HCPL-2602 datasheet(16/16 Pages) HP

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1-181

Figure 25. Illustration of Propagation Delay Skew – tPSK.

50%

1.5 V

I F

50%

I F

tPSK

1.5 V

TPHL

TPLH

Figure 26. Parallel Data Transmission Example.

DATA

t PSK

INPUTS

CLOCK

DATA

OUTPUTS

CLOCK

t PSK

data transmission rate. Figure 26

is the timing diagram of a typical

parallel data application with both

the clock and the data lines being

sent through optocouplers. The

figure shows data and clock

signals at the inputs and outputs

of the optocouplers. To obtain the

maximum data transmission rate,

both edges of the clock signal are

being used to clock the data; if

only one edge were used, the

clock signal would need to be

twice as fast.

Propagation delay skew repre-

sents the uncertainty of where an

edge might be after being sent

through an optocoupler. Figure

26 shows that there will be

uncertainty in both the data and

the clock lines. It is important

that these two areas of uncer-

tainty not overlap, otherwise the

clock signal might arrive before

all of the data outputs have

settled, or some of the data

outputs may start to change

before the clock signal has

arrived. From these considera-

tions, the absolute minimum

pulse width that can be sent

through optocouplers in a parallel

application is twice tPSK. A

cautious design should use a

slightly longer pulse width to

ensure that any additional

uncertainty in the rest of the

circuit does not cause a problem.

The tPSK specified optocouplers

offer the advantages of guaran-

teed specifications for propaga-

tion delays, pulse-width

distortion, and propagation delay

skew over the recommended

temperature, input current, and

power supply ranges.

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