Page 110 - Embedded Microprocessor Systems Real World Design

The stability of a Pierce crystal oscillator usually is in the range of 0.1 percent.
This is accurate enough for many applications, but if you are designing something
that must keep track of the time of day, this will not be close enough. A 0.1 percent
error is a drift of almost 1.5 minutes per day. For applications that are extremely
sensitive to frequency, you will need a very accurate oscillator design, an external
oscillator, or a means to adjust the crystal frequency.

Fundamental Mode

I got a call from the factory one day. The company had a batch of processor boards
that worked, but everything worked funny. The response was slow and the machine
would not function correctly, although all the operator screens looked okay. It
turned out that the 24MHz crystals installed on the boards were overtone crystals.
In our circuit, they operated at their fundamental frequency of 8MHz. Make sure
you spec$ the right crystals and the right circuit around the crystals. This most
likely is a problem with AT-cut crystals in the 20MHz to 30MHz range, where you
can get both types. Below 20MHz, most crystals are fundamental; above 30MHz,
most crystals are overtone. This is because the crystal blank gets very thin when you
try to make a high-frequency, fundamental-mode crystal.

Ceramic Resonators
A ceramic resonator is less expensive than a crystal but less accurate as well. Ceramic
resonators typically have a frequency accuracy about one tenth that of crystals.

External Oscillators
The two considerations for using an external oscillator are the drive level and the
processor connection. Most modern microprocessors have CMOS technology, so
an oscillator with CMOS drive levels must be used. Since most microcontrollers
provide two pins for a crystal, you drive only one when using an external oscilla-
tor. Make sure you drive the right one and terminate the other as specified by the
manufacturer. Some processors need the unused pin to be floating, some want it
tied low, and so on.

Hardware Checklist

The following is a summary of the information scattered through the chapter.
Verify EPROM, RAM, and peripheral access times. Add wait states if necessary.
Vew that setup and hold times are met, both to the peripheral device and to
the microprocessor.

92 Embedded Microprocessor Systems