HD74AC datasheet(5/52 Pages) HITACHI

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FACT Descriptions and Family Characteristics

1.7.2 Dynamic Output Drive

Traditionally, in order to predict what incident wave voltages would occur in a system, the designer was

required to do an output analysis using a Bergeron diagram. Not only is this a long and time-consuming

operation, but the designer needed to depend upon the accuracy and reliability of the manufacturer-supplied

‘typical’ output I/V curve. Additionally, there was no way to guarantee that any supplied device would

meet these ‘typical’ performance values across the operating voltage and temperature limits. Hitachi has

taken the necessary steps to guarantee incident wave switching on transmission lines with impedances as

low as 50

Ω.

Figure 2 shows a Bergerton diagram for switching both high-to-low and low-to-high. On the right side of

the graph (I

out > 0), are the VOH and IIH curves for FACT logic while on the left side (Iout < 0), are the curves

for V

OL and IIL. Although we will only discuss here the low-to-high transition, the information presented

may be applied to a high-to-low transition.

Current (A)

0.1

0.2

–0.1

–0.2

–2

–1

VOH/IOH

HIGH-to-LOW

LOW-to-HIGH

Line 1

Slope = 50

Ω

Line 2

Slope = 50

Ω

VIN/IIN

VOL/IOL

Figure 2 Gate Driving 50

Ω Line Reflection Diagram

Begin analysis at the V

OL (quiescent) point. This is the intersection of the VOL/IOL curve for the output and

the V

IN/IIN curve for the input. For CMOS inputs and outputs, this point will be approximately 100 mV.

Then draw a 50

Ω load line from this intersection to the V

OH/IOH curve as shown by Line 1. This

intersection is the voltage that the incident wave will have. Here it occurs at approximately 3.95 V. Then

draw a line with a slope of –50

Ω from this first intersection point to the V

IN/IIN curve as shown by Line 2.

This second intersection will be the first reflection back from the input gate. Continue this process of

drawing the load line from each intersection to the next. Lines terminating on the V

OH/IOH curve should

have positive slopes while lines terminating on the V

IN/IIN curve should have negative slopes.

Each intersection point predicts the voltage of each reflected wave on the transmission line. Intersection

points on the V

OH/IOH curve will be waves travelling from the driver to the receiver while intersection points

on the V

IN/IIN curve will be waves travelling from the receiver to the driver.

Figure 3 and 4 show the resultant waveforms. Each division on the time scale represents the propagation

delay of the transmission line.

HD74AC Datasheet(PDF) 5 Page - Hitachi Semiconductor

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