PCB Layout for multiple SPI devices. 1

[JohnDW]

Hi,
Working on my first project that has multiple slave SPI devices served by a single microcontroller.
When laying out the PCB tracks for the SPI control lines between the devices what are best design practices?

Do I simply;
Daisy-chain the control lines between the devices.
Leave absolute minimum spurs to each device.
Avoid hard right angles in the track.

I assume you should not;
Have a star type layout.
Need any form of termination resistor.

Any pointers appreciated,
Thanks, John.

 

[zappedagain]

SPI on the PCB will be fairly forgiving. At 10 Mbps you'll want clean clock pulses, so you'll want at least the third harmonic (70 MHz). Your wavelength in air for 70 MHz is over 4 meters (and longer on the PCB - velocity of propagation). You don't have to worry about stubs and transmission line effects until your trace lengths get to be about 1/10 your wavelength.

Have fun! Sounds like a good introductory project.

John D without the W

 

[VEBill]

...Your wavelength in air for 70 MHz is over 4 meters (and longer on the PCB - velocity of propagation). ...

Longer? Is that correct?

 

[itsmoked]

I thought it was shorter too..

SPI is pretty forgiving as zapped sez, wire it up any way you want it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[JohnDW]

Thanks for the reassurance guys, good to know SPI is not super picky, I'll just go for a tidy layout on the prototype board and see how we fair. I'm sure there will be plenty of other bits that'll trip me up in the layout!
Cheers,
John.

 

[zappedagain]

Ooops, brain fart. wavelength = C / freq -> Slower velocity means shorter wavelength... seems kind of counterintuitive if you think about the energy side of things... As my brother would say "there you go thinking again!".

Z

 

[hrcooper]

Remember each SPI device needs to have its one chip select line. Good PCB layout comes from experience, having done it for the last 20 years (started with Bishop graphics and mylar). Right angles are avoided because its more difficult to etch..the chemicals can sometimes sit in the tight corners, I always do 45's. As others have said..slow speeds, pretty forgiving. Just keep them away from other traces that might have strong dynamic signals and watch for ground currents as well.