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MC100E211

5V ECL 1:6 Differential Clock Distribution Chip

ON Semiconductor 

MC10E211, MC100E211

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.

E111

Q0

E211

Q0

Q8

E111

Q0

Q5

Q8

Figure 1. Standard E211 Application

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. 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 50kΩ 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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