HD74AC datasheet(39/52 Pages) HITACHI

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Design Considerations

3. CMOS Bus Loading

CMOS logic devices have clamp diodes from all inputs and outputs to V

CC and ground. While these diodes

increase system reliability by damping out undershoot and overshoot noise, they can cause problems if

power is lost.

Figure 10 exemplifies the situation when power is removed. Any input driven above the V

CC pin will

forward-bias the clamp diode. Current can then flow into the device, and out V

CC or any output that is high.

Depending upon the system, this current, I

IN, can be quite high, and may not allow the bus voltage to reach

a valid high state. One possible solution to eliminate this problem is to place a series resistor in the line.

VCC

Input

Output

ICC

IIN

Iout

Figure 10 Clamp Diode Operation

4. Noise Effects

FACT offers the best noise immunity of any competing technology available today. With input thresholds

specified at 30% and 70% of V

CC and outputs that drive to within 100 mV of the rails, FACT devices offer

noise margins approaching 30% of V

CC. At 5 V VCC, FACT’s specified input and output levels give almost

1.5 V of noise margin for both ground and V

CC-born noise. With realistic input thresholds closer to 50% of

CC, the actual margins approach 2.5 V.

However, even the most advanced technology cannot alone eliminate noise problems. Good circuit board

layout techniques are essential to take full advantage of the superior performance of FACT circuits.

Well-designed circuit boards also help eliminate manufacturing and testing problems.

Another recommended practice is to segment the board into a high-speed area, a medium-speed area and a

low-speed area. The circuit areas with high current requirements (i.e., buffer circuits and high-speed logic)

should be as close to the power supplies as possible; low-speed circuit areas can be furthest away.

Decoupling capacitors should be adjacent to all buffer chips; they should be distributed throughout the

logic: one capacitor per ship. Transmission lines need to be terminated to keep reflections minimal. To

minimize crosstalk, long signal lines should not be close together.

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