Just For Starters
and some less). Capacitors take a finite time to charge
and slow down the rise and fall times of an IC's output
driver. AC specifications, such as propagation delays,
are often accompanied with a maximum load capacitance. Values from 20 to 50 pF are typical. A rough fan-out estimate is obtained by dividing the maximum
capacitance by 10 pF. This drops the fan-out to just several inputs rather than 20 or 30.
If an IC has much faster AC specifications than your
application requires, you may experiment with higher
fan-out than what is actually allowed by the capacitance
numbers. It is important to understand the compromise
being made.
As seen in Table 1, HCT has lower input current
than 74LS and 74F, which enables it to consume less
power at steady-state. (Capacitance dominates power
consumption during signal transitions due to the
charge/discharge process.) 74HCT also has symmetrical output drive strength.
Table 1 shows that 74HCT is slower than 74LS and
74F. 74ACT increases the speed and drive strength of
CMOS logic, though it is not as fast as 74F. Both CMOS
variants do a better job than TTL of driving outputs closer to the power rails.
Low-Voltage Devices
CMOS Logic
Complementary MOS (CMOS) logic is a lower
power alternative to bipolar. Several CMOS 7400 families were developed to complement the TTL variants.
Some families such as C and HC are function-compati-ble with TTL, but have different voltage thresholds.
Different thresholds make it difficult to mix different
technologies on the same circuit board. 74HCT was
developed for the low-power benefits of CMOS while
retaining TTL-compatible voltage thresholds.
Discrete logic has taken a diminished role in mainstream system design due to high-density ICs. More and
more systems have abandoned 5-volt logic in favor of
3. 3, 2.5, 1.8, and 1.5-volt technologies. The 7400 family has been energized in recent years by new variants
from several manufacturers. Fairchild Semiconductor
offers the 74LCX family that operates at lower supply
voltages with relatively fast propagation delays.
Which Family to Pick?
Choosing a logic family is highly application specific. If you are prototyping with mature technologies,
5-volt logic may be appropriate. The next aspect to consider is how fast your ICs need to operate. Generally
speaking, it is best to use slower devices with lower
power consumption whenever possible.
Slower devices have fewer tricky problems and it is
easier to work with lower currents. Then there are handling concerns of CMOS versus TTL. CMOS devices are
sensitive to static electricity.
If you're not careful, you can zap a chip and destroy
it. Finally, the choice may be made for you by the other
devices that you are working with. If you need to
work with a 3.3-volt IC, you'll have to pick a low-voltage
family. NV
About the Author
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NUTS & VOLTS
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Mark Balch is the author of Complete
Digital Design (see www.completedigitaldesign.com) and
works in the Silicon Valley high-tech industry. His responsibilities
have included PCB, FPGA, and ASIC design. Mark has designed
products in the fields of telecommunications, HDTV, consumer
electronics, and industrial computers. In addition to his work in
product design.
Mark has actively participated in industry standards committees and has presented work at technical conferences. Mark holds
a bachelor's degree in electrical engineering from The Cooper
Union in New York City. He can be reached via Email at
mark@completedigitaldesign.com.
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