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MC10E211 데이터시트(PDF) 4 Page - ON Semiconductor |
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MC10E211 데이터시트(HTML) 4 Page - ON Semiconductor |
4 / 7 page MC10E211 MC100E211 MOTOROLA ECLinPS and ECLinPS Lite DL140 — Rev 4 2–4 APPLICATIONS INFORMATION General Description The MC10E/100E211 is a 1:6 fanout tree designed explicitly for low skew high speed clock distribution. The device was targeted to work in conjunction with the E111 device to provide another level of flexibility in the design and implementation of clock distribution trees. The individual synchronous enable controls and multiplexed clock inputs make the device ideal as the first level distribution unit in a distribution tree. The device provides the ability to distribute a lower speed scan or test clock along with the high speed system clock to ease the design of system diagnostics and self test procedures. The individual enables could be used to allow for the disabling of individual cards on a backplane in fault tolerant designs. Because of lower fanout and larger skews the E211 will not likely be used as an alternative to the E111 for the bulk of the clock fanout generation. Figure 1 shows a typical application combining the two devices to take advantage of the strengths of each. Figure 1. Standard E211 Application E111 Q8 Q0 E211 Q5 Q0 E111 Q8 Q0 Using the E211 in PECL Designs The E211 device can be utilized very effectively in designs utilizing only a +5V power supply. Since the internal switching reference levels are biased off of the VCC supply the input thresholds for the single-ended inputs will vary with VCC. As a result the single-ended inputs should be driven by a device on the same board as the E211. Driving these inputs across a backplane where significant differences between the VCC’s of the transmitter and receiver can occur can lead to AC performance and/or significant noise margin degradations. Because the differential I/O does not use a switching reference, and due to the CMR range of the E211, even under worst case VCC situations between cards there will be no AC performance or noise margin loss for the differential CLK inputs. For situations where TTL clocks are required the E211 can be interfaced with the H641 or H643 ECL to TTL Clock Distribution Chips from Motorola. The H641 is a single supply 1:9 PECL to TTL device while the H643 is a 1:8 dual supply standard ECL to TTL device. By combining the superior skew performance of the E211, or E111, with the low skew translating capabilities of the H641 and H643 very low skew TTL clock distribution networks can be realized. Handling Open Inputs and Outputs All of the input pins of the E211 have a 50k Ω to 75kΩ pulldown resistor to pull the input to VEE when left open. This feature can cause a problem if the differential clock inputs are left open as the input gate current source transistor will become saturated. Under these conditions the outputs of the CLK input buffer will go to an undefined state. It is recommended, if possible,that the SCLK input should be selected any time the differential CLK inputs are allowed to float. The SCLK buffer, under open input conditions, will maintain a defined output state and thus the Q outputs of the device will be in a defined state (Q = LOW). Note that if all of the inputs are left open the differential CLK input will be selected and the state of the Q outputs will be undefined. With the simultaneous switching characteristics and the tight skew specifications of the E211 the handling of the unused outputs becomes critical. To minimize the noise generated on the die all outputs should be terminated in pairs, ie. both the true and compliment outputs should be terminated even if only one of the outputs will be used in the system. With both complimentary pairs terminated the current in the VCC pins will remain essentially constant and thus inductance induced voltage glitches on VCC will not occur. VCC glitches will result in distorted output waveforms and degradations in the skew performance of the device. The package parasitics of the 28-lead PLCC cause the signals on a given pin to be influenced by signals on adjacent pins. The E211 is characterized and tested with all of the outputs switching, therefore the numbers in the data book are guaranteed only for this situation. If all of the outputs of the E211 are not needed and there is a desire to save power the unused output pairs can be left unterminated. Unterminated outputs can influence the propagation delay on adjacent pins by 15ps - 20ps. Therefore under these conditions this 15ps - 20ps needs to be added to the overall skew of the device. Pins which are separated by a package corner are not considered adjacent pins in the context of propagation delay influence. Therefore as long as all of the outputs on a single side of the package are terminated the specification limits in the data sheet will apply. |
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