High-speed digital systems today can operate at clock frequencies reaching hundreds of megahertz, and their fast-slope transitions, high operating speeds, and dense circuitry make them behave very differently from low-speed designs. These factors can lead to signal integrity issues that directly affect the quality of signals, resulting in distortion, timing errors, and incorrect data, address, or control signals. Such problems can cause system malfunctions or even crashes. As a result, signal integrity has become a critical concern for designers working on high-speed digital circuits.
1. Signal Integrity Problem and Its Mechanism
Signal Integrity (SI) refers to the accuracy of signal quality and timing along a transmission path. In digital systems, logic 1 and 0 are defined relative to reference voltages, as shown in Figure 1(a). Signals above ViH are considered logic 1, while those below ViL are logic 0. The area between these thresholds is an indeterminate region. In reality, signals often exhibit overshoot, undershoot, and ringing, which can fall into this uncertain zone. This can lead to timing inaccuracies, especially when the signal delay is too long, causing logic errors at critical clock edges.
Most digital chips require that data be stable before the tsetup time of the clock edge to ensure correct timing. In high-speed systems, electromagnetic interference and other factors can worsen signal quality, making the tsetup requirements even stricter. Therefore, maintaining signal integrity is essential during hardware design.
As illustrated in Figure 1(b), signal waveform distortions and transmission delays significantly impact timing accuracy. These issues are primarily caused by three main factors:
- Reflection Noise: Caused by impedance mismatches due to discontinuities in transmission lines, vias, and other interconnections.
- Crosstalk: Occurs when closely spaced signal lines cause electromagnetic coupling, leading to unwanted interference.
- Power and Ground Noise: Arises from parasitic inductance and resistance in chip packages and power planes, causing voltage fluctuations during high current draw.
High-speed systems often include complex structures such as daughter boards, motherboards, and backplanes, connected via cables or connectors. Signal transmission occurs through transmission lines, vias, and chip I/O pins. Due to differences in transmission characteristics, signal integrity can be compromised. To ensure reliable operation, it's crucial to minimize negative effects from improper physical connections.
2. Ways to Ensure Signal Integrity
When the length of a signal line exceeds the wavelength of the transmitted signal, it should be treated as a transmission line. At high speeds, signal lines are typically modeled as RLC ladder networks. Impedance mismatches at the chip’s input and output pins can cause significant reflections. A common approach to reduce reflection is to keep high-speed signal lines as short as possible. Alternatively, matching resistors can be used at both ends to achieve impedance matching and eliminate reflections.
When multiple high-speed signals are close together, crosstalk becomes a major concern. As shown in Figure 2, crosstalk between parallel signal lines increases with shorter spacing and longer parallel lengths. Inductive and capacitive coupling between the lines leads to greater interference. To reduce crosstalk, increasing the distance between parallel lines and minimizing their parallel length is the most effective method. Altering dielectric properties of the PCB can also help, though it may increase costs.
In cases where space is limited, adjusting the routing strategy is necessary. Important signal lines should be protected, and termination techniques can be used to mitigate crosstalk. Different routing topologies require different termination methods: serial termination is common for single-load networks, AC parallel termination is used for daisy chains, and star wiring typically employs AC parallel termination (see Figure 3).
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