USB Bus Switch Circuit Parasitic Capacitance Reduction
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Solution Overview
Problem
USB signal characteristics degrade due to parasitic capacitance and resistance in HS mode transmission circuits, affecting signal quality and compliance with USB standards, especially in long cable routes and vehicle-mounted systems.
Innovation Solution
A circuit device with a bus switch circuit that connects and disconnects USB buses between a host and a device, using switch circuits and signal lines with specific layouts and configurations to reduce parasitic capacitance and improve signal integrity, including a processing circuit for packet transfer and signal processing to enhance signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a signal line is provided between the first switch circuit and the second switch circuit to enable direct connection, then connection speed is improved, but parasitic capacitance increases causing signal characteristic degradation
Solution Approach 1:
The bus switch circuit performs preliminary switching actions before high-speed packet transmission begins. The first and second switch circuits are turned on in advance during the first period (before packet transmission start timing) to establish direct connection paths, allowing the signal line to be bypassed during critical HS mode transmission, thus avoiding parasitic capacitance effects while maintaining connection readiness
2Reliability
If the bus switch circuit is used to switch connection on and off between buses, then signal characteristics are improved by reducing parasitic capacitance effects, but device complexity increases
Solution Approach 1:
The bus switch circuit is segmented into two independent switch circuits: a first switch circuit connected to the first bus and a second switch circuit connected to the second bus. These switch circuits can be controlled independently, allowing flexible connection configurations. This segmentation reduces the overall complexity compared to a single complex switch while achieving the same signal characteristic improvement through selective bypassing
3Reliability
If wiring capacitance of signal lines is reduced to improve signal quality, then signal characteristics are improved, but manufacturing precision requirements increase
Solution Approach 1:
The bus switch circuit acts as an intermediary device that provides two distinct transmission paths: a direct path (bypassing the processing circuit) and an indirect path (through the processing circuit). By enabling the direct path during HS mode transmission, the system avoids the need for extremely precise wiring design, as the signal line capacitance is bypassed rather than minimized. This intermediary switching mechanism relaxes manufacturing precision requirements while maintaining signal quality
Data Source
AI summary
A circuit device includes first and second physical layer circuits, a bus switch circuit that switches connection between a first bus and a second bus that comply with a USB standard on in a first period and off in a second period, and a processing circuit that performs, in the second period, packet transfer processing on a transfer route that includes the first bus, the first and second physical layer circuits, the second bus. The bus switch circuit includes a first switch circuit, a second switch circuit, and a signal line connected between the first switch circuit and the second switch circuit.