What are the Basic Differences Between CMOS and TTL Signals?

Updated Jul 17, 2018

Reported In

Hardware

  • PCI-6230

Issue Details

What are some of the differences between CMOS and TTL signals and how do they compare?

Solution

Characteristics of CMOS logic:
  • Dissipates low power: The power dissipation is dependent on the power supply voltage, frequency, output load, and input rise time. At 1 MHz and 50 pF load, the power dissipation is typically 10 nW per gate.
  • Short propagation delays: Depending on the power supply, the propagation delays are usually around 25 nS to 50 nS.
  • Rise and fall times are controlled: The rise and falls are usually ramps instead of step functions, and they are 20 - 40% longer than the propagation delays.
  • Noise immunity approaches 50% or 45% of the full logic swing.
  • Levels of the logic signal will be essentially equal to the power supplied since the input impedance is so high.
  • Voltage levels range from 0 to VDD where VDD is the supply voltage. A low level is anywhere between 0 and 1/3 VDD while a high level is between 2/3 VDD and VDD.

Characteristics of TTL logic:
  • Power dissipation is usually 10 mW per gate.
  • Propagation delays are 10 nS when driving a 15 pF/400 ohm load.
  • Voltage levels range from 0 to Vcc where Vcc is typically 4.75V - 5.25V. Voltage range 0V - 0.8V creates logic level 0. Voltage range 2V - Vcc creates logic level 1.


CMOS compared to TTL:
  • CMOS circuits do not draw as much power as TTL circuits while at rest. However, CMOS power consumption increases faster with higher clock speeds than TTL does. Lower current draw requires less power supply distribution, therefore causing a simpler and cheaper design.
  • Due to longer rise and fall times, the transmission of digital signals becomes simpler and less expensive with CMOS chips.
  • CMOS components are more susceptible to damage from electrostatic discharge than TTL components.
 
Resource: National Semiconductor CMOS Logic Databook and National Semiconductor LS/S/TTL Logic Databook

Additional Information

CMOS components are typically more expensive than TTL equivalents. However, CMOS technology is usually less expensive on a system level due to CMOS chips being smaller and requiring less regulation.

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