In high-speed design, we usually need to match the signal line length based on the consideration of timing. For the signal path with only two pins, the process of calculating and matching line length is very direct and clear. However, for most high-speed designs, the situation will be more complicated, including but not limited to:
1. The termination resistor is needed in the signal path. In this case, the wire length to be modulated is different networks including the termination resistor.
2. T-point topology commonly used in data bus. As shown in the figure below, it is the different branches of the same network that need to be adjusted to the same length: 2-3 = 2-4 = 2-5 = 2-6.
3. In DDR3 / DDR4, the address bus and control bus use the fly by topology, which needs to match the line length of different branches in the same network
Before the introduction of xsignal, the net length and matched net length of ad could only be used for the same network, and could not modulate a certain path in the same network, nor could it process the termination resistance. Although there is always a “from to” object, there is always a bug. The “from to” object is invalid for the rule Engineers have no choice but to use Excel to calculate the length of each section, and then slowly adjust, the efficiency is very low.
After AD15, xsignal can not only solve the problem of branch line length modulation, but also solve the problem of signal path modulation with termination resistance. In short, xsignal can define the signal path between two nodes, whether they are the same network or different networks.
Create a new xsignal
There are several ways to create xsignal
Using the run xsignal Wizard
Select the source device and click “create xsignals between components” to define the xsignal between the two devices
Select the source device and click “create xsignals from connected nets” to define xsignal through the source device and the network connected with the source device
Click “create xsignals” to create xsignal manually.
The use of the wizard is a bit complicated. I’ll talk about it later. Here are two other ways.
Using the create xsignals between components dialog box
Click menu design » xSignals » Create xsignals: in the dialog box, the left side is the source device and the right side is the target device; Multiple target devices can be selected.
Follow these steps to create xsignal:
Select source component
Select destination component
Choose the network you are interested in
Click “analyze” and ad will automatically analyze all paths of the network of interest between the two devices. Please check the results in source and destination carefully for correctness (especially in case of termination resistance)
Enter the name of the created xsignal class in the include created xsignals into class column, and it will be classified into all xsignals class by default.
Click OK to complete the creation. After completion, you can see all the created xsignal in the PCB panel.
Using the create xsignals from connected nets dialog box
If your xsignal contains termination resistors, it is more direct to create xsignal in this way. Select the termination resistor and click “create xsignals from connected nets”:
The figure above shows the drain rn33 with four pin angles selected, and the source component nets on the right shows all the networks connected with drain. After clicking “analyze”, the system will automatically analyze the possible xsignal. Note: please carefully check whether the network in source and destination is correct. If not, you can adjust it manually. After confirmation, complete the creation of xsignal.
The function of net topology
Xsignal defines the signal path between two nodes (or pads). Ad analyzes the signal path according to the connection line between two nodes, and the definition of the connection line is determined by the routing topology rules. The default routing topology rule is shortest, that is, the shortest.
The following figure shows the animation of CPU connected to four memories in fly by topology. You can clearly see the definition of four xsignal
The definition of fulcrum in balance t
T-point topology not only needs the same length of trunk, but also the same length of branch.
The red point in the figure above is the branch point. It is necessary to ensure that each branch point is equal to dram1, 2, 3 and 4.
The problem is that xsignal in ad must be created with pin pins of solid devices, and does not support independent pads or vias. In mentor and cadence, a pad or via can be defined as a “virtual T-point” to modulate the line length of a branch. However, in AD, this method cannot be supported at present. An alternative is to make a separate device (schematic symbol + package) for the independent pad / via, and then place the device at the t point. Although there are some troubles, it solves the embarrassment that the branch network can’t define the line length rule and modulate the line length before ad.
Length & matched net length rule
After xsignal is created, the next use method is exactly the same as net. Remember, xsignal class corresponds to net class; Xsignal corresponds to net. The difference lies in the path between pin and pin defined by xsignal. Net is a network with the same properties.
The following figure shows the schematic diagram of xsignal in PCB panel and a screenshot of defining matched net length rule
The creation of specific rules will be introduced in detail in later chapters.
To sum up, xsignal is a very important function, which basically solves the problem that Altium designer could not accurately modulate the signal line length in high-speed design before. Combined with how to accurately measure the signal length, the improvement of the accuracy of line length calculation mentioned in this paper greatly reduces the time of layout in high-speed design, and improves the accuracy of design.