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Affinity designer xor free.VLSI Design - Quick Guide
During the system level planning phase of any major hardware project, at least one Ethernet communication link is often included as a standard option, and it is this Ethernet interface on the circuit board that we are going to discuss in depth. In my Altium community , the question of how to implement Ethernet comes up every few months. It is often met with some generic answers as to impedance , but without anyone having a fantastic resource they can link to which covers everything from the ground up.
There are screenshots of these schematics in this article. It should be noted that these transmission rates are theoretical maximum figures. You may already have some idea about implementing gigabit Ethernet, perhaps you have even succeeded in implementing a working gigabit Ethernet interface, or this may be the first time that you have dived into high-speed digital interface design.
This article is intended as a guide for designers, from the theoretical basics to the practical aspects of schematic and layout design. Even if you are an expert in digital interfaces, this article may be useful as a checklist or a reminder of the theory.
You should be aware that to aid the readability of this article, some blocks or components will not be described in some sections, but these gaps will be filled in some of the following sections.
Before jumping straight into the hardware design, it may be helpful to have a brief insight into what kinds of data are traveling from the real world to the controller from the perspective of the network.
By definition, Ethernet data carried on twisted-pair copper circuit cable is part of the physical layer until it reaches a device. In the data link layer, the data is decomposed into a format that can be understood by a network stack embedded in the controller. In simple terms, the physical layer is analogous to roads and trucks that carry the mail.
In contrast, the data link layer corresponds to the envelope that has the address information needed to distinguish each item of mail from another. We will go into a more detailed explanation of how these network layers correspond with the equivalent IC level information further into the article. Once the history of the Ethernet protocol evolution is examined, the significant speed improvements that come with each new generation clearly stand out. Looking at circuit board hardware speed and bandwidth capabilities, the clear choice of generation to implement into a modern design is gigabit Ethernet.
When it comes to different mediums, let us say you choose WiFi to avoid the need for cables, there are definitely some advantages and disadvantages when compared with Ethernet, as can be seen in the following examples. Except when designing IoT devices, a hardware designer will often use an Ethernet interface to communicate with other systems, particularly for the transfer of bulky monitoring data and files.
The reliability and speed of Ethernet is difficult to match, and this reliability and speed simplifies engineering decisions and development of the circuit board hardware and firmware. Using a wired connection also offers another advantage: certification costs can be much lower if there is no radio transmission from the device as the device will be certified as an unintentional radiator.
What about using a USB interface instead of an Ethernet connection you might be thinking? Should we replace all the Ethernet equipment with USB 3. Before making your choice, think about if you are happy to settle for the following:.
If you can live with those restrictions, then why not give USB3. Note that these restrictions are not intended to denigrate the USB3. For Ethernet, a game-changer is the use of an optical communication link instead of copper cable , an option that expands nearly all speed, latency , and cable length limits. However, fiber gigabit Ethernet is a subject for another time and will not be covered in this article.
Ethernet is a very convenient technology , allowing direct access to standard networking protocols and systems. Ethernet allows you to leverage existing infrastructure. WiFi offers a lot of convenience but comes with risks and penalties which may or may not be acceptable to your application. USB is a prolific standard available on many devices. However, your device needs to be in close proximity to a host or client device which will typically need custom software installed on that device to provide communication to the product you are developing.
Since time immemorial, RJ type sockets and plugs with twisted-pair copper cables have been used for Ethernet interfaces. The carrier frequency determines the transmission speed , and to get the correct speeds, cable rated as Cat5 or higher should always be used for your gigabit Ethernet. Tip: Pay attention when selecting an RJ socket for your PCB, as some sockets have a low-profile option, which will require a board cut-out under the connector. Also note that some RJ jacks include the required magnetics termination circuit known as Bob Smith termination integrated into the connector sometimes called MagJack connectors.
As can be seen in Figure 2, UTP cables have four twisted pairs where each pair is assigned one positive and one negative signal. At this point, two questions are likely to have sprung to mind: Why do they use twisted pairs, and why is there one positive and one negative signal for each pair? The short answer is that both of these features are used to reduce the effects of electromagnetic radiation and interference. Parallel cables in a bundle not twisted may easily inject noise into each other, as the cable acts as a current-carrying inductor and creates a magnetic field.
A differential transmission technique is an excellent starting point in preventing this magnetic field effect since this method uses two cables, one for the original signal and one for an inverted copy of the signal that each induces an equal and opposite magnetic field that cancels the other out.
Although differential receivers are resistant to common-mode noise by design, if the positive and negative signal cables are not equally distanced from the noise source, the common-mode noise could be converted into a differential mode noise. This problem is solved by twisting the positive and negative signal pairs together.
This makes sure they are close to each other within the entire length of the cable. A variation of this technique, differential pair routing, is a widespread technique used in PCB layout for critical signals.
Another problem seen in high-speed communications is signal reflection. If there are any impedance mismatches along the signal path, the maximum power will not be transferred beyond that point, and some of the signal energy will be reflected back to the source. In summary, a UTP cable has four balanced twisted pairs that have ohms characteristic impedance to reduce reflection , and they are twisted at different turn-ratios to reduce crosstalk between pairs.
