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What is the best way to route a SPI bus with four loads?

  • One bus with multiple drops.
  • A star pattern with equal length branches to each peripheral.
  • Or other configuration.

All 4 peripherals and controller are on the the same board. The peripherals are from 2" to 8" from controller (master). The SPI bus will be run at 3.3V and between 30 - 40 MHz.

I am primarily worried about signal integrity and reflection, on CLK mainly but data also, due to the long lengths to each peripheral. I have seen some examples, but they usually only deal with 2 peripherals.

Roger P
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  • Do the considerations in https://electronics.stackexchange.com/questions/33372/spi-bus-termination-considerations answer your questions? – kruemi Mar 24 '22 at 11:18
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    40MHz = controlled impedance (transmission line) bus configuration with source or/and end termination, likely over a dedicated ground plane. Stubs (drops) as short as possible. – rdtsc Mar 24 '22 at 12:47
  • @kruemi. Some of the discussions and the considerations in electronics.stackexchange.com/questions/33372/… have some application to SPI bus with multiple loads, but usually in a general manner. And some of them I couldn't tell if they were talking about a single load or multi-load design, or they were running at much slower frequency. But none of of them really address how to route and terminate the SPI bus with multiple load at higher frequency in any detail. Yes it will be 50-ohm controlled impedance over a solid ground reference plane. – Roger P Mar 25 '22 at 19:26
  • From reading some of the answers and other info in other links it appears that a multi-drop with short stubs and some kind of termination may be a possible solution. But what kind of termination scheme is best without drawing too much current is a problem. Some of the comments recommend series resistor, but it was also stated that series resistor does not work with multiple load designs. – Roger P Mar 25 '22 at 19:27
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    I did finally find a post with similar issue and a possible solution. https://electronics.stackexchange.com/questions/165512/spi-bus-termination-issue?rq=1. This does use the daisy-chain/multi-drop routing with end termination. Thanks. Roger – Roger P Mar 25 '22 at 20:10

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