Author Topic: 25 MHz spi routing  (Read 1932 times)

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exapod

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25 MHz spi routing
« on: November 16, 2014, 03:19:26 pm »
I need to route a 25 MHz spi interface on a two layer board between three device 100 mm apart.
I never routed a spi with this frequency and distance so i would like some advice on what to avoid.
Should i put resistors in series with the signal and design termination resistors?
I will try to route all the signal on the top layer with a solid ground plane under them but to connect three device i have to use vias, what should i avoid?
Thanks for the help

 

Offline Scrts

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Re: 25 MHz spi routing
« Reply #1 on: November 16, 2014, 07:34:12 pm »
25MHz is SLOW. You can put series resistors, however you should route SCK wire at least 2x the track width away from the data lines in order to avoid cross-talk. Even 125Mhz is no issue, unless you're targeting DDR designs.
 

Offline markb82

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Re: 25 MHz spi routing
« Reply #2 on: November 17, 2014, 08:35:48 pm »
Ditto.   Check out the Saturn PCB Design Toolkit for cross-talk calculations.  Or: http://www.eeweb.com/toolbox/microstrip-crosstalk .  And keep the cross-talk between the clock line and any datalines to < -15dB.  Series resistors would help slow down the rise time so I would add them (right at the driver, ie mcu for sck, mosi, and at your devices for miso), even a 25 MHz design can have problems if you have a 1ns driver.

Edit:

Since this is likely a 2-layer board?, I think you may need more than 2x the trace width.  Or you could put a ground trace (or copper pour) between the clock line and the data lines?
« Last Edit: November 17, 2014, 08:41:45 pm by markb82 »
 


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