MT9043 Zarlink Semiconductor, MT9043 Datasheet - Page 14

• Manufacturer Part Number: MT9043
• Description: T1/E1 System Synchronizer
• Manufacturer: Zarlink Semiconductor

The output clock should be connected directly (not AC coupled) to the OSCi input of the MT9043, and the OSCo
output should be left open as shown in Figure 8.
Crystal Oscillator - Alternatively, a Crystal Oscillator may be used. A complete oscillator circuit made up of a
crystal, resistor and capacitors is shown in Figure 9.
The accuracy of a crystal oscillator depends on the crystal tolerance as well as the load capacitance tolerance.
Typically, for a 20MHz 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 Figure 9 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 - not an overtone. 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:
Tolerance:
Oscillation Mode:
Resonance Mode:
Load Capacitance:
Maximum Series Resistance:
Approximate Drive Level:
e.g., R1B23B32-20.0MHz
(20ppm absolute,
Reset Circuit
A simple power up reset circuit with about a 50us reset low time is shown in Figure 10. Resistor R
only and limits current into the RST pin during power down conditions. The reset low time is not critical but should
be greater than 300ns.
±
6ppm 0C to 50C, 32pF, 25
20MHz
As required
Fundamental
Parallel
32pF
35
1mW
1uH inductor: may improve stability and is optional
Ω
Figure 9 - Crystal Oscillator Circuit
Ω
OSCo
OSCi
1MΩ
)
MT9043
Zarlink Semiconductor Inc.
MT9043
56pF
14
100Ω
20MHz
39pF
1uH
3-50pF
P
is for protection
Data Sheet