 Crystal Unit Application Notes |
| This application note describes the selection of a crystal used with any type of micro-controller that accepts a parallel mode, AT or BT cut crystal, fundamental or third-overtone mode. |
| Circuit Description |
| Most chips include an inverter design with a positive feedback resistor (typical 1 M ohm) with an optional series resistor with value varied from 100 to 1k ohm (See Fig 8). |
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| Figure8.Oscillation Circuit |
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It has an input port, normally called (XIN) and an output port (XOUT) for crystal connections between those two ports. Most chips are designed with an option either driven by an external crystal oscillator fed to the crystal input port, or with an external crystal. |
| Depending on frequency, crystals can be selected as fundamental or an overtone mode. Normally, frequency above 35 MHz requires the third overtone mode for price advantage and delivery. In parallel mode, where the crystal reactance is inductive, two external capacitors Cl and C2 are required for a necessary phase shift in oscillation. Cl and C2 are needed whether the crystal is in fundamental mode or overtone mode. Values of Cl and C2 are specified by the chip manufacturer and vary from 6pF to 47pF. Cl and C2 may not be balanced, e g, equal in value, but sometimes are offset in a particular ratio (Cl/C2) for best performance, depending on crystal and amplifier characteristics and board layout. (See Fig 9) shows a typical configuration for a fundamental mode operation. |
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| Figure 9. Fundamental Mode Oscillation Circuit |
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| In an overtone mode, an additional inductor Ll and capacitance Cc is required to select the third overtone mode while suppressing or rejecting the fundamental mode. Choose Ll and Cc component values in the third overtone crystal circuit to satisfy the following conditions: |
1. The LlCc components from a series resonant circuit at a frequency below the fundamental frequency, which makes the circuit look inductive at fundamental frequency. This condition does not favor to oscillation at fundamental mode.
2. The LlCc and C2 components form a parallel resonant circuit at a frequency about half-way between the fundamental and third overtone frequency. This condition makes the circuit capacitive at the third-overtone frequency, which favors the oscillation at the desired overtone mode. (See Fig 10).
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| Figure 10. Third Overtone Mode Oscillation Circuit |
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3. In a standard overtone mode, C2 value varies from 10pF to 30pF. Cc value should be chosen at least 10 times the value of C2, so its equivalent Cequiv. will be approximately the value of C2.
4. Typical values of Ll for different crystal frequencies:
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25 MHz 4.7uH, 6.8uH, 8.2uH, l0uH
32 MHz 2.7uH, 3.9uH, 4.7uH, 5.6uH
40 MHz 1.5uH, 1.8uH, 2.2uH, 2.7uH, 3.3uH |
| Negative Resistance |
| For optimum performance, it is recommended to measure the negative resistance of oscillation circuit. As Fig. 11 show below, raise one end of the crystal from the oscillation circuit and insert a variable resistor beginning with a low value. Monitor the waveform with the oscilloscope, and continue increasing the value of inserted Vr (Variable Resistor), until the circuit will show no oscillation signal on oscilloscope. The value at which oscillation stops represents negative resistance. It is recommended that the negative resistance value of the oscillation circuit should generally be at least five ~ ten times of ESR max. |
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| Figure 11 Negative Resistance Measuring |
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