When things go wrong, listen for the heartbeat
By Bonnie Baker, Microchip Technology Inc -- EDN, January 22, 2004
Selecting the right resonator for your controller or processor is like taking care of your application's heartbeat. This selection may or may not be easy, depending on how critical clock performance is to your application circuit. However, cycling over all conditions, including start-up and overtemperature, is a requirement. If precision or real-time operation is not a concern, you can use the controller or processor's on-chip internal RC oscillator. Otherwise, you will be searching the catalog for the right resonator.
How do you identify the right resonator? A resonator is any piezoelectric device, such as a crystal or ceramic type. The required operating frequency (or the heartbeat of your application) and package type are easy to choose from a catalog. But dig a little deeper before you implement the circuit. After you choose the required frequency, you need to define the frequency tolerance, temperature stability, and external-capacitor requirements. If you don't go through this exercise, you may be one of the many who call the controller or processor vendor thinking that the controller is at fault, when many times the problem is the oscillator circuit.
As an example, take a look at a resonator circuit. Assume that you are using an AT-cut crystal resonator in its fundamental mode (no overtones) and in a parallel resonant-frequency configuration. A typical resonator configuration for a controller or processor is a Pierce oscillator (Figure 1).
The oscillator circuit in the figure must meet the Barkhausen criteria. These criteria define the status of the system-loop gain at the resonant frequency at start-up and steady state. During start-up, the loop gain exceeds unity. In steady-state conditions, the loop gain is unity with a phase equal to 2πn, where n is an integer.
The manufacturer defines the frequency, or "make," tolerance at 25°C and specifies it in parts per million (ppm). The calculation of this specification equals (high frequency-low frequency)/nominal frequency. A typical magnitude for the make tolerance of standard, off-the-shelf resonators is about 50 ppm. You may also want to pay attention to the tolerance of the frequency overtemperature. Add this error to the make tolerance as the application's temperature changes. Off-the-shelf parts are typically rated at 50 ppm/°C over the specified temperature range. The frequency-aging specification is again an additive. For an off-the-shelf part, this specification would be less than 5 to 10 ppm in the first year. If your application is demanding and the specifications of your crystal resonator need to be tighter than catalog devices, you should contact the resonator manufacturer. Although test time and lower yields will increase the manufacturer's cost, you may see your quoted price increase by only a few pennies.
Finally, you must specify the load capacitance that you need for your circuit implementation. The load capacitance is a conglomerate of several capacitors on your board, including C1 and C2 and stray capacitance (Figure 1). The formula for total load capacitance, CLOAD EQUIVALENT, in your circuit is:
CLOAD EQUIVALENT=[(C1+C2)/(C1*C2)]+CSTRAY.
The resonator manufacturer specifies the total load capacitance. CSTRAY consists of the collective capacitance from both your layout and the controller or processor pins. You usually derive this capacitance's value through bench testing. Note that C1 and C2 are usually equal in value.
So, take good care of the heartbeat of your circuit. Select your resonator with an eye on the specifications. Implement your circuit to the manufacturer's requirements. Complete the loop by verifying operation overtemperature on the bench.
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| Author Information |
 Bonnie Baker is the analog/mixed-signal-applications engineering manager for Microchip Technology's microperipherals division. You can reach her at Bonnie.Baker@microchip.com.
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