
1.10 Crystal Oscillator Consideration
Introduction
STD150
1-36
Samsung ASIC
1.10
Crystal Oscillator
Consideration
1.10.1 OVERVIEW
The STD150 library contains a cell commonly referred to as an
on-chip oscillator
.
The on-chip oscillator itself is not really an oscillator, but is an amplifier suitable
for being used as the feedback amplifier in an oscillator circuit. With proper
selection of off-chip components (crystal or ceramic resonator, resistors and
capacitors) this oscillator circuit performs better than other types of clock
oscillators.
It is very important to select suitable off-chip components for the on-chip oscillator
circuitry. It should be noted, however, that Samsung cannot assume
responsibility for writing specifications for the off-chip components or for the
performance of the finished oscillator design in production since the optimization
of the crystal oscillator circuit will be specific to a given application. Samsung
does, however, spec and guarantee the performance of the on-chip oscillator cell.
1.10.2 OSCILLATOR DESIGN CONSIDERATIONS
SoC designers have a number of options for clocking the system. A primary
decision is whether to use an on-chip oscillator or an external oscillator. If the
choice is to use an on-chip oscillator, the designer must then choose the type of
oscillator and off-chip component values. These decisions will be based on both
economic and technical requirements. The following section discusses some of
the factors to be considered.
1.10.2.1 On-Chip Oscillator
In most cases, the on-chip oscillator with the appropriate external components
provides the most economical solution to the clocking problem. Exceptions may
arise in server environments when frequency tolerances are tighter than about
0.01%.
The external components commonly used for the oscillator circuit are a positive
reactance (normal crystal oscillator), two capacitors, C1 and C2, and two
resistors, Rf and Rx, as shown in the Figure below.
Figure 1-14.
CMOS Oscillator
1.10.2.2 Crystal Specifications
Specifications for an appropriate crystal are not very critical. Any fundamental
mode crystal of medium or better quality can be used. Crystal resistance affects
start-up time and steady state amplitude but can be compensated by the choice
of C1 and C2, however, the lower the crystal resistance, the better. A discussion
of external R and C components follows below.
C1
C2
Rx
Rf
PADA
PADY
Feedback
Amplifier
Inside of a Chip