Introduction to TTL and CMOS Logic
Transistor-transistor logic (TTL) and complementary metal-oxide-semiconductor (CMOS) are two of the most widely used logic families in integrated circuits. Both implement Boolean logic functions to perform computation, but they have distinct characteristics that make them suitable for different applications.
TTL was developed in the early 1960s and became the dominant logic family through the 1970s. It is based on bipolar junction transistors (BJTs) and offers high speed and good drive capability. However, TTL consumes significant static power.
CMOS emerged in the late 1960s as a lower power alternative to TTL. It is based on metal-oxide-semiconductor field-effect transistors (MOSFETs) and has come to dominate modern digital integrated circuits due to its low static power consumption, high noise immunity, and dense circuitry.
This article will compare the key characteristics, advantages, and disadvantages of TTL and CMOS logic families. It will also discuss variants of each family and typical applications.
Key Characteristics of TTL and CMOS
The following table summarizes the main characteristics of TTL and CMOS logic:
| Parameter | TTL | CMOS |
|---|---|---|
| Maximum supply voltage | 5.5 V | 18 V |
| Typical supply voltage | 5 V | 3.3 V to 15 V |
| Static power consumption | High | Very low |
| Dynamic power consumption | Medium | Low to medium |
| Maximum toggle frequency | 25-125 MHz | 0-200 MHz |
| Propagation Delay | 10-100 ns | 5-50 ns |
| Output current | 16-20 mA | 0.1-1.6 mA |
| Input logic levels | 0.8 V (low), 2.0 V (high) | 30% VCC (low), 70% VCC (high) |
| Noise immunity | Poor to fair | High |
| Density | Medium | Very high |
| Cost | Low to medium | Low to high |
Supply Voltage
TTL typically operates at a 5 V power supply, with an absolute maximum rating of 5.5 V. Exceeding this limit can damage the device. The minimum supply voltage is around 4.5 V for most TTL parts.
In contrast, CMOS can operate over a much wider supply voltage range, from 3 V to 18 V for common parts. Lower supply voltages reduce power consumption, while higher voltages can be used for enhanced noise immunity. Modern CMOS is commonly used at 3.3 V or lower.
Power Consumption
One of the main differences between TTL and CMOS is power consumption. TTL has a relatively high static power consumption, meaning it draws significant current even when not switching. This is because the BJTs are always partly on, allowing current to flow continuously.
CMOS, on the other hand, has very low static power consumption. The MOSFETs are fully off when not switching, so only tiny leakage currents flow. Dynamic power is consumed when the circuit switches states. CMOS dynamic power is proportional to frequency and the square of the voltage.
Overall, CMOS consumes much less power than equivalent TTL circuits. This makes CMOS well-suited for low-power and battery-operated devices.
Speed
TTL is generally faster than 4000 series CMOS, with typical propagation delays in the 10-30 ns range compared to 20-50 ns for CMOS. However, 74HCT series CMOS, which is pin and function compatible with TTL, has similar speed to TTL.
The maximum toggle frequency of TTL ranges from around 25 MHz for standard parts to 125 MHz for advanced Schottky TTL. CMOS toggle frequency depends on supply voltage and can exceed 200 MHz at higher voltages.
Drive Capability
TTL has much higher output drive capability than CMOS. A typical TTL output can sink 16 mA and source 400 µA. CMOS, in contrast, can typically sink or source only 0.4-1.6 mA. This means TTL is better able to drive high fanouts and capacitive loads.
Special CMOS families like 74HC and 74HCT have higher output current than 4000 series CMOS to improve drive. However, even these sink only about 4-8 mA, still well below TTL levels.
Noise Immunity
CMOS has much better noise immunity than TTL due to its high-impedance inputs and symmetric input threshold. TTL input low and high logic levels are fixed at around 0.8 V and 2.0 V respectively. Any noise pulse exceeding 0.4 V can cause malfunction.
For CMOS, the input thresholds are a percentage of the supply voltage, typically 30% for low and 70% for high. This means the noise margin scales with supply voltage, providing better immunity at higher VCC. Slow input rise and fall times also have less effect on CMOS than TTL.
Circuit Density
CMOS allows very high circuit density, as MOSFET transistors are smaller than BJTs. Also, the low power consumption of CMOS means less heat is generated, so components can be packed closer together without overheating.
TTL circuit density is more limited by the size of BJTs and the heat generated at high integration levels. However, advanced TTL families using Schottky transistors and low-power techniques achieve higher density than standard TTL.
Cost
The cost of TTL and CMOS depends on the specific part and logic family. In general, standard TTL and 4000 series CMOS are low cost and widely available. More advanced families like 74LS TTL and 74HC/HCT CMOS are low to moderate in cost.
Some specialized CMOS families for very low power or high speed applications can be more expensive. The cost of both technologies has decreased over time as manufacturing processes have improved.
TTL Logic Families
Several variants of TTL logic have been developed to improve speed, power consumption, and other characteristics. The main TTL logic families are:
Standard TTL
The original TTL family, designated 74xx, provides medium speed and a good balance of characteristics. It operates at 5 V and has a typical propagation delay of 10 ns. Standard TTL is widely used in general-purpose digital logic applications.
Schottky TTL
Schottky TTL (74Sxx) uses Schottky diodes to improve switching speed and reduce power consumption. It has a propagation delay of around 3 ns and can toggle at up to 125 MHz. Schottky TTL is used in high-speed applications such as processors and memory.
