Crystal Oscillator Hysteresis Control for Faster Start-Up
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Solution Overview
Problem
The early application of hysteresis in AC-coupled, self-biased inverter comparators within crystal oscillator circuits hinders the growth of the EXTAL signal, resulting in longer start-up times in System-on-Chip (SoC) applications with low-jitter requirements.
Innovation Solution
A controlled hysteresis application system and method that initially disables hysteresis during start-up and enables it only when the EXTAL signal reaches a selected level, using an Automatic Loop Control (ALC) circuit and hysteresis control circuit to manage the gain and apply hysteresis once the signal has grown sufficiently, thereby preventing hindrance to the signal's growth and reducing start-up time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If hysteresis is applied early in the crystal oscillator circuit, then noise immunity is improved, but the EXTAL signal growth is hindered resulting in longer start-up times
Solution Approach 1:
The patent applies preliminary action by enabling hysteresis only after the EXTAL signal has grown to a sufficient level, rather than applying it immediately at circuit power-up. The hysteresis control circuit monitors the EXTAL signal amplitude and delays hysteresis activation until the signal exceeds a predetermined threshold, allowing the oscillator to start up quickly without initial hysteresis hindrance while still providing noise immunity once the signal is established.
Solution Approach 2:
The patent implements dynamics by making the hysteresis characteristic variable rather than fixed. The hysteresis control circuit dynamically adjusts the hysteresis state of the inverter comparator based on the real-time amplitude of the EXTAL signal. This dynamic control allows the circuit to transition from a low-hysteresis state during start-up to a high-hysteresis state during steady-state operation, optimizing both start-up time and noise immunity at different operational phases.
2Stability of the object's composition
If hysteresis is applied to the inverter comparator, then signal stability is improved, but the oscillator start-up process is delayed
Solution Approach 1:
The patent uses preliminary action by preparing the hysteresis control circuit to monitor EXTAL signal amplitude before hysteresis is actually applied. The control circuit is pre-configured to detect when the signal reaches a sufficient level, and hysteresis is activated at the optimal moment rather than being applied continuously from power-up, thus maintaining signal stability without delaying start-up.
Solution Approach 2:
The patent implements feedback through the hysteresis control circuit that continuously monitors the EXTAL signal amplitude and adjusts the hysteresis state of the inverter comparator accordingly. This feedback mechanism ensures that hysteresis is applied only when the signal has grown sufficiently, maintaining signal stability during normal operation while avoiding hysteresis-induced delays during the start-up phase.
Data Source
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Figure 3A
AI summary
Systems and methods for controlled application of hysteresis in crystal oscillator circuits are discussed. In various embodiments, an Integrated Circuit, IC, includes: an inverter comparator (110) coupled to a crystal oscillator (106), where the inverter comparator (110) is configured to: (i) receive an input (EXTAL) of the crystal oscillator, and (ii) output a clock signal (clk_out); and a hysteresis control circuit (112) coupled to the inverter comparator (110), wherein the inverter comparator is configured to: (i) start up with hysteresis disabled, and (ii) enable hysteresis in response to a hysteresis enable signal provided by the hysteresis control circuit.