Analytical cabling techniques Improve PCB signal integrity in Embedded Systems

PCBPrint circuit board (Printed circuit board) is the basic support for circuit components and devices in electronic products. Its Design Quality often directly affects the reliability and compatibility of embedded systems. In the past, in some low-speed circuit boards, the clock frequency is generally about 10 MHz. The main challenge of the circuit board or encapsulation design is how to distribute allSignalLine and how to avoid damaging encapsulation during assembly.

Since the interconnection line has never affected the system performance, the electrical characteristics of the interconnection line are not important. In this sense, the Interconnection Lines in low-speed signal circuit boards are smooth and transparent. However, with the development of embedded systems, the circuits used are basically high-frequency circuits. As the clock frequency increases, the rising edge of the signal is also shortened, the capacitance and inductance produced by the printed circuit are much larger than those of the printed circuit, seriously affecting the signal integrity. For embedded systems, when the clock frequency exceeds 100 MHz or the rising edge is less than 1 ns, the signal integrity effect becomes important.

In PCB, the signal line is the main carrier for signal transmission. The cabling of the signal line directly determines the superior signal transmission, thus directly affecting the performance of the entire embedded system. Unreasonable wiring will seriously lead to various signal integrity problems, and generate timing, noise, and electromagnetic interference EMI for the circuit, which will seriously affect the Embedded performance. In this paper, based on the actual electrical characteristics of signal lines in high-speed digital circuits, an electrical characteristic model is established to find out the main reasons affecting signal integrity and solutions to the problems, the problems that should be paid attention to in cabling and the methods and skills that should be followed are given.

Signal Integrity

Signal integrity refers to the quality of the signal on the signal line, that is, the ability of the signal to respond with the correct timing and voltage levels in the circuit, good signal integrity refers to the value of the required voltage level when necessary. Poor signal integrity is not caused by a single factor, but by a variety of factors in the board design. Signal integrity problems are embodied in many aspects, including latency, reflection, crosstalk, overhead, oscillation, and ground play.

Latency (Delay): A Delay occurs when a signal is transmitted at a limited speed on the PCB transmission line. A Delay occurs when the signal is sent from the sending end to the receiving end. The signal delay will affect the embedded time sequence. The transmission delay mainly depends on the length of the wire and the dielectric constant of the media around the wire. In a high-speed digital system, the length of the signal transmission line is the most direct factor affecting the phase difference of the clock pulse. the phase difference between the clock pulse is that the time when two clock signals arrive at the receiving end at the same time is not synchronized. The phase difference between the Clock Pulse reduces the predictability of the arrival of the signal edge. If the phase difference between the clock pulse is too large, an incorrect signal will be generated at the receiving end.

Reflection): Reflection is the echo of a signal on the signal line. When the signal delay time is much greater than the signal jump time, the signal line must be treated as a transmission line. When the characteristic impedance of the transmission line does not match the load impedance, some of the signal power voltage or current are transmitted to the line and reach the load, but some are reflected. If the load impedance is less than the original impedance, the reflection is negative. Otherwise, the reflection is positive. Such reflection is caused by changes in the geometric shape of the cabling, incorrect wire connections, transmission through the connector, and power plane disconnections.

Crosstalk): the Crosstalk is the coupling between two signal lines, the mutual inductance between the signal lines and the mutual capacity, causing noise on the signal lines. Capacitive coupling leads to coupling current, while inductive coupling leads to coupling voltage. Crosstalk noise is caused by electromagnetic coupling between signal lines, between signal systems and power distribution systems, and between passing holes. String winding may cause false clock and intermittent data errors, which may affect the transmission quality of adjacent signals. In reality, crosstalk cannot be completely eliminated, but it can be controlled within the range that the system can afford. The PCB layer parameters, signal line spacing, electrical characteristics of the drive end and the receiving end, and baseline connection modes have certain effects on crosstalk.

Overcharge Overshoot) and undercharge Undershoot): Overcharge refers to the first peak or valley value exceeding the set voltage. For the rising edge, it refers to the highest voltage; For the falling edge, it refers to the lowest voltage. Downstream refers to the next valley or peak value exceeding the set voltage. Excessive overhead can cause protection diode operation, resulting in premature failure. A false clock or incorrect data operation can be caused by an excessive underspeed ).

Oscillation Ringing) and surround oscillation Rounding): the phenomenon of oscillation is repeated over and down. The signal oscillation is the oscillation caused by the inductance and capacitance of the online transition. It belongs to the state of underdamping, And the surround oscillation is the state of over-damping. Oscillation and surround oscillation are also caused by multiple factors like reflection. The Oscillation can be reduced by appropriate ends, but it cannot be completely eliminated.

Ground Level bounce noise and reflux noise: when a large amount of current surge in the circuit will cause the ground level bounce noise, such as when a large number of chip outputs are enabled at the same time, there will be a large transient current flowing through the power plane of the chip and the Board. The inductance and resistance of the chip encapsulation and power plane will lead to power noise, this will generate voltage fluctuations and changes on the real ground plane. This noise will affect the movements of other components. The increase of the load capacitance, the decrease of the load resistance, the increase of the local inductance, and the increase of the number of switching devices will lead to the increase of the ground Bomb.

Electrical Characteristics Analysis of Transmission Channels

In multi-layer PCB, the vast majority of transmission lines are arranged on a single layer, but are staggered on multiple layers. Therefore, in multilayer PCB, a typical transmission channel consists of transmission lines, strip corners, and passing holes. In the case of low frequency, the printed line and the strip through the hole can be seen as the ordinary electrical connection connecting the pins of different devices, which will not have a great impact on the signal quality. However, in high-frequency situations, the connectivity of printed wires, corners, and passing holes cannot be considered, but the electrical characteristics and parasitic parameters at high frequencies should also be considered.

Electrical Characteristics of transmission lines in High-Speed PCB

In high-speed PCB design, a large number of signal connection lines are inevitably used, and the length is different. The delay time of the signal passing through the connection line cannot be ignored compared with the change time of the signal itself, signals are transmitted over the connection line at the speed of electromagnetic waves. At this time, the connection line is a complex network with resistors, capacitors, and inductors, which needs to be described by the distributed parameter system model, that is, the transmission line model.

Transmission lines are used to transmit signals from one end to the other end. They are composed of two wires with a certain length. One is called a signal path and the other is called a return path. In low-frequency circuits, transmission lines are characterized by Electrical Properties of pure resistance. In high-speed PCB, with the increase of transmission signal frequency, the capacitive impedance between wires decreases, and the inductive impedance increases on the wires. The signal lines no longer only show as pure resistance, that is, the signal will be transmitted not only on the wire, but also in the medium between the conductor. If the signal frequency increases further, when j ω L> R, 1/j ω C) <

For even wires, the average distribution of the resistance R, the parasitic inductance L of the transmission line, and the parasitic capacitance C is L1 = L2 =... = Ln; C1 = C2 =... = Cn + 1 ). Assume that the transmission line is a lossless transmission line, that is, when R = 0, if the line parameter is used: unit length capacitor C1, unit length inductance L1, and the total length of the transmission line is Len, there are:

Wiring skills to improve the signal integrity of Embedded System PCB is a very practical skill, I hope you can master.

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