IC Specification & Simple Interfacing – Quick Review

Digital IC Families

  • Bipolar (TTL, RTL, DTL, ECL, HTL) vs Unipolar (CMOS, NMOS, PMOS)
  • Integration scales: SSI <12<12 gates, MSI 12!–!9912!–!99, LSI 100!–!9,999100!–!9{,}999, VLSI 104!–!10510^{4}!–!10^{5}, ULSI >105>10^{5}

TTL Basics

  • Fixed supply VCC=+5 VV_{CC}=+5\,\text{V}
  • Input LOW V<em>IL=0!–!0.8 VV<em>{IL}=0!\text{–}!0.8\,\text{V}, HIGH V</em>IH=2!–!5 VV</em>{IH}=2!\text{–}!5\,\text{V}
  • Output LOW V<em>OL,max=0.4 VV<em>{OL,max}=0.4\,\text{V}, HIGH V</em>OH,min=2.4 VV</em>{OH,min}=2.4\,\text{V}

CMOS Basics (4000, 74C/HC/HCT)

  • Supply range VDD=+3!–!18 VV_{DD}=+3!\text{–}!18\,\text{V}
  • Input LOW 0!–!0.3V<em>DD0!\text{–}!0.3V<em>{DD}, HIGH 0.7V</em>DD!–!VDD0.7V</em>{DD}!\text{–}!V_{DD}
  • Output swing ≈ 0 V0\,\text{V} to VDDV_{DD}

Noise Margin

  • High-level V<em>NH=V</em>OH,min−VIH,minV<em>{NH}=V</em>{OH,min}-V_{IH,min}
  • Low-level V<em>NL=V</em>IL,max−VOL,maxV<em>{NL}=V</em>{IL,max}-V_{OL,max}
  • Typical at 5 V5\,\text{V}: TTL V<em>NH=0.4 VV<em>{NH}=0.4\,\text{V}, V</em>NL=0.4 VV</em>{NL}=0.4\,\text{V}; CMOS V<em>NH=0.9 VV<em>{NH}=0.9\,\text{V}, V</em>NL=1.17 VV</em>{NL}=1.17\,\text{V} (larger ⇒ better immunity)

Propagation Delay tPt_P

  • t<em>PLHt<em>{PLH}: LOW→HIGH, t</em>PHLt</em>{PHL}: HIGH→LOW
  • Smaller t<em>Pt<em>P ⇒ higher max clock: f</em>max≈12tPf</em>{max}\approx\dfrac{1}{2t_P}
  • Range: Schottky TTL ≈ 1.5 ns1.5\,\text{ns}; HTL ≈ 125 ns125\,\text{ns}

Loading, Fan-Out/Fan-In

  • Fan-out = max inputs one output can drive without spec violation
    • TTL ≈ 1010, CMOS ≈ 5050
  • TTL outputs: source current in HIGH, sink in LOW (sink > source)
  • CMOS inputs mainly capacitive; delay rises with CLC_L

Power Dissipation

  • TTL ≈ 10 mW10\,\text{mW}/gate (standard)
  • Low-power TTL ≈ 1 mW1\,\text{mW}/gate
  • CMOS static ≪ TTL (µW range); rises with switching frequency

MOSFET & CMOS Structure

  • Complementary pair (P-channel top, N-channel bottom) forms inverter
  • Static hazards: thin SiO2SiO_2 susceptible to ESD ⇒ handle with grounding, conductive foam, off-power insertion

TTL ↔ CMOS Interfacing

  • Problem: TTL V<em>OH,min=2.4 VV<em>{OH,min}=2.4\,\text{V} < CMOS V</em>IH,min≈3.5 VV</em>{IH,min}\approx3.5\,\text{V} (@5 V5\,\text{V})
  • Solutions:
    • Pull-up resistor to +5 V+5\,\text{V} on TTL output
    • Use HCT series (TTL-compatible CMOS inputs)
    • Buffer/level-shifter (4049/4050, 74HCT, open-collector + resistor, NPN translator)
  • CMOS→TTL usually OK; one CMOS gate drives one LS-TTL input or use buffer for more drive

Switch Interfaces

  • Active-LOW: pull-up resistor, switch to GND
  • Active-HIGH: pull-down resistor, switch to VCCV_{CC}
  • Debouncing: RS latch (cross-coupled NAND), Schmitt trigger (40106), or open-collector with pull-up

LED Interfacing

  • At VDD=5 VV_{DD}=5\,\text{V} CMOS can drive LED directly (≤2 mA2\,\text{mA}) else add ≈1 kΩ\approx1\,\text{k}\Omega series resistor
  • TTL: use resistor, obey sink/source limits (sink ≤16 mA16\,\text{mA} std TTL)
  • Higher drive: NPN (source), PNP (sink) transistors; protect with base resistor

Driving Buzzers, Relays, Motors, Solenoids

  • Logic gate → transistor switch → load
  • Use flyback diode across inductive loads (relay coil, solenoid, motor, buzzer)
  • Relay isolates high-power side; NPN driver, separate supplies possible
  • Ensure transistor rated for load current; piezo buzzer: 3!–!5 mA3!–!5\,\text{mA}, relay/motor higher

Quick Formulas & Limits

  • Noise margins: as above
  • Fan-out: Fan-out=I<em>OHI</em>IH\text{Fan-out}=\dfrac{I<em>{OH}}{I</em>{IH}} (HIGH), I<em>OLI</em>IL\dfrac{I<em>{OL}}{I</em>{IL}} (LOW)
  • Max frequency: f<em>max=12t</em>Pf<em>{max}=\dfrac{1}{2t</em>P}

Exam Pointers

  • CMOS ≪ TTL power; CMOS higher noise margin
  • Interfacing TTL→CMOS needs voltage lift; CMOS→TTL needs current drive
  • Always debounce mechanical switches
  • Add series resistor with LED; diode across coils/inductors
  • Propagation delay, fan-out, power dissipation often trade-off