MT90812 Datasheet, PDF (33/105 Pages) Mitel Networks Corporation – Integrated Digital Switch (IDX)

Advance Information
MT90812
9.5 C8P Pin Timing Source
The MT90812 can use either a clock or crystal, connecting to pins C8P_C16i and OSCo, as a reference timing
source.
9.5.1 Clock Oscillator
Fig. 19 shows a 8.192MHz clock oscillator, with 32 ppm tolerance, directly connected to C8P_C16i pin of the
MT90812. The output clock should be connected directly (not AC coupled) to the C8P_C16i pin, and the OSCo
output should be left open.
MT90812
C8P_C16i
+5V
+5V
8.192MHz
GND
0.1uF
OSCo
No Connection
Figure 19 - Clock Oscillator Circuit
9.5.2 Crystal Oscillator
Alternatively, a Crystal Oscillator may be used. A complete oscillator circuit made up of a crystal, resistor and
capacitors is shown in Fig. 20.
The accuracy of a crystal oscillator depends on the crystal tolerance as well as the load capacitance tolerance.
Typically, for a 8.192MHz crystal specified with a 32pF load capacitance, each 1pF change in load capacitance
contributes approximately 9ppm to the frequency deviation. Consequently, capacitor tolerances, and stray
capacitances have a major effect on the accuracy of the oscillator frequency.
The trimmer capacitor shown in Fig. 20 may be used to compensate for capacitive effects. If accuracy is not a
concern, then the trimmer may be removed, the 39pF capacitor may be increased to 56pF, and a wider
tolerance crystal may be substituted.
The crystal should be a fundamental mode type. The fundamental mode crystal permits a simpler oscillator
circuit with no additional filter components and is less likely to generate spurious responses. The crystal
specification is as follows:
Frequency:
8.192MHz
Tolerance:
As required
Oscillation Mode:
Fundamental
Resonance Mode:
Parallel
Load Capacitance:
20pF
Maximum Series Resistance: 35Ω
Approximate Drive Level:
1mW
e.g. CTS R1B23B32-8.192MHz
(20ppm absolute 6 ppm 0C to 50C, 32pF, 25
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