Transmitter Circuit Impedance Control for CAN Bus Signal Integrity
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Solution Overview
Problem
Current CAN bus networks face challenges with data rate limitations due to ringing and reflections, especially with advancements in protocols like CAN FD, which increase data rates and make termination resistance and impedance matching critical.
Innovation Solution
A transmitter circuit is designed with a voltage setting circuit and an impedance setting circuit, allowing for independent control of differential driver voltage and driver impedance. The impedance setting circuit has adjustable pull-up and pull-down resistances, enabling precise adjustment of driver impedance to target profiles during data transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If data rate is increased to support CAN FD protocol, then productivity is improved, but ringing and reflections occur causing signal integrity degradation
Solution Approach 1:
The transmitter circuit proactively adjusts the driver impedance to a target impedance profile before and during the recessive period of data transmission. This preliminary impedance matching action prevents reflections and ringing from occurring in the first place, rather than attempting to correct them after they occur. The controller detects the dominant-to-recessive transition and responds by adjusting the impedance setting circuit to match the characteristic impedance of the bus, thereby eliminating the root cause of signal integrity degradation at higher data rates.
2Reliability
If termination resistance is adjusted to reduce reflections, then signal integrity is improved, but hardware complexity and control precision requirements increase
Solution Approach 1:
The patent implements dynamic impedance adjustment rather than static termination. The impedance setting circuit uses adjustable resistances (such as switched resistor networks or variable resistors) that can change their resistance values in real-time based on the transmission state. The controller dynamically switches between different impedance configurations - using a target impedance profile during recessive periods and a different configuration during dominant periods. This dynamic approach simplifies the overall system by eliminating the need for complex external termination networks while maintaining signal integrity through adaptive impedance matching.
Solution Approach 2:
The transmitter circuit integrates multiple functions into a single device: it simultaneously performs voltage level switching and impedance matching. The impedance setting circuit is incorporated directly into the transmitter structure, allowing the same hardware block to handle both data transmission and reflection suppression. This multi-functionality reduces the need for separate termination components and simplifies the overall network architecture.
3Reliability
If driver impedance is precisely controlled to match bus impedance, then reflections are minimized, but timing accuracy requirements become more stringent
Solution Approach 1:
The patent changes the impedance parameter dynamically based on the transmission phase rather than requiring continuous precise control. The impedance setting circuit is configured to switch between discrete impedance states - specifically, a target impedance profile during the recessive period that matches the bus characteristic impedance. By changing the impedance parameter in sync with the data transmission phases (triggered by dominant-to-recessive transitions), the system achieves effective reflection suppression without requiring sub-nanosecond timing precision, as the impedance adjustment is coordinated with the natural bit timing of the CAN protocol.
Data Source
AI summary
A transmitter circuit including an impedance setting circuit having first and second legs, wherein each leg includes an adjustable pull-up resistance and an adjustable pull-down resistance connected in series between a supply terminal and a reference terminal. A first-leg-node, between the adjustable resistances of the first leg, is connected to a first bus terminal. A second-leg-node, between the adjustable resistances of the second leg, is connected to a second bus terminal. The controller detects a transition in a transmission data signal, and in response to a dominant to recessive transition the controller controls a voltage setting circuit to set the differential driver voltage on the bus to a recessive value; adjusts each of the adjustable pull-up resistances and the adjustable pull-down resistances with the same target impedance profile such that the transmitter circuit drives the bus with a target driver impedance for an active recessive period of a bit time.