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I have a system where I will connect up to 128 identical magnometer ICs up to an FPGA. I am trying to decide between SPI and I2C for comms. If I use SPI the setup will require 128 + 3 wires IF all sensors are on the same bus.

How do I decide/calculate if this is feasible and the sensors and wire won't load the circuit with too much capacitance or resistance?

The sensors will be distributed across several circuit boards so most of the wire length will be connecting sensor boards to each other.

I guess a general answer would be nice but just in case I am using these sensors and FPGA.

EDIT: I appreciate any helpful advice on changing the system but what I'm looking for is how to calculate how many wires and sensors I can add before the capacitance is too great.

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  • \$\begingroup\$ Why not use a multiplexer and differential signaling? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 4 '14 at 15:00
  • \$\begingroup\$ What data rate are you planning to use? Can it operate in daisy chain mode? \$\endgroup\$ – Andy aka Oct 4 '14 at 15:00
  • \$\begingroup\$ I haven't decided the data rate but 1Mhz or so would be ideal. Down to 200kHz might be doable. Daisy chaining would make the system too slow \$\endgroup\$ – ballaw Oct 4 '14 at 15:19
  • \$\begingroup\$ Please tell me how would a multiplexer and differential signaling work? The sensors take single-ended signals. \$\endgroup\$ – ballaw Oct 4 '14 at 15:20
  • \$\begingroup\$ You have a driver on one side, and a receiver on the other. Or just on one side if the FPGA can generate/process a differential signal. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 4 '14 at 15:26

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