(2) Equal length design of common PCB traces
1. Isometric design of serpentine lines
In PCB design, the purpose of network equal length adjustment is to reduce the difference of signal transmission delay on PCB as much as possible. The method of implementing network isometric adjustment in Altium Designer is as follows:
(1) In Altium Designer, the equal length adjustment of the network can be realized through the serpentine routing. Before performing the equal length of the snake, it is necessary to complete the connection of the corresponding traces of the PCB, and then execute "Wiring" → "Network Equal Length Adjustment" in the menu bar command, or press the shortcut key U+R, click the line that needs to be equal in length and press the Tab key to call out the "equal length attribute setting" panel, as shown in the figure.
① Target Length provides the following three target length types:
Manual: Manually set the equal target length.
From Net: Select the equal target length according to the network.
From Rules: Set the target length according to the rules.
② Pattern provides the following three isometric patterns:
Mitered Lines: oblique line pattern.
Mitered Ares Mitered Ares pattern.
Rounded: Semicircle mode.
The effects of the three serpentine equal-length modes are shown in the figure, and the oblique arc mode is generally used.
(2) After setting the equal length parameters, slide on the signal line that needs equal length to pull out the serpentine line. In the state of equal length, press the < and > keys on the keyboard to adjust the up and down amplitude of the serpentine line respectively, the number key 1 decreases the corner amplitude, the number key 2 increases the corner width, the number key 3 decreases the Space interval, and the number key Key 4 increases the Space interval.
(3) After completing a section of serpentine equal length, if you need to adjust the serpentine line, you can use the mouse to drag the modulation line to adjust, as shown in the figure. The equal length of the differential pair serpentine line is similar to the equal length of the single-ended serpentine line. Execute the "Wiring" → "Differential Pair Network Equal Length Adjustment" command in the menu bar, or press the shortcut key U+P, and click the differential pair that needs to be equal in length. And press the Tab key to bring up the "Differential Pair Equal Length Setting" panel, the setting method is similar to the setting of single-ended equal length, as shown in the figure.
2. Equal-length packets of DDR
DDR wiring grouping is generally divided into two types: data group and address group.
Data group: every group of 8 data lines (D0 D7/D8 D15), plus the corresponding DQS (data latch) differential pair and DQM (data mask). Note: The DDR data line is latched by DQS, and the data line wiring priority is the highest.
Address group: including address lines (A0~A14), clock lines (CLK, CLKN) and control lines (BA0, BA1, BA2, WE, CAS, ODT, RESET, CKE, RAS, CS, etc.). Note: Address and control lines need to be processed with equal lengths based on CLK. CLK controls the acquisition of data signals, so it is longer than the longest data line.
3. Isometric topology
3.1 Point-to-point connection
Point-to-point connection, that is, one pad is connected to another pad, and the connection channel is relatively unique, mainly for single-chip DDR, as shown in the figure.
Point-to-point equal length is relatively simple, just check the length in Nets, and equal length with reference to the longest line in the group, as shown in the figure. If there is exclusion between the main control and DDR, as shown in the figure. When the length is equal, you can back up a schematic diagram file, then modify the schematic diagram, short the two ends of the resistor, and introduce it into the PCB, so that the length can be equalized in a point-to-point manner. After the length is completed, add the backup schematic to the project and re-import the PCB. Note: When performing isometric operation, you can use the screening tool in the PCB panel, select Mask or Dim mode, and perform highlighting operation, as shown in the figure.
3.2 T-shaped topology
Compared with DDR, DDR2 and later series lack delay compensation technology. In order not to make the length error of the clock line and data strobe signal of each piece of DDR2 too large, T-shaped structure can generally be used for winding equal length.
The T shape is equal in length, and the simple structure is shown in the figure. In fact, the length of the wiring from the CPU to each DDR is equal, and the branch wiring on both sides of the T point is required to be as short as possible and the length is as equal as possible, as shown in the figure. Realize A+A1=A+A2=B+B1=B+B2.
3.3 Daisy Chain Topology
The structure of the daisy chain is relatively simple, and the impedance is relatively easy to control. Its simple structure is shown in the figure. It can be seen from the figure that the signal is transmitted between two adjacent components, from the CPU to the first DDR, and then to the second DDR. The actual connection diagram is shown in the figure, realizing A=B, A1=B1.
Note: After simulation verification, when the DDR rate reaches 1.6Gb/s, compared with the T-type connection, the daisy chain can better meet the signal integrity requirements.