The industry is doing its best with cable manufacturing, and this article will guide you through how to get the best PCB layout to avoid any signal noise or loss side-effects. All information traveling along a cable must be suitably digitized according to the required controller protocol, regardless of its architecture. The hardware designer usually has three options when implementing a Gigabit Ethernet interface into their system:.
MX6 and i. Generally speaking, the second option we listed is always preferred if the processing unit has a sufficient MAC interface MII for the required gigabit interface quantity for the design.
Even with the limited number of MAC interfaces on the processor side, the use of IC level Ethernet switches may solve any problems if all the Ethernet interfaces operate at the same network confidentiality level.
Use by the defense industry may require physical separation of the interfaces for security reasons. Based on the information we have covered so far, we have chosen a design example that will be based on using a discrete PHY and integrated MAC. Note that the specific selection criteria and consideration of their features will be covered in the following sections. Tip: Pay attention to the environmental requirements of the integrated circuits when you make your selection. Double-check your needs in terms of operating temperature, ROHS compliance, and moisture sensitivity in addition to the electrical requirements such as voltage level, device footprint, etc.
However, the IEEE There are two fundamental reasons for this isolation requirement. The first is due to the possible ground offset between devices that are located far away from each other.
The second is to protect all devices from line failures such as a short to a high-voltage rail , a surge-spike , or an ESD strike. Although the Ethernet standard does not strictly define the isolation method, using either a transformer or an optoisolator is usually the preferred option in the first instance.
However, transformer isolation has some great advantages when used in Ethernet applications, and it is widely used in circuit designs. The benefits of using a isolation transformer are:. A couple of disadvantages of using a transformer is that it blocks the DC component and is not very efficient at low frequencies. However, these can be easily solved by the modulation scheme and selecting an appropriate transformer that meets the chosen Ethernet protocol standard definitions.
After deciding to proceed with using the transformer option, and after a brief supplier search, the first question you are most likely to have is whether you should use discrete magnetics or a connector with integrated magnetics. Unfortunately, there is no perfect answer, and the trade-off between these options needs to be analyzed in detail by the designer. A comparison of the two options is summarized in Table-1 below The bold text denotes the winner. Table 1. The trade-Off between Discrete and Integrated Magnetics.
For mass-produced commercial projects and hobby-level electronics, integrated magnetics are a perfect fit as they reduce costs and simplify the design process.
Here, the discrete magnetics option will be selected for our design example. The internal structure, selection criteria, and connection diagrams for the discrete magnetics will be described below. Firstly, the selected magnetics should have a transformer block for each of the four pairs that are used in Gigabit Ethernet applications.
Also, even though it is not mandatory, having a common-mode choke CMC to increase the common-mode noise immunity is always a good option.
Although differential receivers on their own are good at rejecting common-mode CM noise, with the help of CMC, the signal-to-noise ratio and, as a result, the bit error rate will be improved on the receiver side. Another optional component in the magnetics is an auto-transformer that creates a high-impedance path for the differential signals while creating a low-impedance path for the CM signals.
To summarize, as shown in Figure 5 above, a isolation transformer and common-mode choke are always included in the magnetics available in the market. Selecting the winding magnetics option will increase costs while decreasing the risk of failure on EMC tests. Alternatively, the 8-winding magnetics option is cheaper and allows a good layout design, but the EMC test failure risk may need to be mitigated. It is good practice to select the winding magnetics option if the Ethernet interface is a part of a digital system that generates a lot of noise.
If an 8-winding is desired in such circumstances, consider connecting the CMC side to the cable side for better EMI performance note that connecting these the other way around will also work.
Where a winding is selected, the auto-transformer should be connected to the cable side for the correct operation. Therefore, a designer needs to use every noise reduction technique available and have some alternative enhancement options ready to mitigate the risk to ensure that the levels are low enough in the final design.
Independent from the topology in magnetics, both the isolation transformer and the auto-transformer have their center taps routed to pins to provide additional termination, filtering, and biasing options.
According to the patent of Robert Bob W. Smith, the UTP cable pair-to-pair relationships form transmission lines relative to each other.
If the transmission line is not terminated correctly, then there is a possibility of a reflection that will degrade the signal quality. To prevent reflections, it is recommended that each center-tap on the cable side including 8- or winding components are separately terminated using a 75 ohms resistor to the magnetics chassis ground. It is also good practice to add one high-voltage capacitor between the termination resistor and the chassis to form an additional filter for common-mode noise reduction, similar to split termination topology.
Note that each center-tap should have an individual termination resistor, while just one capacitor is adequate for all four chassis connections. See Figures 6 and 7 below. When it comes to the center tap on the PHY side, this should generally be connected to the signal ground using a capacitor for additional filtering purposes.
Like the Bob-Smith termination resistors, each center-tap for the pairs should have their own capacitors to prevent any stray current flow between each pair.
Please check the PHY datasheet carefully to identify which biasing and line-driver configurations are applicable.
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