SI52142 Datasheet, PDF (8/20 Pages) Silicon Laboratories – PCI-EXPRESS GEN 1, GEN 2 & GEN 3 CLOCK TWO OUTPUT GENERATOR WITH 25 MHZ REFERENCE CLOCK

Si52142
Use the following formulas to calculate the trim capacitor values for Ce1 and Ce2.
Load Capacitance (each side)
Ce = 2 x CL – (Cs + Ci)
Total Capacitance (as seen by the crystal)
CLe
=
1
(
Ce1
+
1
Cs1
+
Ci1
+
1
)
Ce2 + Cs2 + Ci2
CL: Crystal load capacitance
CLe: Actual loading seen by crystal using standard value trim capacitors
Ce: External trim capacitors
Cs: Stray capacitance (terraced)
Ci : Internal capacitance (lead frame, bond wires, etc.)
2.2. OE Clarification
The OE pins are active high inputs used to enable and disable the output clocks. To enable the output clock, the OE
pin needs to be logic high and the I²C output enable bit needs to be logic high. There are two methods to disable
the output clocks: the OE is pulled to a logic low, or the I²C enable bit is set to a logic low. The OE pins is required
to be driven at all time and even though it has an internally 100 k resistor.
2.3. OE Assertion
The OE signals are active high input used for synchronous stopping and starting the output clocks respectively while
the rest of the clock generator continues to function. The assertion of the OE signal by making it logic high causes
stopped respective output clocks to resume normal operation. No short or stretched clock pulses are produced when
the clock resumes. The maximum latency from the assertion to active outputs is no more than two to six output clock
cycles.
2.4. OE Deassertion
When the OE pin is deasserted by making its logic low, the corresponding output clocks are stopped cleanly, and
the final output state is driven low.
2.5. SS[1:0] Clarification
SS[1:0] are active inputs used to select differential output frequency and enable spread of –0.5% on all DIFF
outputs as per Table 5.
Table 5. SS0 and SS1 Frequency/Spread Selection
SS1
SS0
Differential Differential Configuration
Frequency
Spread
0
0
100 MHz
Spread Off
Default
0
1
100 MHz
–0.50%
1
0
125 MHz
Spread Off
1
1
200 MHz
Spread Off
8
Preliminary 0.1