DDR Write-Leveling Clock Phase Calibration for Fly-By Skew
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Solution Overview
Problem
High-speed double data-rate (DDR) memory interfaces face challenges in maintaining accurate timing between clock and data strobe signals due to increasing speed, leading to potential errors caused by skew in fly-by topologies, which existing technologies struggle to efficiently compensate for across various interface standards.
Innovation Solution
A programmable memory write interface utilizing a tapped delay line and multiplexers to generate multiple phases of the clock signal, allowing for precise calibration and distribution to DQ/DQS groups, ensuring minimal skew by selecting the optimal phase for each receiving device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the clock signal is provided to multiple memory devices in series using a fly-by topology, then the clock signal reaches all devices, but skew between clock and DQS signals increases with distance
Solution Approach 1:
The clock signal distribution is segmented into multiple phases using a tapped delay line, where each phase corresponds to a specific memory device's timing requirement. This segmentation allows each device to receive a clock phase that is precisely aligned with its DQS signal, compensating for the cumulative skew in fly-by topology without requiring a single unified clock distribution.
Solution Approach 2:
Each memory device receives a locally optimized clock phase that is specifically tailored to its position in the fly-by topology. The tapped delay line provides different clock phases to different devices, ensuring that each device has the optimal timing alignment between its clock and DQS signals, rather than using a uniform clock distribution.
2Productivity
If the interface speed is increased to improve data throughput, then productivity increases, but timing margins become smaller and more susceptible to error
Solution Approach 1:
The system dynamically changes the clock phase parameter for each memory device based on its position in the fly-by topology and the interface speed. By adjusting the phase shift amount in the tapped delay line, the system maintains optimal timing margins even at higher data transfer rates, preventing timing errors while maximizing productivity.
3Manufacturing precision
If multiple phases of clock signal are generated and distributed to multiple DQ/DQS groups, then skew compensation precision improves, but device complexity increases
Solution Approach 1:
The tapped delay line generates more clock phases than the minimum required, providing a range of phase options that exceeds the basic compensation need. This excessive action ensures that the optimal phase is available for each memory device, achieving high skew compensation precision. The multiplexers then select only the necessary phases, managing complexity by providing a systematic way to choose from the available options.
4Manufacturing precision
If calibration is performed to minimize skew for each phase, then timing precision improves, but calibration time increases
Solution Approach 1:
The system performs calibration in advance during manufacturing or initialization, determining the optimal clock phase for each memory device before normal operation begins. This preliminary action stores the calibration results in configuration data, allowing the system to quickly switch between pre-determined optimal phases during operation without requiring time-consuming real-time calibration, thus minimizing skew while reducing operational calibration time.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively reduces skew between clock and DQS signals, ensuring data recovery and adaptability to different interface standards with high precision, thereby enhancing the reliability of data transfer in DDR memory interfaces.
Implementation Method 1
an input clock signal is generated using a phase-locked loop
Implementation Method 2
The input clock signal is sequentially delayed using a tapped delay line to generate a number of clock signals, each having a different phase
Data Source
AI summary
Circuits, methods, and apparatus for memory interfaces that compensate for skew between a clock signal and DQ/DQS signals that may be caused by a fly-by routing topology. The skew is compensated by clocking the DQ/DQS signals with a phase delayed clock signal, where the phase delay has been calibrated. In one example calibration routine, a clock signal is provided to a receiving device. A DQ/DQS signal is also provided and the timing of their reception compared. A delay of the DQ/DQS signal is changed incrementally until the DQ/DQS signal is aligned with the clock signal at the receiving device. This delay is then used during device operation to delay a signal that clocks registers providing the DQ/DQS signals. Each DQ/DQS group can be aligned to the clock, or the DQS and DQ signals in a group may be independently aligned to the clock at the receiving device.