Lab3update - Input VOH Non-inverting Output tr Inverting...

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38 Lab 3 D R A F T DRAFT CONTINUATION OF SECTION 3.1.4 OF LAB MANUAL: As an example of navigating a datasheet to f nd the correct values for propagation delay, consider the excerpt from the CD74HCT132 datasheet in Figure 3.9, where shading has been added: 1. Start by observing that this datasheet also includes information for HC types. Since we are interested in the HCT, ignore the upper shaded rows and skip to the bottom section. 2. Notice that there are three groups of columns with different temperature headings. The “25°C” columns represent ideal operation at room temperature, and they include idealized “typical” values. Ignore those three columns. The “-40°C TO 85°C” columns are realistic for normal industrial operation, but unless you have speci f c instructions to design something for those conditions, you have to look further. The “-55°C TO 125°C” columns are the largest propagation delays and transition times, because they cover the largest extremes, so our focus now is on a very small section of the table that is not shaded. 3. Although there is a MIN column with no values, we would not be interested even if there were values there. Only worst-case (slowest) values matter. Also, there are no values for the 5V V CC V OH V OL V OH V OL Input Non-inverting Output Inverting Output 90% 90% 10% 10% t f t r t r t f Figure 3.8. This diagram done in the style of a typical datasheet shows the rise and fall time definitions, consistent with the earlier 10%/90% discussion. Often, this information will be shown on the SAME diagram with propagation delays, but here it is separated for clarity.
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Lab 3 39 power condition. Normal 5V logic assumes that the power supply can vary +/- 10%, from 4.5V to 5.5V. Logic runs slower at lower voltages, so 4.5V represents the worst-case. Had there been values for the 5V condition, it would be another example of typical, not worst-case conditions. 4. Thus, the worst case propagation delay is 50 ns, and the worst-case transition time (rise/fall time) is 22 ns. This example has illustrated most of the common pitfalls of reading switching characteristics in datasheets. One remaining complication is that some datasheets, such as those for LSTTL parts, will give separate values for t PHL and t PHL . You should pick the larger of the two values in such cases. 3.1.5 Tri-state outputs and busses Busses are an extremely important aspect of modern digital design because they enable a large number of devices to communicate with each other while minimizing the amount of wiring involved. For Figure 3.9. Switching specification for the Texas Instruments CD74HCT132 Quad 2-input NAND Schmitt Trigger, a device not used in these exercises, but with similar datasheets (Copyright © 2004, Texas Instruments, used by permission with inclusion of all information in Appendix E).
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This note was uploaded on 04/19/2010 for the course ECE 2040 taught by Professor Yili during the Spring '08 term at Georgia Institute of Technology.

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Lab3update - Input VOH Non-inverting Output tr Inverting...

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