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Hello,
I see in the guideline document for USB design using the MSP430F5659 that it is recommended to use 27 ohm resistors in series with D- and D+. I am using a design that don't have those resistors and where the USB is working. Because of that, I was wondering why those resistors are mentionned, especially considering the following:
- USB FS is 12Mbit/s, so quite low speed for USB, meaning transmission line characteristics are not really important. This fact is even stated in the document : "At 12 MHz, full-speed USB usually is not highly sensitive to transmission line characteristics, but it is recommended to keep D+ and D- reasonably short". So I don't expect those resistors to be for source termination. Even if they were for line termination, I would expect a bigger value, because USB is 90 Ohm differential.
Are those resistors only to damp overshoot? I am really curious for their purpose as it seems to work without them, but I would rather follow best practice. That being said, I would like to understand what makes the topology presented "best practice" and not only repeat a layout.
Thank you,
Vincent
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