8.4.1 General Routing Guidelines
The following paragraphs detail the routing guidelines that must be observed when routing the various functional LVCMOS interfaces.
- Line spacing:
- For a line width equal to W, the spacing between two lines must be 2W, at least. This minimizes the crosstalk between switching signals between the different lines. On the PCB, this is not achievable everywhere (for example, when breaking signals out from the device package), but it is recommended to follow this rule as much as possible. When violating this guideline, minimize the length of the traces running parallel to each other (see Figure 8-28).
Figure 8-28 Ground Guard Illustration
- Length matching (unless otherwise specified):
- For bus or traces at frequencies less than 10 MHz, the trace length matching (maximum length difference between the longest and the shortest lines) must be less than 25 mm.
- For bus or traces at frequencies greater than 10 MHz, the trace length matching (maximum length difference between the longest and the shortest lines) must be less than 2.5 mm.
- Characteristic impedance
- Unless otherwise specified, the characteristic impedance for single-ended interfaces is recommended to be between 35-Ω and 65-Ω.
- Multiple peripheral support
- For interfaces where multiple peripherals have to be supported in the star topology, the length of each branch has to be balanced. Before closing the PCB design, it is highly recommended to verify signal integrity based on simulations including actual PCB extraction.