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Introduction
1.12 Crystal Oscillator Consideration
SEC ASIC
1-45
STD110
1.12
Crystal Oscillator
Consideration
1.12.1 OVERVIEW
STD110 contains a circuit commonly referred to as an “on-chip oscillator.” The
on-chip circuit itself is not an oscillator but an amplifier which is suitable for being
used as the amplifier part of a feedback oscillator. With proper selection of off-
chip components, this oscillator circuit performs better than any other types of
clock oscillators.
It is very important to select suitable off-chip components to work with the on-
chip oscillator circuitry. It should be noted, however, that SEC cannot assume the
responsibility of writing specifications for the off-chip components of the
complete oscillator circuit, nor of guaranteeing the performance of the finished
design in production, any more than a transistor manufacturer, whose data
sheets show a number of suggested amplifier circuits, can assume responsibility
for the operation, in production, of any of them.
We are often asked why we don’t publish a list of required crystal or ceramic
resonator specifications, and recommend values for the other off-chip
components. This has been done in the past, but sometimes with consequences
that were not intended.
Suppose we suggest a maximum crystal resistance of 30ohms for some given
frequency. Then your crystal supplier tells you the 30ohm crystals are going to
cost twice as much as 50ohm crystals. Fearing that SEC will not “guarantee
operation” with 50ohm crystals, you order the expensive ones.
In fact, SEC guarantees only what is embodied within an SEC product. Besides,
there is no reason why 50ohm crystals couldn’t be used, if the other off-chip
components are suitably adjusted.
Should we recommend values for the other off-chip components Should we do
for 50ohm crystals or 30ohm crystals With respect to what should we optimize
their selection Should we minimize start-up time or maximize frequency
stability
In many applications, neither start-up time nor frequency stability is particularly
critical, and our “recommendations” are only restricting your system to
unnecessary tolerances. It all depends on the application.
1.12.2 OSCILLATOR DESIGN CONSIDERATIONS
ASIC designers have a number of options for clocking the system. The main
decision is whether to use the “on-chip” oscillator or an external oscillator. If the
choice is to use the on-chip oscillator, what kinds of external components are to
use an external oscillator, what type of oscillator would it be
The decisions have to be based on both economic and technical requirements.
In this section we will discuss some of the factors that should be considered.