HCPL-261A Datasheet, PDF (16/16 Pages) Agilent(Hewlett-Packard) – HCMOS Compatible, High CMR, 10 MBd Optocouplers

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.
I_F
50%
V_O
1.5 V
TPHL
I_F
50%
V_O
1.5 V
TPLH
t _PSK
Figure 25. Illustration of Propagation Delay Skew – t_PSK.
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 t_PSK. 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.
DATA
INPUTS
CLOCK
DATA
OUTPUTS
CLOCK
t _PSK
t _PSK
Figure 26. Parallel Data Transmission Example.
The t_PSK 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.
1-181