DDR wiring in PCB design applications, what do you think?
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DDR cabling is a critical aspect of PCB design, and the key to achieving a successful layout lies in ensuring sufficient timing margin within the system. One of the most important elements in maintaining this timing margin is line length matching. This is particularly crucial for signals such as address, control/command, and clock lines, which must be matched in length. For data signals, the DQS (Data Strobe) signal should be matched in length with the corresponding DQ (Data) signals.
Why is it necessary to match signal lengths? The primary reason is that all signals in a group need to arrive at the receiver at the same time so that the receiving chip can process them simultaneously. When the clock and address signals arrive at the same time, their waveforms must align properly to ensure correct timing. To better understand this, we conducted a simulation of the DDR3 address and clock channels.
In the simulation, we assumed a 500MHz clock frequency, meaning the address signal rate is also 500Mbps. Although DDR is a double-data-rate interface, the address and control signals operate at a single data rate. The waveform results showed that when the address and clock signals are perfectly matched in length, the received signals appear synchronized.
To analyze the setup and hold times more clearly, we used an eye diagram. The results showed a setup time of approximately 891ps and a hold time of 881ps. However, if the address signal is delayed by 200ps relative to the clock, the setup time increases to 1.1ns, while the hold time decreases to 684ps. This shows how critical it is to maintain proper signal alignment.
For double-rate data signals, such as DQ and DQS, the same principle applies. In our simulation, we observed that the DQS edge should ideally align with the center of the DQ bit window to ensure adequate setup and hold times. If the DQ and DQS lines are not matched in length, the clock edge may not fall in the middle of the data pulse, reducing the timing margin.
The Freescale MPC8572 DDR controller manual provides specific timing relationships between DQS and DQ. On the memory side, Micron DDR chips define required setup and hold times. By calculating the delay skew (T_pcbskew) between DQ and DQS, we can determine the acceptable length difference. Assuming a transmission speed of 6 mils per picosecond, the allowable skew is around ±960 mils. While this seems large, real-world factors like jitter, crosstalk, and signal integrity issues reduce the effective margin.
In conclusion, the goal of timing control in DDR designs is to ensure that data has enough setup and hold time at the receiver. Understanding these principles allows designers to confidently manage signal matching and achieve reliable high-speed performance.