Low-power Schottky TTL
Low-power Schottky TTL (74LSxx) offers a compromise between the speed of Schottky TTL and the lower power of standard TTL. It has a propagation delay of 5-10 ns and reduces power consumption by about 20% compared to standard TTL.
Advanced Schottky TTL
Advanced Schottky TTL (74ASxx) further improves speed and reduces power compared to Schottky TTL. It has a propagation delay of 1.5-2 ns and toggle frequency up to 200 MHz, at the cost of increased power consumption.
Fairchild Advanced Schottky TTL
Fairchild Advanced Schottky TTL (74Fxx) is a proprietary variant of advanced Schottky TTL with even higher speed. It offers propagation delays as low as 1 ns and toggle frequencies up to 350 MHz, the highest of any TTL family.
CMOS Logic Families
Like TTL, CMOS has several logic families optimized for different characteristics:
4000 Series
The original CMOS logic family, 4000 series operates at 3-15 V and has very low power consumption. It is also very low speed, with typical propagation delays of 50-100 ns. 4000 series CMOS is used in low-power, low-frequency applications like battery-operated devices.
74HC
74HC is a high-speed CMOS family designed as a pin-compatible replacement for TTL. It has a propagation delay of 6-10 ns, comparable to low-power Schottky TTL, with much lower power consumption. 74HC operates at 2-6 V supply voltage.
74HCT
74HCT is a version of 74HC with TTL-compatible input logic levels. This allows 74HCT to interface directly with TTL parts without level-shifting circuitry. Like 74HC, it operates at 2-6 V and has similar speed.
74AC
74AC is an advanced CMOS family with very high speed, operating at 3.3-5 V. It has a propagation delay of just 1-3 ns, similar to advanced Schottky TTL, with much lower power consumption. 74AC is used in high-performance memory and processor circuits.
74ACT
74ACT is a TTL-compatible version of 74AC, with the same high speed and low power. It interfaces directly with both 74AC and TTL components.
Applications of TTL and CMOS
TTL and CMOS are used in a wide variety of digital logic applications. Some typical uses of each family include:
TTL
- Computers and peripherals
- Industrial control systems
- Instrumentation
- Military and aerospace systems
- Telecommunications
CMOS
- Microprocessors and microcontrollers
- Memory devices
- Mobile and portable electronics
- Automotive electronics
- Consumer appliances
- Medical devices
- Watches and clocks
In many cases, CMOS has replaced TTL due to its lower power consumption and higher integration density. However, TTL is still used in applications requiring its unique characteristics, such as high current drive and compatibility with legacy systems.
Interfacing TTL and CMOS
TTL and CMOS can be interfaced with each other, but some considerations are necessary due to their different characteristics. The main issues are:
Logic levels
TTL and CMOS have different input and output logic levels. TTL outputs are typically not high enough to reliably drive CMOS inputs, especially at higher supply voltages. CMOS outputs at 5 V are usually sufficient to drive TTL inputs, but 3.3 V CMOS may not be.
Current drive
TTL inputs require more current than CMOS inputs, and CMOS outputs typically cannot source enough current to drive TTL inputs reliably. Special CMOS families like 74HC and 74HCT have higher drive capability to interface with TTL.
Noise
TTL is more susceptible to noise than CMOS, so care must be taken to avoid coupling noise from CMOS to TTL circuitry. Proper grounding, decoupling, and shielding techniques should be used.
To interface between TTL and CMOS, level-shifting circuitry may be needed. This can be as simple as a resistor divider or transistor, or a dedicated level-shifting IC can be used. Alternatively, using a TTL-compatible CMOS family like 74HCT eliminates the need for level-shifting.
Conclusion
TTL and CMOS are two important logic families used in a wide range of digital electronics. TTL offers high speed and drive capability, while CMOS provides low power consumption and high density. Each has advantages and disadvantages that make them suitable for different applications.
Over time, CMOS has become the dominant logic family due to its low power and scalability. However, TTL remains in use for certain applications, and understanding both families is important for digital designers. Interfacing between TTL and CMOS is possible with proper consideration of their electrical characteristics.
As technology continues to advance, new logic families and circuit techniques are being developed to push the boundaries of speed, power, and density. However, the fundamental principles of TTL and CMOS continue to underlie much of modern digital electronics.
Frequently Asked Questions
What is the main difference between TTL and CMOS?
The main difference is in their power consumption and speed. TTL has higher static power consumption but is generally faster than CMOS, especially at higher integration levels. CMOS has very low static power but historically lower speed, although modern CMOS families rival TTL in performance.
Can TTL and CMOS be mixed in the same circuit?
Yes, TTL and CMOS can be mixed, but level-shifting circuitry may be needed to ensure reliable interfacing. Using a TTL-compatible CMOS family like 74HCT can simplify mixing the two.
Which logic family is best for low-power applications?
CMOS is the best choice for low-power applications due to its very low static power consumption. The 4000 series and some newer CMOS families are optimized for low-power operation.
What is the difference between 74HC and 74HCT CMOS?
74HC and 74HCT are both high-speed CMOS families with similar performance. The difference is that 74HCT has TTL-compatible input logic levels, while 74HC inputs are CMOS levels. 74HCT can interface directly with TTL parts, while 74HC may need level-shifting.
Are TTL parts still used in new designs?
TTL is still used in some new designs, particularly those requiring high drive current or compatibility with legacy TTL systems. However, CMOS has largely replaced TTL in most new applications due to its lower power consumption and higher circuit density. Most new TTL designs use advanced Schottky families for their improved speed and lower power compared to standard TTL.
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