Internal RC Oscillator Circuit for Crystal-Free RTC Precision
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Solution Overview
Problem
Current crystal-based real-time clock (RTC) solutions in wireless devices are bulky, expensive, and introduce design constraints due to the need for additional board space and pins, while also experiencing frequency drift and noise issues that affect synchronization precision.
Innovation Solution
A non-crystal-based oscillator is integrated into the RTC using an oscillation circuit with a first comparator for low frequency drift and a second comparator for adaptive bias current generation, along with a chop switch matrix and multiplexer to minimize jitter and power consumption, achieving stable clock generation with low Allan Deviation and frequency stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a crystal-based RTC is used to achieve precise time synchronization, then measurement precision is improved, but device complexity and board space requirements increase
Solution Approach 1:
The patent extracts the essential oscillation function from the crystal-based system and implements it using an RC oscillator circuit with comparator-based frequency stabilization. This removes the need for physical crystal components and load capacitors, reducing board space and pin requirements while maintaining time synchronization precision through electronic frequency control mechanisms
Solution Approach 2:
The patent replaces the mechanical crystal oscillator system with an electronic RC oscillator system. The mechanical resonance of crystal is substituted by electronic RC time constants and comparator-based frequency detection, eliminating the need for mechanical components while achieving comparable timekeeping precision through electronic stabilization
2Measurement precision
If a crystal oscillator is added to achieve precise RTC functionality, then measurement precision is improved, but manufacturing cost increases
Solution Approach 1:
The patent replaces expensive crystal components with inexpensive RC circuit elements (resistors and capacitors) that can be easily manufactured and integrated. The RC oscillator components are standard electronic parts with lower cost than crystal oscillators, while the comparator-based frequency stabilization provides sufficient precision for time synchronization applications without requiring premium components
3Reliability
If a crystal-based RTC is used to achieve precise time tracking, then reliability is improved, but power consumption increases
Solution Approach 1:
The patent employs a periodic chopping mechanism that alternates between active oscillation phases and low-power states. The comparator-based frequency stabilization operates periodically to detect and correct frequency drift, rather than continuously consuming power. This periodic action maintains time tracking reliability while significantly reducing average power consumption compared to continuous crystal oscillator operation
Data Source
AI summary
An oscillation circuit includes resistors with tap points for high/low reference voltages. An RC network coupled in parallel with the resistors includes a first capacitor to vary a first voltage input and a second capacitor to generate a second voltage input. A first comparator alternately compares the voltage inputs with the low reference voltage to generate oscillation outputs. A PTAT current DAC supplies an injection current to a resistor of the series of resistors that variably modulates the reference voltages. A second comparator alternately compares the voltage inputs with the high reference voltage and controls generation of an adaptive bias current to first comparator near a switching threshold voltage range thereof. A chop switch matrix alternately flips voltage reference inputs to input terminals of first comparator. A multiplexer alternately inverts a polarity of the oscillation outputs in concert with alternately flipping the voltage reference inputs by the chop switch matrix.