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I2C bus needs capacitance?

I think when "intent" is mentioned, it gets away from the technical issues involved. You can have a poorer electrical environment on a PCB than you do on a ribbon cable. It depends on the details. I would not automatically preclude transferring data on by I2C this way. I'm sure it can work fine if handled correctly.

I know. But I think in this case it is not needed unless there is more he is not telling. I've never seen 5 volt logic that wouldn't work when driven from a 3.3 volt source... well, I shouldn't say "never", but it seems to work in the general case. Depends on the electrical specs of the devices or course.

The point is if he can't do anything about the 5 volt pullups, he can limit the voltage seen by the 3.3 volt device.

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Rick C
Reply to
rickman
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The MAX7219 requires 3.5v min for a logic high. I tried 3.3v and it doesn't work. At least the data sheet specifies 3.5v so you can figure out why it doesn?t work :-).

Reply to
Dennis

s/correctly/carefully

Don't forget the ground returns and be carefully of the edge rates of other signals in the cable.

CMOS high is guaranteed at .7VDD. Even at nominal power supplies, that's 3.5V and even then you have no noise margin. At VDD(Max) you're even in worse shape. Even if it switches, you may get excessive DC shoot-through current from the gate. It is *not* a good plan to run 5V CMOS with a 3.3V signal.

Reply to
krw

yep, and if you have to find something 74HCT

Reply to
Lasse Langwadt Christensen

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