Shared I/O Buffer Networks for Dynamic Drive and Split Termination
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Solution Overview
Problem
High-speed signal integrity issues arise in chip-to-chip interconnects due to transmission line effects like reflections, attenuation, and cross-talk, particularly in SDRAM memory systems, where existing solutions lack dynamic impedance control and often require external resistors, increasing costs and power consumption.
Innovation Solution
A combined drive and termination circuit with variable impedance pull-up and pull-down networks, dynamically configurable between drive and termination modes, using transistors in parallel to adjust impedance, and a calibration mechanism to set optimal transistor enablement for different operating conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fixed external resistors are used for termination, then signal integrity is improved, but device complexity and cost increase
Solution Approach 1:
The patent merges the drive function and termination function into a single integrated circuit block. The termination resistors are integrated on-die rather than being external components, and the same circuit resources are shared between driving the transmission line and providing termination, eliminating the need for separate external resistors and reducing overall system complexity.
Solution Approach 2:
The I/O buffer circuit is designed to perform multiple functions: it can operate as a drive buffer when the output enable signal is active, and as a termination resistor when the output enable signal is inactive. This multi-functionality allows the same hardware resources to serve both driving and termination purposes, reducing the need for additional components.
2Reliability
If fixed external resistors are used for termination, then signal reflections are reduced, but power consumption increases
Solution Approach 1:
The termination function is activated periodically or conditionally based on the output enable signal state rather than being continuously active. When the I/O buffer is in high-impedance state (not driving), the termination resistors are enabled to absorb reflections. When driving, the termination is disabled to avoid unnecessary power consumption. This conditional activation reduces overall power consumption while maintaining signal integrity when needed.
3Strength
If I/O buffers have low output impedance, then signal swing is improved, but signal reflections increase
Solution Approach 1:
The patent implements dynamic impedance control where the output impedance of the I/O buffer can be adjusted based on operating conditions. The circuit can switch between low-impedance drive mode (for strong signal swing) and high-impedance termination mode (to match transmission line impedance and reduce reflections). This dynamic adjustment allows the circuit to optimize for signal swing during driving and minimize reflections during high-impedance states.
Data Source
AI summary
A system and method of performing off chip drive (OCD) and on-die termination (ODT) are provided. A common pull-up network composed of transistors and a common pull-down network composed of transistors are employed to implement both of these functions. In drive mode, the pull-up network is configured to produce a calibrated drive impedance when an “on” output is to be generated, and the pull-up network is configured to produce a calibrated drive impedance when an “off” output is to be generated. In termination mode, the pull-up network and the pull-down network are configured to produce a calibrated pull-up resistance and pull-down resistance respectively such that together, they form a split termination.