Functional Description

The 11C90 contains four ECL Flip-Flops an ECL to TTL

converter and a Schottky TTL output buffer with an active

pull-up Three of the Flip-Flops operate as a synchronous

shift counter driving the fourth Flip-Flop operating as an

asynchronous toggle The internal feedback logic is such

that the TTL output and the Q ECL output are HIGH for six

clock periods and LOW for five clock periods The Mode

Control (M) inputs can modify the feedback to make the

output HIGH for five clock periods and LOW for five clock

periods as indicated in the Count Sequence Table

The feedback logic is such that the instant the output goes

HIGH the circuit is already committed as to whether the

output period will be 10 or 11 clock periods long This

means that subsequent changes in an M input signal in-

cluding decoding spikes will have no effect on the current

output period The only timing restriction for an M input sig-

nal is that it be in the desired state at least a setup time

before the clock that follows the HHLL state shown in the

table The allowable propagation delay through external log-

ic to an M input is maximized by designing it to use the

positive transition of the 11C90 output as its active edge

This gives an allowable delay of ten clock periods minus

the CP to Q delay of the 11C90 and the M to CP setup time

If the external logic uses the negative output transition as its

active edge the allowable delay is reduced to five clock

periods minus the previously mentioned delay and setup

time

Capacitively coupled triggering is simplified by the 400X re-

By connecting this to the CP input as shown in

Figure 3 the

clock is automatically centered about the input threshold A

clock duty cycle of 50% provides the fastest operation

since the Flip-Flops are Master-Slave types with offset clock

thresholds between master and slave This feature ensures

that the circuit will operate with clock waveforms having

very slow rise and fall times and thus there is no maximum

frequency restriction Recommended minimum and maxi-

mum clock amplitude as a function of a frequency and tem-

perature are shown in the graph labeled

Figure 2 When the

CP or any other input is driven from another ECL circuit

normal ECL termination methods are recommended One

method is indicated in

Figure 4 Other ECL termination

methods are discussed in the F100K ECL Design Guide

(Section 5 of Databook)

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FIGURE 2 AC Coupled Triggering Characteristics

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FIGURE 3 Capacitively Coupled Clocking

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50

75

100

806

121

162

130

196

261

FIGURE 4 Clocking by ECL Source via Terminated Line

When an M input is to be driven from a TTL output operating

resistor can be used to pull the TTL output up as shown in

Figure 5 Some types of TTL outputs will only pull up to

11C90 inputs The resistor will pull the signal up through the

threshold region although this final rise may be somewhat

slow depending on wiring capacitance A resistor network

that gives faster rise and also lower impedance is shown in

Figure 6

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FIGURE 5 Using Internal Pull-Up with TTL Source

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FIGURE 6 Faster Low Impedance TTL to ECL Interface

6