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CMOS Logic Circuits

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**Logic Values Logic values = {0, 1}**

A logic value, 0 or 1, is called as BInary DigiT or BIT. Physical states representing bits in digital technologies:

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**Encoding Bits Information can be encoded using:**

Current, Voltage, Phase, Frequency Digital systems use two voltage levels for encoding bits. LOW: A signal close to the GND HIGH: A signal close to the VCC

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**Encoding Bits Positive logic Negative logic High: 1 and Low: 0**

Our convention in this course Negative logic High: 0 and Low: 1

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**Logic Gates Gates are basic digital devices.**

A gate takes one or more inputs and produces an output. Inputs are either 0 or 1. Although they may have very different values of voltage. Output is either 0 or 1. A logic gate’s operation is fully described by a truth table.

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**Logic Families – What is inside of a logic gate?**

A logic family is a collection of different integrated-circuit chips that have similar input, output, and internal circuit characteristics, but that perform different logic functions. Logic gates are made from transistors. TTL (Transistor-Transistor Logic) family gates are made from bipolar transistors. CMOS (Complementary Metal Oxide Semiconductor) family logic gates are made from MOS transistors.

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**MOS Transistors – N-type MOSFET**

OFF (open circuit) : when gate is logical zero ON (short circuit) : when gate is logical one Passes a good logical zero Degrades a logical one

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**MOS Transistors – P-type MOSFET**

OFF (open circuit) : when gate is logical one ON (short circuit) : when gate is logical zero Passes a good logical one Degrades a logical zero

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CMOS Logic Gates

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Inverter

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Inverter

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NAND – Not AND

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NAND

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NOR – Not OR

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Non-inverting Buffer

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AND Gate

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OR Gate

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**CMOS Electrical Characteristics**

Digital analysis works only if circuits are operated in specs: Power supply voltage Temperature Input-signal quality Output loading Must do some “analog” analysis to prove that circuits are operated in spec. Fan-out specs Timing analysis (setup and hold times) Analysis involves only consequences of V = IR (static) and q = CV (dynamic)

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**CMOS Electrical Characteristics**

Logic voltage levels DC noise margin Fan-in Fan-out Speed Power consumption

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Data Sheets

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**Logic Levels Typical transfer characteristic of a CMOS inverter:**

LOW input level: < 2.4 Volt HIGH input level: > 2.6 Volt Transfer characteristic depends on power-supply voltage, temperature and output loading.

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Logic Levels Should use more conservative specifications for LOW and HIGH VILmax: max input voltage guaranteed to be recognized as a LOW level 30% of VCC VOLmax: max output voltage in the LOW level GND V VOHmin: min output voltage in the HIGH level VCC – 0.1 V VIHmin: min input voltage guaranteed to be recognized as a HIGH level 70% of VCC

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Logic Levels VIHmin VILmax VOHmin VOLmax

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DC Noise Margins DC noise margin is a measure of how much noise it takes to corrupt a worst-case output voltage into a value that may not be recognized properly by an input. Noise Margin Low = VILmax – VOLmax = 1.35 – 0.1 = 1.25 V Noise Margin High = VOHmin – VIHmin = 4.4 – 3.15 = 1.25 V

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Input Currents CMOS gate inputs consume very little current, only the leakage current of the two transistors’ gates. IIH: max current that flows into the input in HIGH state IIL: max current that flows into the input in LOW state

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DC Output Loading An output must sink current from a load when the output is in the LOW state. An output must source current to a load when the output is in the HIGH state.

