>> Also regards grounding, you need to watch out for ground loops that
>> let high frequency noise currents flow between the 7I90 and the CPU.
>
> Humm, would one of those clamp-on ferrite chokes help? Packrat that I am,
> I'm pretty sure I could find one.
A ferrite bead on the connections to the RPI and 7I90 will help, if you put it
on the cable between them you will make things worse (raising ground
impedance)
>
>> The most critical signal is the clock from the SPI master. Depending
>> on the drive strength it may require a series termination resistor
>> (the 7I90 SPI output are series terminated so need no termination )
>
> Thank you Peter, this points me in the right direction. And I'll term the
> signals _from_ the r-pi at the r-pi. Would it be too much loss if I used
> 120 ohm r's? I am pretty sure I can find enough of those. