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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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HCPL0738 датащи(PDF) 7 Page - Agilent(Hewlett-Packard) |
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HCPL0738 датащи(HTML) 7 Page - Agilent(Hewlett-Packard) |
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7 / 8 page  7 the optocouplers at different times. If this difference in propa- gation delays is large enough, it will determine the maximum rate at which parallel data can be sent through the optocouplers. Propagation delay skew is defined as the difference between the minimum and maximum propaga- tion delays, either t PLH or tPHL, for any given group of optocouplers which are operating under the same conditions (i.e., the same supply voltage, output load, and operating temperature). As illustrated in Figure 8, if the inputs of a group of optocouplers are switched either ON or OFF at the same time, t PSK is the differ- ence between the shortest propa- gation delay, either t PLH or tPHL, and the longest propagation de- lay, either t PLH or tPHL. As mentioned earlier, t PSK can determine the maximum parallel data transmission rate. Figure 8 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 sig- nal 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 7 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 out- puts may start to change before the clock signal has arrived. From these considerations, the absolute minimum pulse width that can be sent through optocouplers in a parallel appli- cation 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. 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, and power supply ranges. Figure 7. Propagation delay skew waveform. Figure 8. Parallel data transmission example. 50% 50% tPSK VI VO VI VO 2.5 V, CMOS 2.5 V, CMOS DATA INPUTS CLOCK DATA OUTPUTS CLOCK tPSK tPSK |
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