SY89841U datasheet(10/15 Pages) MICREL

Part #	SY89841U

Description	Precision LVDS Runt Pulse Eliminator 2:1 Multiplexer

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Manufacturer	MICREL [Micrel Semiconductor]

Direct Link	http://www.micrel.com

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10 / 15 page

Micrel, Inc.

SY89841U

February 2005

M9999-021405

hbwhelp@micrel.com or (408) 955-1690

10

Case #4: Input Clock Failure: Switching from the

selected clock input stuck in an undetermined state

to a valid clock input (RPE enabled).

If CLK1 fails to an undetermined state (e.g.,

amplitude falls below the 100mV (VIN) minimum

single-ended input limit, or 200mV differentially)

before the RPE MUX selects CLK2 (using the SEL

pin), the switchover to the valid clock CLK2 will

occur either following Case #2 or Case #3,

depending upon the last valid state at the CLK1

If the selected input clock fails to a floating, static, or

extremely low signal swing, including 0mV, the FSI

function will eliminate any metastable condition and

guarantee a stable output signal. No ringing and no

undetermined state will occur at the output under

these conditions.

Please note that the FSI function will not prevent

duty cycle distortions or runt pulses in case of a

slowly deteriorating (but still toggling) input signal.

Due to the FSI function, the propagation delay will

depend on rise and fall time of the input signal and

on its amplitude. Refer to “Typical Operating

Characteristics” for detailed information.

CLK1

CLK2

SEL

OUTPUT

Select CLK1

Select CLK2

as in case #2

as in case #3

Timing Diagram 4

Power-On Reset (POR) Description

The SY89841U includes an internal power-on reset

(POR) function to ensure the RPE logic starts-up in

a known logic state once the power-supply voltage is

stable. An external capacitor connected between VCC

and the CAP pin (pin 11) controls the delay for the

power-on reset function.

The required capacitor value calculation is based

upon the time the system power supply needs to

power up to a minimum of 2.3V. The time constant

for the internal power-on-reset must be greater than

the time required for the power supply to ramp up to

a minimum of 2.3V.

The following formula describes this relationship:

C( F)

t

(ms)

12(ms/ F)

dPS

µ

≥

µ

As an example, if the time required for the system

power supply to power up past 2.3V is 12ms, then

the required capacitor value on pin 11 would be:

C( F)

12ms

12(ms/ F)

µ

≥

µ

C1 F

≥ µ

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