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I'm trying to do some crosstalk calculations between signals and I'm trying to find the minimum rise times for the STM32F768IIT6 - all I see are the maximum rise/fall times for the specified test conditions. I've looked in the datasheet and reference manual but all I can find are maximum rise/fall times. How do I find the minimum rise/fall time? Why are these not included?

For example I have SPI traces around 35mm long going to TLC5922 LED driver. How can I calculate what my fastest rise time will be ?

Thanks in advance!

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The minimum rise-time depends on as much on the output resistance of the STM32 as it depends on the capacitance on the wire. 35mm is not long. Just put the traces over a ground plane and you are good. If you think there could be some issues, put 10R resistors into the path (make sure that you still get a fast enough rise-time for the frequency you have chosen), but I doubt that this will be necessary.

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  • \$\begingroup\$ How do I find the output resistance of the MCU? \$\endgroup\$ – VanGo Sep 21 '17 at 17:03
  • \$\begingroup\$ By reverse engineering the datasheet. Look for voltage drop vs source/sink current. But be aware that this is at best an estimate and you can easily be off by an order of magnitude. Dynamic behavior of the output transistors is different from static loading condition. Also, the rise time will depend on an accurate guess on the capacitance on the trace and the impedance. \$\endgroup\$ – Attila Kinali Sep 21 '17 at 17:13
  • \$\begingroup\$ Ah I see. For my purposes, can I assume load capacitance of 50pf and assume that rise times won't go lower than 0.5ns ? Based on the datasheet the maximum rise time is 2ns EDIT: and they give their specs based on 50pf load (and 10 pf load as well) \$\endgroup\$ – VanGo Sep 21 '17 at 17:33
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    \$\begingroup\$ 50pF is probably a bit on the high side. The usual pin capacitance of a chip package is usually in the order of 1-10pF, most non-high speed packages are around 5pF...ish... if you have only one chip connected I would guestimate the total capacitance around 10-20pF. As for rise time, 500ps is probably the lower limit. 1-2ns is likely to be closer to reality. But if you really want to know it, measure it. \$\endgroup\$ – Attila Kinali Sep 21 '17 at 21:14

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