3 TCAN455x Clock Structure

The TCAN455x clock circuit contains three main blocks, an Amplifier and Bias Control block, a Filter and Comparator block, and a Clock Input Detection block as shown in Figure 1-1. A Comparator is used to compare the voltage difference between the OSC1 and OSC2 pins and generate the Clock Output signal used by the device when either a quartz crystal or single-ended clock input is used. The positive input of the comparator is connected to the OSC1 pin, and the negative input is connected to the OSC2 pin. When a single-ended clock is used, the clock signal is applied to the positive input of the comparator through the OSC1 pin, and the negative input of the comparator is held low by connecting the OSC2 pin to ground.

The Amplifier and Bias Control block provides the source current needed to start and sustain the crystal oscillation through the OSC1 pin when a quartz crystal is used, but it must be disabled when a single-ended clock input is used instead of a crystal. This is handled by the Clock Input Detection block that monitors the voltage level on the OSC2 pin and disables the Amplifier block if the voltage is below the detection threshold which is typically between 90mV to 150mV, such as when the pin is connected to ground. If the OSC2 pin is not connected to ground, the current supplied by the Amplifier and Bias block through the crystal will ideally keep the minimum voltage level of the oscillation waveform above 400mV and includes a peak detector circuit that will sense the oscillation envelope formed by the voltage difference between the OSC1 and OSC2 pins.

The crystal oscillator transconductance amplifier adjusts the amplifier bias current to maintain a consistent and stable oscillation when the voltage amplitude of the OSC1 and OSC2 waveforms are nominally 1 Vpp for each of the individual OSC1 and OSC2 signals, or 2 Vpp between the differential OSC1 and OSC2 signal resulting from their 180° phase shift. However, the actual voltage levels will depend on the total load connected to the TCAN455x oscillator circuit.

When the total external load on the amplifier created by the reactance of the load capacitance and the crystal equivalent series resistance is too small, the amplifier's minimum output current may cause the oscillation voltage amplitude levels to become too large by allowing too much power to flow through the crystal. If the voltage amplitude on the OSC2 pin becomes too large, the minimum peak voltage level of the signal could drop below the single-ended clock input detection threshold causing the amplifier and bias block to become disabled and stop the oscillation. When the external load is configured properly, the amplifier will be able to adjust the bias current to maintain a smaller voltage amplitude level in the nominal range and ensure the single-ended clock detection circuit will not disable the amplifier and bias control block.

Establishing the proper balance in the oscillation circuit is critical to stable reliable operation, preventing mechanical damage to the crystal, and guarantee several other factors not previously discussed. Ensuring the frequency of oscillation and the gain in the transconductance amplifier has enough margin to create the negative resistance needed to reliably establish the crystal oscillation must also be considered when selecting the values of the components used in the circuit. The remainder of this document will discuss how to avoid unreliable operation and determine the proper component values needed to meet all parametric concerns through a combination of measurements and calculations.

Common schematic representations of the Basic Pierce Oscillator circuit are shown in the following figure with the key components identified.