USB SOF Clock Calibration Without an External Oscillator
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Solution Overview
Problem
The high cost of accurate external oscillating signal sources for USB interfaces is a challenge in low-cost applications, as they are necessary for maintaining frequency accuracy in USB chip operations.
Innovation Solution
A USB apparatus that utilizes the Start of Frame (SOF) packet signal as a reference clock to calibrate its internal frequency, eliminating the need for an external accurate frequency oscillator by incorporating a frequency error detecting circuit and a frequency generator, which includes a fractional-N frequency synthesizer and a reference clock generator, to achieve frequency accuracy within USB specifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an external accurate oscillating signal source is used to maintain frequency accuracy in USB chip operations, then frequency accuracy is improved, but manufacturing cost increases
Solution Approach 1:
The USB apparatus generates its own frequency signal using an internal oscillating signal source and calibrates it using the received SOF packet signal, eliminating the need for external accurate oscillating components. The frequency error detecting circuit continuously monitors and adjusts the internal frequency to maintain accuracy requirements.
Solution Approach 2:
The system uses the SOF packet signal as a reference and implements a feedback mechanism where the frequency error detecting circuit compares the internal oscillating signal against the reference, detects frequency errors, and adjusts the frequency generator accordingly to maintain required frequency accuracy.
2Measurement precision
If an external accurate oscillating signal source is used to maintain frequency accuracy, then frequency error requirements are met, but device complexity increases
Solution Approach 1:
The invention extracts the frequency reference function from an external component and implements it using the received SOF packet signal itself, thereby eliminating the need for external accurate oscillating components while maintaining frequency error requirements.
Solution Approach 2:
The SOF packet signal serves multiple functions: it acts as both the data communication signal and the frequency reference signal for calibration, eliminating the need for separate external oscillating components and reducing overall device complexity.
Data Source
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AI summary
The present invention disclosed a Universal Serial Bus (USB) apparatus. The apparatus includes: a signal detecting unit for detecting a packet signal transmitted from a USB host and generating an acknowledgment signal according to a detection result; an error detecting unit, coupled to the signal detecting unit, for generating a control signal according to the acknowledgment signal; and a frequency generating unit, coupled to the error detecting unit, for generating an output clock signal according to the control signal. In this way, the USB apparatus can precisely detect the SOF packet under various operating modes and the SOF signal can be used as reference clock signal, by measuring its period, to calibrate an internal operating frequency. No external accurate oscillator is necessary.