400 µA @ VIL = 0.4VDC IIH 20 µA @ VIH = 2.7 VDC
For such analysis Ohm’s law must hold If high drive required to supply additional current Voltage must drop If low drive required to sink additional current Voltage must increase
For this device Logic 0 State When output at logic 0 Can sink 8mA When input at logic 0 Sources -400 µA Thus Can sink current from 20 devices
Logic 1 State When output at logic 1 Can supply -400 µA When input at logic 1 Requires 20 µA Thus Can source current to 20 devices
Thus fan out for device is 20
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Can connect up to 20 devices Can support up to 20 unit loads
Observe on input Device typically requires 20 µA for logic 1 -400 µA for logic 0 Call this a unit load State fan in is 1 unit load If values double State fan in is 2 unit loads
Increasing Drive Capability Are occasions when require more drive capability Handle several ways Buffers / Drivers Such devices designed for purpose of driving Often open collector configuration High sink - intended to drive low Such devices tend to be slower
Parallel Devices Another common technique Connect several standard devices in parallel Assumption If one device will sink/source x mA Then n devices will sink/source nx mA
Is belief valid? Let's consider following circuit configuration
A
B
Let's put a step input into the circuit
input
device A
device B
tpd1
tpd2 - 10 of 20 -
From the diagram we see Propagation delay through two devices is different If device A faster that device B When A tries to go high B still (partially) off Tries to hold signal low Two devices fighting
Conflict in two logic states can potentially damage part May not occur immediately More often will appear as premature failure
Problem In any random sample of parts Have no guarantee of device speed Hand selecting parts not solution
Possible solution If paralleled devices in same package Thus on same small portion of die Prop delays much closer than on arbitrary parts Still real world parts and will be different
Unused Inputs Gate inputs should always be defined Never let them float Problem with TTL Larger problem with CMOS Floating input Has parasitic capacitors connected To ground Surrounding circuitry
Such capacitors Charge up to threshold voltage of device Cause device to change state Uncontrolled or unknown ways
Defining Inputs Direct connection to VDD, VCC, VSS permitted Both CMOS and LS TTL
Direct connection to VDD, VCC, prohibited Standard TTL
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Must be done through pullup
Preferred method Connect to VCC or VDD Through pullup resistor CMOS 10 - 100K Since input current negligible size insignificant LS TTL Remember IIH and IIL Must calculate current to ensure voltage drop not too large With such scheme Can connect to ground for test Only works if several inputs not connected to same pullup
Unused Gates Similarly must define inputs to unused devices Goal to place device in low power state Devices left undefined Can enter state in which all internal devices conducting to ground
Basic Logic Devices Classified according to function Will include SSI - Small Scale Integrated circuits MSI - Medium Scale Integrated Circuits Combinational Logic SSI Devices
Basic Logic Gates Comprises basic logic gates AND / NAND OR / NOR Invertor Exclusive OR / NOR
Bus Drivers and Transceivers Serve two main purposes Provide high current drive Perform multiplexing function Output configuration Open collector Tristate Inverting / Noninverting
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If tristate output configuration Control may be high or low
TTL Configuration
Driver Transceiver
ENB ENB
ENB ENB ENB
ENB
ENB
ENB ENB ENB
ENB CMOS Configuration ENB
Driver Transceiver
ENB
ENB
ENB
ENB
ENB
ENB ENB
ENB
MSI Devices
Encapsulate standalone piece of functionality Types Arithmetic circuits Data formatters and convertors Selection devices Simple communication circuits When we choose to use MSI parts Give up some flexibility Must use features the manufacturer has designed in
Top Level View
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Can represent as a box
Signals Inputs Outputs Input Data input signals Control Output Data output signals May or may not be same a input signals Control Provide timing reference Clock or strobe Selection Select From alternate input sources Different outputs Enable / Disable Inputs or Outputs
Data Encoding and Decoding These are simply combinational logic problems We work with truth table Identify input patters Corresponding output patterns Design logic to do mapping Control Usually these devices Have some form of selection or enable Output enable / disable Perhaps tri state control Output Logic level, tristate, or open collector Logic level may be High true Output high when desired logical condition satisfied Low true Output low when desired logical condition satisfied
Let’s consider a couple of examples Encoders Devices that Accept one set of signals Encode them into second set Generally go from 2n inputs to n outputs 8 Line to 3 Line Priority Encoder
