Electronic – SPI to USB converter Question (for STM32F105)

4-20ns rise time is pretty slow - each nanosecond is about 15cm at PCB trace speeds. Various rules of thumb exist - Howard Johnson proposes that if the length of your interconnect is < 1/6 of the risetime, you can treat it as a lumped circuit and neglect most of the signal integrity problems that rear their heads in longer circuits.

For you, 4ns is 60cm, so if your wires are shorter than 10cm, don't worry about it.

Regarding mitigations, without access to a simulator, the easy things to do are:

• Series terminate the serial-clock line, some value between 27 and 47 ohms is usual.
• Ensure that there is a ground return next to each signalling wire in the ribbon/flex cable. If you can't do that, ensure that the clock line is at least next to a ground line.

I've more deeply studied the field, and found that 15 kOhm pull-down resistors are required to hold the data line at 0 V (D- in the full-speed mode or D+ in the low-speed mode). In my particular case, all devices are full-speed, so I will try to use them on the D- lines. I still not figured the role of the series resistors, but there are built-in resistors in CP2102 and I will add 22 Ohm resistors to the lines to my uC, just in case. CP2102 and ATMega32U4 are both have pull-up resistors on their D+ "inputs", it tells the downstream device (USB Hub in my case) that it is a full-speed device.

So, I will try to use only one pull-down resistor and will post here results, but, by now, my advice for the on-board usb interconnection is the following:

• carefully read the documentation on the upstream device internal USB transceiver circuit. What components it already have?
• if some external resistors (series and pull-up) are required by the upstream device, use them
• use all resistors (series and pull-down) on the down-stream termination
• capacitors and ESD protection are not requiered