HD74AC Datasheet, PDF (39/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

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.
I_IN
Input
I_CC ^VCC
Iout
Output
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 V_CC, 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
V_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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