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This device Inputs 0 A0 Has 8 data inputs 1 Outputs 2 A1 3 A2 3 data outputs 4 2 Output for cascading 5 6 E0 Controls 7 GS 1 Input enable Function EI Map each input to encoded output pattern Output pattern based upon priority of input Assume 7 highest and 0 lowest Input lines 5-7 inactive 4 active 0-3 don't care Output code A0-A2 → 011 If 5 goes active A0-A2 → 010 EI is logical 0
Decoders Opposite of encoders Devices that Accept one set of signals Decode them into second set
Generally go from n inputs to 2n outputs A B C Output = 1 3 to 1 of eight Decoder 0 0 0 O0 0 0 1 O1 This device 0 1 0 O2 Inputs 0 1 1 O3 Has 3 data inputs 1 0 0 O4 1 0 1 O5 Three inputs represent 8 combinations 1 1 0 O6 As reflected in 3 variable truth table 1 1 1 O7 Outputs 8 data outputs Controls 2 active low 1 active high Usually these are low true
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Function 0 A Map each input combination to one of 8 1 B 2 outputs 3 C Corresponding output to be logical 1 4 G1 5 when G2A 6 Enable is satisfied G2B 7
Data Selection Data selection devices permit one to select from Several alternate data sources Usually because of pin limitations
Number of choices limited to 2 D00 Combine devices to increase number of D0 D alternatives 01
4 Bit Data Selector D10 D1 Inputs D11 Has 8 data inputs D Grouped as 2 sets of 4 20 D Outputs 2 D21 4 data outputs D Controls 30 D 2 3 D31 Enable Select Device Logical 0 active Select Selector Select one of 2 input sets ~enable Function Cause one or the other of the 2 input data sets to Appear on output When enable is logical 0 Selector = 0 Selects one group Selector = 1 Selects other group
1 of N Data Selector - Multiplexer We can build a variation on the above device Multiplex number of data bits onto single line Inputs
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Has 4 data inputs D0 Outputs 1 data output D1 Controls 3 D2 Enable Dout Select Device Logical 0 active D3 2 Selectors Select one of 4 input bits Function A Cause one of the four input data bits to Appear on output B
Based upon selector pattern Enable When enable is logical 0
Storage Elements Two basic types Latch Flip flop Distinguished by clocking scheme Latch Control signal When control signal Active Output tracks input Inactive Output holds last value Flip flop Control signal called clock Input signal stored on output Clock transitions from High to low Low to high Types D, JK, SR, T
Uses Basic Component Use flip flops to build more complex sequential circuits Will cover these shortly
Registers or Latches Use either as Register
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Latch Will hold data or address type information
Registers or Latches Packaged in 4 or 8 bit sizes
May have set of control lines Select or address Enable or disable tristate outputs Permit use in bus type applications
Counters Collection of flip flops Configured to count transitions on input line May count Up, down, or both Asynchronously or synchronously Async Each element changes with respect to another element in the collection Sync Each element changes with respect to a common signal May have Carry in and / or carry out Permit devices to be cascaded Clear / Preset Constrain device into Reset state Known state Synchronous or asynchronous Typical configurations BCD HEX
Shift Register Collections of flip flops Configured to store collections of bits Shift the bits Left, right, or selectable left/right Shift by one position with each clock
Inverted Logic We often will use to indicate active state of signal In large systems clarifies intent of designer Let's examine on simple gates
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Invertor Observe the invertor truth table and function NOT If input a ~a A get ~A out 0 1 ~A get A out 1 0
We use the bubble to indicate the two different forms With bubble on input
Implies 0 in gives 1 out ~A A With bubble on output Implies 1 in gives 0 out
AND Observe the AND truth table and function AND We have A B C 0 0 0 3 zero terms 0 1 0 1 one term 1 0 0 Writing logic equation for 1 term 1 1 1
C = 1 = A * B
Writing the equation for the 0 term
~C = 0 = ~A~B + ~A B + A~B = ~A (~B + B) + ~B ( ~A + A) = ~A + ~B
Observe we now have a OR type function If A is 0 or false or if B is 0 or false Then C is 0 or false If we are to express this using bubbles to indicate inversion First the basic logic function is an OR so we draw that
Then we want 0’s on the inputs and the output
~A ~C ~B Thus our two symbols now look like
C ~A ~C A ~B B Thus the gate implements AND of logical 1’s OR of logical 0’s - 19 of 20 -
OR Observe the OR truth table and function OR A B C We have 0 0 0 1 zero term 0 1 1 3 one terms 1 0 1 Following same reasoning as with AND 1 1 1
Thus our two symbols now look like
A C B ~A ~C ~B
Thus the gate implements AND of logical 0’s OR of logical 1’s
NAND Observe the NAND truth table and function AND NAND We have a b a * b ~(a * b) 3 one terms 0 0 0 1 1 zero term 0 1 0 1 1 0 0 1 1 1 1 0 Thus our two symbols now look like
A ~A ~C C B ~B
Thus the gate implements AND of logical 1’s OR of logical 0’s
NOR Observe the NOR truth table and function OR NOR We have a b a + b ~(a + b) 1 one term 0 0 0 1 3 zero terms 0 1 1 0 1 0 1 0 1 1 1 0
Thus our two symbols now look like
A ~A C C ~B B Thus the gate implements AND of logical 0’s OR of logical 1’s
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