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NAND Flash - PCB layout tips

navry , 01-02-2024, 10:52 AM
Hey, I need to design a PCB with two NAND Flash chips as an extension to a board with an FPGA. The thing seems quite simple, but I have very little experience with PCB design and wanted to ask a few things. The memories will work in asynchronous mode. The minimum time of Read/Write Enable signals is 20 ns (50 MHz).

What in particular do I need to pay attention to when laying out the board?
In Micron's documentation, I don't see typical PCB design guidelines. I found on the web that I should pay attention to:
1. matching the length of the paths - I understand that the data lines must be fairly consistent with the RE/EC lines. RE/WE should be as long as the longest data path or as long as approximately the average length of the data paths?
2 Matching the impedance of the paths to 50 Ohms - I am planning a 4 layer board with a stack as below - in that case, am I calculating correctly (according to the PCB calculator) that the digital paths to memory should be 0.694 mm thick?

Could you please help?
QDrives , 01-02-2024, 08:32 PM
I think your findings are correct.
If you want narrower traces, you should change the stack-up to have the prepreg to something like 2x1080, which would get you something like 0.15mm (H) and trace width of about 0.27mm
navry , 01-03-2024, 12:56 PM
Thank you for your confirmation. This is really very helpful 🙂
I only have a stack-up to choose from yet with 1x1080 and 1x7628 -> 256 um and in that case I can narrow the paths to about 0.47 mm. That's always something 🙂
navry , 01-03-2024, 04:22 PM
And I would still like to add a question. Should I then add a series resistor on the data and RE/WE lines? Next to the memory chip?
Or if I am going to route the paths of the appropriate width, do I no longer need to add it?
QDrives , 01-03-2024, 08:45 PM
I worked with 2 designs (one was my own) where there were >= 50MHz signals outside of the MCU. In both cases the EMC was terrible. Neither of them had termination.
However, both projects were canceled before I could figure out exactly what caused it and how to fix it.
I also have little experience with external flash. So perhaps @Robert Feranec can help you with this one, unless either datasheet (MCU / flash) already mentions something.
navry , 01-04-2024, 01:06 PM
Thank you! I found such a description in the Microchip SAMA5D3 Layout Recommendations. It appears that it is worth adding these resistors when there is a longer path (then treated as a transmission line). I need to educate myself on these issues, I remember little from my studies on this subject. As if Robert would find a moment to suggest something would be great 🙂
Robert Feranec , 01-05-2024, 07:09 AM
I would only used resistors if it is recommended. Resistors may be needed to tweak impedance, but if the pins already have som capabilities (e.g. to set them as week, normal, strong ... slow, fast, ...) then resistors may not be required
navry , 01-05-2024, 08:48 AM
Thank you, I really appreciate your help
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