Surgery is very important, and postoperative recovery is also essential! After the various PCB layouts are completed, is it ok? Obviously, no! After the PCB wiring inspection work is also very necessary, then how to check the wiring in the PCB design, paving the way for the subsequent PCB design, circuit design? This article will teach you how to complete the inspection after PCB wiring from the various characteristics of PCB design, and do the final check!
Before explaining the inspection work after the PCB layout is completed, we will introduce the special wiring skills of the three PCBs. The PCB LAYOUT trace will be explained from three aspects: right angle trace, differential trace, and serpentine line:
First, right angle routing (three aspects)
The effect of the right-angled trace on the signal is mainly reflected in three aspects: First, the corner can be equivalent to the capacitive load on the transmission line, slowing down the rise time; second, the impedance discontinuity will cause the signal to reflect; third, the right-angle tip is generated. EMI, to the RF design field above 10GHz, these small right angles may become the focus of high-speed problems.
Second, the differential trace ("equal length, equidistance, reference plane")
What is the differential signal (DifferenTIal Signal)? In layman's terms, the driver sends two equal-valued, inverted signals. The receiver compares the difference between the two voltages to determine whether the logic state is "0" or "1". The pair of traces carrying the differential signals is called a differential trace. Compared with ordinary single-ended signal traces, the most obvious advantages of differential signals are reflected in the following three aspects:
1. Strong anti-interference ability, because the coupling between the two differential traces is very good. When there is noise interference from the outside, it is almost simultaneously coupled to two lines, and the receiving end only cares about the difference between the two signals. Therefore, the common mode noise of the outside world can be completely offset.
2, can effectively suppress EMI, the same reason, because the polarity of the two signals is opposite, their external electromagnetic field can cancel each other, the closer the coupling, the less electromagnetic energy vented to the outside.
3. Timing positioning is accurate. Since the switching change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal, which depends on the high and low threshold voltages, it is less affected by the process and temperature, and can reduce the timing error. It is also more suitable for circuits with low amplitude signals. The currently popular LVDS (low voltage differenTIal signaling) refers to this small amplitude differential signaling technique.
Third, the snake line (adjustment delay)
Serpentine lines are a type of routing that is often used in Layout. Its main purpose is to adjust the delay and meet the system timing design requirements. The two most important parameters are the parallel coupling length (Lp) and the coupling distance (S). It is obvious that when the signal is transmitted on the serpentine trace, the coupling between the parallel segments will occur in the form of differential mode. The smaller the Lp, the greater the degree of coupling. It may cause a reduction in transmission delay and greatly reduce the quality of the signal due to crosstalk. The mechanism can be referred to the analysis of common mode and differential mode crosstalk. Here are some suggestions for dealing with Python engineers when dealing with snake lines:
1. Try to increase the distance (S) of the parallel line segment, at least greater than 3H. H refers to the distance from the signal to the reference plane. In layman's terms, it is to follow the big bend line. As long as S is large enough, the mutual coupling effect can be almost completely avoided.
2. Reduce the coupling length Lp. When twice the Lp delay approaches or exceeds the signal rise time, the resulting crosstalk will reach saturation.
3. The stripe line of the Strip-Line or Embedded Micro-strip causes less signal transmission delay than the Micro-strip. In theory, the stripline does not affect the transmission rate due to differential mode crosstalk.
4, high-speed and signal lines with stricter timing requirements, try not to take the serpentine line, especially in the small range.
5, can often use snake-shaped lines at any angle, can effectively reduce the coupling between each other.
6. In high-speed PCB design, the serpentine line has no so-called filtering or anti-interference ability, and it can only reduce the signal quality, so it is only used for timing matching and has no other purpose.
7. Sometimes the winding can be considered in the way of spiral routing. The simulation shows that the effect is better than the normal serpentine routing.
Surgery is very important, and postoperative recovery is also essential! After finishing the PCB layout, is it finished? Obviously, no! Post-PCB inspection is also necessary, so how to check the wiring in the PCB design, paving the way for later design? Please see below!
General PCB design inspection project
1) Has the circuit been analyzed? In order to smooth the signal circuit into basic units?
2) Does the circuit allow for short or isolated key leads?
3) Is the place that must be shielded effectively shielded?
4) Make full use of the basic grid graphics?
