sx20ac Parallax, Inc., sx20ac Datasheet - Page 25

• Manufacturer Part Number: sx20ac
• Description: Configurable Communications Controllers With Ee/flash Program Memory, In-system Programming Capability And On-chip Debug
• Manufacturer: Parallax, Inc.

Parallax SX20AC/SX28AC
9.2.
It is a good engineering practice to design your system to
operate as conservatively as possible; that is as slowly as
possible.
clock rate and there is no way around it. Consider also
that since the physical size of the elements within ceramic
resonators vary inversely with the operational frequency,
you will not find a great selection of resonators designed
to operate over 50MHz.
To aid our customers who need to exploit the full-speed
capabilities of the SX, we have specified a custom TTL
oscillator that performs well throughout the industrial
temperature range. Figure 9-3 depicts how the SX is used
with the Transko 75MHz TTL oscillator.
9.3.
The external RC oscillator mode provides a cost-effective
approach for applications that do not require a precise
operating frequency. In this mode, the RC oscillator
frequency is a function of the supply voltage, the resistor
(R) and capacitor (C) values, and the operating
temperature. In addition, the oscillator frequency will
vary from unit to unit due to normal manufacturing
process variations. Furthermore, the difference in lead
© Parallax Inc.
75 MHz Operation
External RC Mode
However, some applications require a high
Figure 9-3: SX28AC/DP with
75 MHz TTL Oscillator
Page 25 of 51
frame capacitance between package types also affects the
oscillation frequency, especially for low C values. The
external R and C component tolerances contribute to
oscillator frequency variation as well.
Figure 9-4 shows the external RC connection diagram.
The recommended R value is from 3 kΩ to 100 kΩ. For R
values below 2.2 kΩ, the oscillator may become unstable,
or may stop completely. For very high R values (such as 1
MΩ), the oscillator becomes sensitive to noise, humidity,
and leakage.
Although the oscillator will operate with no external
capacitor (C = 0 pF), it is recommended that you use
values above 20 pF for noise immunity and stability. With
no or small external capacitance, the oscillation frequency
can vary significantly due to variation in PCB trace or
package lead frame capacitances.
9.4.
The internal RC mode uses an internal oscillator, so the
device does not need any external components. At
4 MHz, the internal oscillator provides typically +/-8%
accuracy over the allowed temperature range. The internal
clock frequency can be divided down to provide one of
eight lower-frequency choices by selecting the desired
value in the FUSE Word register. The frequency range is
from 31.25 KHz to 4 MHz. The default operating
frequency of the internal RC oscillator may not be 4
MHz. This is due to the fact that the SX device requires
trimming to obtain 4 MHz operation. The parts shipped
out of the factory are not trimmed. The device relies on
the programming tool provided by the third party vendors
to support trimming.
Internal RC Mode
Figure 9-4: RC
Oscillator Mode
Rev 1.6 11/20/2006
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