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**DC Output Loading Specs**

VOLmax and VOHmin are specified for certain output-current values, IOLmax and IOHmax. IOLmax: max current that output can sink in the LOW state while still maintaining an output voltage no greater than VOLmax IOHmax: max current that output can source in the HIGH state while still maintaining an output voltage no less than VOHmin

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**DC Output Loading - Output-voltage Drops**

Resistance of “off” transistor is > 1 Megaohm, but resistance of “on” transistor is nonzero, Voltage drops across “on” transistor, V = IR For “CMOS” loads, current and voltage drop are negligible. For TTL inputs, LEDs, terminations, or other resistive loads, current and voltage drop are significant and must be calculated. If too much load, output voltage will go outside of valid logic-voltage range. VOHmin, VIHmin VOLmax, VILmax

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Fan-in The number of inputs that a gate can have in a particular logic family is called the logic family’s fan-in. You could design a CMOS NAND or NOR gates with a very large number of inputs. In practice, additive “on” resistance of series transistors limits the fan-in of CMOS gates – Lower speed. Max fan-in = 4 for NOR, 6 for NAND 7-input NAND gate using 4-input NAND gates 3-input NAND gate

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Fan-out The fan-out of a gate is the number of inputs that the gate can drive without exceeding its worst-case loading specifications. Characteristics of the gate’s output Characteristics of the inputs that it is driving DC fan-out: The number of inputs that an output can drive with the output in a constant state (high or low). AC fan-out: The ability of an output to charge or discharge the stray capacitance associated with the inputs that it drives. If the capacitance is too large, the transition from low to high (or vice versa) may be too slow, causing improper system operation.

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**DC Fan-out Calculation**

LOW state: The sum of the IIL values of the driven inputs may not exceed IOLmax of the driving output. HIGH state: The sum of the IIH values of the driven inputs may not exceed IOHmax of the driving output. Low State Fan-out= 20 μA/1 μA= 20 High State Fan-out= 20 μA/1 μA= 20

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AC Loading AC loading has become a critical design factor as industry has moved to pure CMOS systems. CMOS inputs have very high impedance, DC loading is frequently negligible (low fan-outs). CMOS inputs and related packaging and wiring have significant capacitance. Time to charge and discharge capacitance is a major component of delay. Gate’s speed and power consumption depend on the AC characteristics of the gate and its load.

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Transition Time The amount of time that the output of a logic circuit takes to change from one state to another is called the transition time. tR : rise time – time to chage from low to high tF : fall time – time to chage from high to low

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Transition Time The rise and fall times of a CMOS output depend mainly on “on” transistor resistance capacitive load Capacitive load = Stray capacitance = AC load Output circuits: A gate’s output transistors, internal wiring, packaging The wiring that connects an output to other inputs Input circuits: A gate’s input transistors, internal wiring, packaging

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Propagation Delay The propagation delay is the amount of time that it takes for a change in the input signal to produce a change in the output signal. tPHL : high-to-low propogation time tPLH : low-to-high propogation time tPD : propogation delay; tPD= max (tPHL, tPLH) tPD determines the gate speed

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Power Consumption Static power consumption: Power consumption when the circuit’s output is not changing Very low static power consumption for CMOS circuits Attractive for low-power applications Power consumption due to the leakage currents Dynamic power consumption: Power consumption when the circuit’s output is in transition PD = (CPD + CL) x (VCC)2 x f PT : Dynamic power consumption VCC: Power-supply voltage f: Transition frequency of the output signal CPD : Power-dissipation capacitance – Specified by the device manufacturer and around pF CL : Load capacitance

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**CMOS Logic Family 4000 series**

First commercially successful CMOS family Fairly slow and not easy to interface to TTL CMOS device part number: 74FAMnn or 54FAMnn HC/HCT: High speed CMOS/ High speed CMOS, TTL compatible FCT/FCT-T: Fast CMOS/ Fast CMOS, TTL compatible VHC/VHCT: Very high speed CMOS/ Very high speed CMOS, TTL compatible

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**CMOS-TTL Interface DC noise margin Fan-out Capacitive loading**

CMOS outputs driving TTL inputs: Ok TTL outputs driving CMOS inputs: CMOS device must be HCT, VHCT or FCT Fan-out TTL outputs driving CMOS inputs: Ok CMOS outputs driving TTL inputs: Limited Capacitive loading Increasing delay and power consumption All CMOS families have similar dynamic power consumption TTL outputs have lower dynamic power consumption

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