SY89843U Datasheet, PDF (11/17 Pages) Micrel Semiconductor – Precision LVPECL Runt Pulse Eliminator 2:1 MUX with 1:2 Fanout and Internal Termination

Micrel, Inc.
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 (V_IN) 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
SY89843U
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 more detailed information.
Timing Diagram 4
Power-On Reset (POR) Description
The SY89843U 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
V_CC and the CAP pin (pin 16) controls the delay for
the power-on reset function.
Calculating the required capacitor value is based on
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 equation describes this relationship:
C(µF) ≥ ^{tdPS (ms)}
12(ms / µF )
As an example, if the time required for the system
power supply to power up past 2.3V is 12ms, the
required capacitor value on pin 16 would be:
12ms
C(µF) ≥
12(ms / µF )
C(µF) ≥ 1µF
March 2005
11
M9999-031505
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