Clock signal and address arrive at the receiving end at the same time, simulate specific waveform analysis
Wiring is a critical aspect of circuit design, and ensuring sufficient timing margin is essential for system reliability. One of the key techniques in achieving this is line length matching. The basic principle is that signals such as address, control/command, and clock should be of equal length, while data signals should match the DQS (Data Strobe) signal. Why is this important? Because signals within the same group must arrive at the receiving end simultaneously so that the chip can process them correctly.
But what happens when the clock and address signals reach the receiver at the same time? How do their waveforms align? To understand this, we can look at simulation results. Let’s examine a setup where the address and clock signals are matched in length.
In this example, we simulate the address and clock signals at 500MHz, which corresponds to a data rate of 500Mbps for the address/control signals. Even though DDR memory operates at double data rates, the address and control signals are still single-speed. The waveform shown in Figure 2 illustrates how the address and clock signals appear at the receiver.
From the waveform, it might not be immediately clear how the clock and address are aligned. By analyzing the eye diagram, we can better understand the setup and hold times. As shown in Figure 3, the setup time for the address signal is approximately 891ps, and the hold time is about 881ps.
Now, let’s consider what happens if the address line is longer than the clock line by 200ps. The resulting eye diagram (Figure 4) shows that the hold time decreases to 684ps, while the setup time increases to 1.1ns. This demonstrates how mismatched lengths affect timing margins.
For double-rate data signals, the relationship between DQ and DQS is crucial. In simulations, we observe that when DQ and DQS are of equal length, their edges align. However, in practice, the master chip adjusts the data signal to be slightly earlier than DQS. This ensures that the DQS edge falls in the center of the DQ bit, maximizing the setup and hold times.
As shown in Figure 8, after adjustment, the DQS edge aligns with the center of the DQ signal, ensuring reliable data sampling. If DQ and DQS are not properly matched, the clock edge may not fall in the middle of the data, reducing the timing margin.
A visual representation of how delay deviations impact timing is shown in Figure 9. Ideally, the clock and data centers should align. However, due to differences in transmission path lengths, the clock may not be centered on the data, leading to reduced setup and hold margins.
To apply these principles practically, let's refer to the Freescale MPC8572 DDR controller manual (Figures 10 and 11). These figures define the phase relationship between DQS and DQ. On the receiving end, the Micron DDR chip requires specific setup and hold times, as shown in Figure 12.
By calculating the allowable skew (T_pcbskew), we ensure enough timing margin. Using the values from the timing diagrams, we find that T_pcbskew should be within ±960 mil (assuming a 6 mil/ps propagation speed). While this seems large in ideal conditions, real-world factors like jitter, crosstalk, and interference reduce the available margin significantly.
In conclusion, the goal of timing control is to ensure that the data has enough setup and hold time at the receiver. Understanding this allows engineers to confidently manage PCB layout and achieve proper signal integrity.
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