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I am designing a circuit board that has an SPI bus with 32 slaves. I plan to drive the bus at 8 MHz. Another individual on this site commented on a previous post of mine that the total capacitance from all the slaves might limit the maximum rate.

I went ahead and tried that calculation. The capacitance of the GPIO pin on the MCU is 30 pF and the capacitance on the CLK lines for the slave is <10 pF. I compute the resistance of the CLK trace to be 0.6 ohms using an online trace calculator (8 mil, 1 oz/ft, 20 cm total length)

C = 30 pF + (32*10 pF) = 350 pF
R = 0.6 ohm
tau = RC = (350 pF)(0.6 ohm) = 0.2 ns
t_rise = 2.2 * tau = 0.44 ns

I was able to probe a similar board with 16 slaves (predecessor to the board I'm designing) and found the rise time to be 18 ns. I made sure to set the probe to 10X.

So my question is: where am I going wrong here? Do I need to take into account the output resistance of the GPIO as well?

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Yes, the rise time will be dominated by the current available from the GPIO pin (it's wrong to think of it as a resistance) and the capacitance. The trace resistance can be ignored.

Look at the guaranteed specs for rise time and what the load is for that guarantee. The capacitance of that output will be in addition to the specified external capacitance. For example, the PIC 16F series is generally specified with a 50pF external load. If your total capacitance exceeds 50pF, then the datasheet limits are not guaranteed.

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You must also take into account the stray capacitance from the traces. This can be relatively large if the traces are long, and the board is multilayer with ground planes or power planes close to the signal wires.

Another potential issue is ringing, which can come into play at 8MHz. You may have to add some series resistance (perhaps tens of ohms) to tame ringing (especially on the clock), which will further slow the bus lines. It's good to hook a fast oscilloscope onto the lines and check the waveforms.

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  • \$\begingroup\$ The datasheet specifies a 2.33 ns rise time for a 8 mA drive strength and 50 pF load. Can I use these numbers to extrapolate what the rise time will be for a higher capacitive load? I.e. solve t_rise = 2.2*R*C for R and then recompute with a different C? \$\endgroup\$ – dgreenheck Sep 15 '15 at 19:07
  • \$\begingroup\$ It's probably not longer than 2.33ns * Cload/50pF, but actually that implies a very high output current. Check the numbers carefully. \$\endgroup\$ – Spehro Pefhany Sep 15 '15 at 19:24
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I think, since you are using single ended lines may be it having 50Ohms impedance. So 350pf*50ohms=17.5ns. be the rise time.

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