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Design Considerations,

Testing and Applications 3

Assistance Form

FAST AND LS TTL

FAST AND LS TTL DATA

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FAN-IN AND FAN-OUT. In order to simplify designing with Motorola TTL devices, the input and output loading parameters of all families are normalized to the following values:

1 TTL Unit Load (U.L.) = 40 μA in the HIGH state (Logic ª1º)

1 TTL Unit Load (U.L.) = 1.6 mA in the LOW state (Logic ª0º)

Input loading and output drive factors of all products described in this handbook are related to these definitions.

EXAMPLES Ð INPUT LOAD

1.A 7400 gate, which has a maximum IIL of 1.6 mA and IIH of 40 μA is specified as having an input load factor of 1 U.L. (Also called a fan-in of 1 load.)

2.The 74LS95B which has a value of IIL = 0.8 mA and IIH of 40 μA on the CP terminal, is specified as having an input LOW load factor of:

3. The 74LS00 gate which has an IIL of 0.4 mA and an IIH of 20 μA, has an input LOW load factor of:

EXAMPLES Ð OUTPUT DRIVE

1.The output of the 7400 will sink 16 mA in the LOW (logic ª0º) state and source 800μA in the HIGH (logic ª1º) state. The normalized output LOW drive factor is therefore:

16 mA = 10 U.L.

1.6 mA

and the output HIGH drive factor is

800 μA

or 20 U.L.

40μA

2.The output of the 74LS00 will sink 8.0 mA in the LOW state and source 400 μA in the HIGH state. The normalized output LOW drive factor is:

8.0 mA = 5 U.L.

1.6 mA

and the output HIGH drive factor is

400 μA

or 10 U.L.

40 μA

Relative load and drive factors for the basic TTL families are given in Table 3.3.

Table 3.3

Values for MSI devices vary significantly from one element to another. Consult the appropriate data sheet for actual characteristics.

FAST AND LS TTL DATA

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LINE DRIVING Ð Because of its superior capacitive drive characteristics, TTL logic is often used in line driving applications which require various termination techniques to maintain signal integrity. Parameters associated with this application are listed in Table 3.4.

It is also often necessary to construct load lines to determine reflection waveforms in line driving applications. The input and output characteristics graphs of section 3 (Figs. 2-4, 2-7 and 2-8) can be very useful for this purpose.

OUTPUT RISE AND FALL TIMES provide important information in determining reflection waveforms and crosstalk coefficients. Typical rise and fall times are approximately 6 ns for LS and about 2.0 ns for FAST with a 50 pF load (measured 10±90%). Output rise and fall times become longer as capacitive load is increased.

INTERCONNECTION DELAYS. For those parts of a system in which timing is critical, designers should take into account the finite delay along the interconnections. These range from about 0.12 to 0.15 ns/inch for the type of interconnections normally used in TTL systems. Exceptions occur in systems using ground planes to reduce ground noise during a logic transition; ground planes give higher distributed capacitance and delays of about 0.15 to 0.22 ns/inch.

Most interconnections on a logic board are short enough that the wiring and load capacitance can be treated as a lumped capacitance for purposes of estimating their effect on the propagation delay of the driving circuit. When an interconnection is long enough that its delay is one-fourth to one-half of the signal transition time, the driver output waveform exhibits noticeable slope changes during a transition. This is evidence that during the initial portion of the output voltage transition the driver sees the characteristic impedance of the interconnection (normally 100 Ω to 200 Ω), which for transient conditions appears as a resistor returned to the quiescent voltage existing just before the beginning of the transition. This characteristic impedance forms a voltage divider with the driver output impedance, tending to produce a signal transition having the same rise or fall time as in the no-load condition but with a reduced amplitude. This attenuated signal travels to the far end of the interconnection, which is essentially an unterminated transmission line, whereupon the signal starts doubling. Simultaneously, a reflection voltage is generated which has the same amplitude and polarity as the original signal, e.g., if the driver output signal is positive-going the reflection will be positive-going, and as it travels back toward the driver it adds to the line voltage. At the instant the reflection arrives at the driver it adds algebraically to the still-rising driver output, accelerating the transition rate and producing the noticeable change in slope.

Table 3.4

Output Characteristics for Schottky TTL Logic

If an interconnection is of such length that its delay is longer than half the signal transition time, the attenuated output of the driver has time to reach substantial completion before the reflection arrives. In the limit, the waveform observed at the driver output is a 2-step signal with a pedestal. In this circumstance the first load circuit to receive a full signal is the one at the far end, because of the doubling effect, while the last one to receive a full signal is the one nearest the driver since it must wait for the reflection to complete the transition. Thus, in a worst-case situation, the net contribution to the overall delay is twice the delay of the interconnection because the initial part of the signal must travel to the far end of the line and the reflection must return.

When load circuits are distributed along an interconnection, the input capacitance of each will cause a small reflection having a polarity opposite that of the signal transition, and each capacitance also slows the transition rate of the signal as it passes by. The series of small reflections, arriving back at the driver, is subtractive and has the effect of reducing the apparent amplitude of the signal. The successive slowing of the transition rate of the transmitted signal means that it takes longer for the signal to rise or fall to the threshold level of any particular load circuit. A rough but workable approach is to treat the load capacitances as an increase in the intrinsic distributed capacitance of the interconnection. Increasing the distributed capacitance of a transmission line reduces its impedance and increases its delay. A good approximation for ordinary TTL interconnections is that distributed load capacitance decreases the characteristic impedance by about one-third and increases the delay by one-half.

FAST AND LS TTL DATA

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