5) Is the size of the printed circuit board the best size?
6) Is the selected wire width and spacing as much as possible?
7) Is the preferred pad size and hole size used?
8) Is the photographic plate and sketch suitable?
What is Car Ethernet
Car Ethernet is a new local area network technology that uses Ethernet to connect the electronic unit in the car. Unlike traditional Ethernet, which uses 4 unshielded twisted pair cables, car Ethernet can achieve a transmission rate of 100Mbit/s or even 1Gbit/s on a single pair of unshielded twisted pair cables. At the same time, it also meets the requirements of the automotive industry for high reliability, low electromagnetic radiation, low power consumption, bandwidth allocation, low latency and synchronous real-time. The physical layer of on-board Ethernet uses BroadRReach technology, and BroadR-Reach's physical layer (PHY) technology has been standardized by the One-pair Ethernet Alliance (OPEN). Therefore, it is sometimes called Broad RReach (BRR) or OABR (Open Alliance BroadR-Reach). The MAC layer of vehicle Ethernet adopts the IEEE 802.3 interface standard and seamlessly supports widely used high-level network protocols (such as TCP/IP) without any adaptation.
On-board Ethernet protocol architecture
Vehicle-borne Ethernet and its supported upper-layer protocol architecture are shown in Figure 1. Vehicle-borne Ethernet mainly involves OSI layer 1 and Layer 2 technologies, while vehicle-borne Ethernet also supports AVB, TCP/IP, DOIP, SOME/IP and other protocols or application forms.
On-board Ethernet framework
Among them, AVB is an extension of traditional Ethernet functions, which enhances the real-time performance of traditional Ethernet audio and video transmission by adding precise clock synchronization, bandwidth reservation and other protocols, and is a network audio and video real-time transmission technology with great development potential. SOME/IP (Scalable Service-Oriented MiddlewarE on IP) specifies the video communication interface requirements for vehicle camera applications, which can be applied to the field of vehicle cameras, and realizes the mode control of driver assistance cameras through apis.
As an extension of AVB protocol, Time-Sensitive Networking (TSN) introduces related technologies of time-triggered Ethernet, which can efficiently realize the transmission of automotive control information. In addition, the on-board Ethernet of the 1Gbit communication standard also supports Power Over Ethernet (POE) function and Energy-Efficient Ethernet (EEE) function. The POE function provides power for connected terminal devices while transmitting data through twisted pair cables, eliminating the need to connect external power cables to terminals and reducing the complexity of power supply.
On-board Ethernet standardization
In terms of in-vehicle Ethernet standardization, the IEEE802.3 and IEEE802.1 working groups, AUTOSAR, the OPEN Alliance and the AVnu Alliance have played a major role in promoting it.
The IEEE802.3 local area network standard represents the mainstream Ethernet standard in the industry, and the on-board Ethernet technology is developed on the basis of IEEE802.3, so the IEEE is currently the most important international standardization body for on-board Ethernet. In order to meet the requirements of the car, it involves the development of a number of new specifications and the revision of the original specifications within the two working groups of IEEE802 and 802.1, including PHY specifications, AVB specifications, and single-wire to data line power supply. In addition, AVB related to AV transmission, timing synchronization and other specifications also need to be standardized by other technical committees of IEEE, such as IEEE1722 and IEEE1588.
OPEN Alliance
The OPEN Industry Alliance was launched in November 2011 by Broadcom, NXP, and BMW to promote the application of Ethernet-based technology standards to in-car connectivity. The main standardization goal is to develop a 100Mbit/s BroadR-R physical layer standard and develop OPEN interoperability requirements.
AUTOSAR
AUTOSAR is a consortium of automotive manufacturers, suppliers, and tool developers that aims to develop an open, standardized automotive software architecture, and the AUTOSAR specification already includes the automotive TCP/UDP/IP protocol stack.
AVnu
The AVnu Alliance was formed by Broadcom in collaboration with Cisco, Harman and Intel to promote the IEEE 802.1 AVB standard and the Time Synchronization Network (TSN) standard, establish a certification system, and address important technical and performance issues such as precise timing, real-time synchronization, bandwidth reservation, and traffic shaping.
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Shenzhen MovingComm Technology Co., Ltd. , https://www.movingcommiot.com