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At what point will I start having trouble with noise and parasitic capacitance/inductance when using standard 0.1in header pins? If I'm using an MCU capable of driving a 20MHz SPI bus, would it be reasonable to run a breakout board or Arduino shield at that speed? I will be using either the ST NUCLEO-F446RE or the NUCLEO-F303K8 development board.

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There is no such thing as a "maximum frequency" for the header itself, unless the wavelength becomes comparable to the total pin length and you need to treat it as a transmission line. This can be a serious issue at 100MHz with a 2-meter long coax cable, but with a sub-10mm connector, you must be talking obscene speeds. You will probably have much more than 10mm of traces on the main board and the breakout board.

I have used breakout boards for prototyping a few times, with an expensive chip on the breakout which I cannot desolder easily. Here is what you can do:

  • Use twice as many pins as you have signals. Connect odd pins to signals, and even pins to the ground plane. (this is for a 1xN header, you can probably do the same for 2xN but I have not tried)
  • Leave space for a 1206 ferrite bead on the breakout board. Place a 0 ohm resistor there. If the scope shows nasties, replace resistor with SMD ferrite. This may sound counterintuitive - as the bead will flatten your nice perfect signal fronts - but it will also greatly dampen the VFH harmonics.
  • Buffer the signals. I have done this on one occasion with great results where the breakout board was "broken out" with a maybe 20cm cable. 74AC14 will do wonders! Mind the level inversion, and also note this won't work for I2C as it requires a pullup resistor and has no 'direction' - the data signal goes both ways.
  • Whatever you do, at 20MHz or more, not only your main board but also your breakout board MUST have a ground plane. If you are designing your own PCB, and dont want to manufacture with more than 2 layers, just leave the bottom layer as a ground plane and do some creative jumper bridging on the top layer.
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The header should not be a problem at 20 MHz but the wires or cables attached to the header may cause problems if they are too long.

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  • \$\begingroup\$ Thanks for the quick response, but that does not quite answer my question. I was asking about the upper limit of the frequency I can send through a header pin. \$\endgroup\$ – Caleb Reister Aug 12 '16 at 18:08
  • \$\begingroup\$ @CalebReister - the point which this answer well makes is that the limit will not be the header pin, but rather whatever you have connected to it. In particular, you will need to worry about transmission line effects and impedance discontinuities, especially their location within the overall signal run, long before you have to worry about the pins themselves. \$\endgroup\$ – Chris Stratton Aug 14 '16 at 18:36

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