DDR wiring in PCB design applications, what do you think?
Guangzhou Winson Information Technology Co., Ltd. , https://www.barcodescanner-2d.com
DDR cabling is a critical aspect of PCB design. The key to achieving a successful design lies in ensuring that the system has enough timing margin. One of the most important techniques for maintaining this margin is line length matching. This ensures that signals arrive at the receiver at the same time, allowing the receiving chip to process them accurately.
When it comes to DDR routing, the basic principle of line length matching is that address and control/command signals should be matched in length to the clock signal. Data signals, on the other hand, should match the length of the DQS (Data Strobe) signal. Why is this important? Because all signals within the same group must arrive at the receiver simultaneously so that the chip can process them correctly. But what exactly does this look like in terms of waveform alignment?
To better understand this, let’s simulate the timing relationship between the address and clock signals in a DDR3 system. We’ll create a simulation setup to analyze how these signals behave.
In the simulation, we assume a DDR clock frequency of 500MHz, which means the address signal operates at 500Mbps. Although DDR is double data rate, the address and control signals are still single-rate. When the address and clock signals are perfectly matched in length, their waveforms align as shown in the figure below.
The eye diagram clearly shows the setup and hold times. From the simulation, we find that the setup time is approximately 891ps, while the hold time is about 881ps. Now, what happens if the address signal is delayed by 200ps compared to the clock? The resulting eye diagram shows a reduced hold time of 684ps and an increased setup time of 1.1ns. This illustrates how even small differences in signal length can significantly impact timing margins.
Now, let’s look at the double-rate data signals, specifically the DQ and DQS lines. In DDR systems, the DQS signal is used to sample the DQ data. If the DQ and DQS signals are not properly matched in length, the sampling point will shift, reducing the available timing margin. In practice, the master chip adjusts the phase of the DQ signals so that the DQS edge aligns with the center of the DQ bit, ensuring optimal setup and hold times.
To illustrate this, we conducted a simulation using IBIS models from a chip manufacturer. The results showed that when the transmission paths of DQ and DQS are equal in length, the edges of the two signals align. However, in real-world scenarios, the DQ signal is typically offset by a quarter cycle relative to DQS. This adjustment ensures that the DQS edge falls in the middle of the DQ bit window, maximizing timing margins.
If the lengths of DQ and DQS are mismatched, the clock edge may no longer be centered on the data, leading to reduced setup or hold time margins. This can result in unreliable data sampling and potential system failure.
To calculate the required line length difference, we convert the timing parameters into physical distances. For example, using a transmission speed of 6 mil/ps, a skew of ±960 mil would provide sufficient margin under ideal conditions. However, real-world factors such as clock jitter, data jitter, crosstalk, and intersymbol interference reduce this margin significantly.
In conclusion, the goal of timing control in DDR design is to ensure that the receiver has enough setup and hold time to correctly capture the data. Understanding these principles allows engineers to confidently manage signal matching and achieve reliable high-speed performance.