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HCPL-M601 Datasheet(PDF) 10 Page - AVAGO TECHNOLOGIES LIMITED |
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HCPL-M601 Datasheet(HTML) 10 Page - AVAGO TECHNOLOGIES LIMITED |
10 / 11 page Propagation Delay, Pulse-Width Distortion and Propagation Delay Skew Propagation delay is a figure of merit which describes how quickly a logic signal propagates through a sys- tem. The propagation delay from low to high (tPLH) is the amount of time required for an input signal to propagate to the output, causing the output to change from low to high. Similarly, the propagation delay from high to low (tPHL) is the amount of time required for the input sig- nal to propagate to the output, causing the output to change from high to low (see Figure 7). Pulse-width distortion (PWD) results when tPLH and tPHL differ in value. PWD is defined as the difference between tPLH and tPHL and often determines the maximum data rate capability of a transmission system. PWD can be expressed in percent by dividing the PWD (in ns) by the minimum pulse width (in ns) being transmitted. Typi- cally, PWD on the order of 20-30% of the minimum pulse width is tolerable; the exact figure depends on the par- ticular application (RS232, RS422, T-1, etc.). Propagation delay skew, tPSK, is an important param- eter to consider in parallel data applications where synchronization of signals on parallel data lines is a con- cern. If the parallel data is being sent through a group of optocouplers, differences in propagation delays will cause the data to arrive at the outputs of the optocou- plers at different times. If this difference in propagation delays is large enough, it will determine the maximum rate at which parallel data can be sent through the op- tocouplers. Propagation delay skew is defined as the difference be- tween the minimum and maximum propagation delays, either tPLH or tPHL, for any given group of optocouplers which are operating under the same conditions (i.e., the same drive current, supply voltage, output load, and op- erating temperature). As illustrated in Figure 15, if the in- puts of a group of optocouplers are switched either ON or OFF at the same time, tPSK is the difference between the shortest propagation delay, either tPLH or tPHL, and the longest propagation delay, either tPLH or tPHL. As mentioned earlier, tPSK can determine the maximum parallel data transmission rate. Figure 11 is the timing diagram of a typical parallel data application with both the clock and the data lines being sent through opto- couplers. 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 represents the uncertainty of where an edge might be after being sent through an op- tocoupler. Figure 16 shows that there will be uncertainty in both the data and the clock lines. It is important that these two areas of uncertainty not overlap, otherwise the clock signal might arrive before all of the data out- puts have settled, or some of the data outputs 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 applica- tion is twice tPSK. A cautious design should use a slightly longer pulse width to ensure that any additional uncer- tainty in the rest of the circuit does not cause a prob- lem. The tPSK specified optocouplers offer the advantages of guaranteed specifications for propagation delays, pulse- width distortion and propagation delay skew over the recommended temperature, and input current, and power supply ranges. |
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