SX28ACSLASHDP datasheet(25/51 Pages) PARALLAX

25 / 51 page

Parallax SX20AC/SX28AC

www.parallax.com

Page 25 of 51

Rev 1.6 11/20/2006

9.2.

75 MHz Operation

It is a good engineering practice to design your system to

operate as conservatively as possible; that is as slowly as

possible.

However, some applications require a high

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.

Figure 9-3: SX28AC/DP with

75 MHz TTL Oscillator

9.3.

External RC Mode

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

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.

Internal RC Mode

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.

Figure 9-4: RC

Oscillator Mode

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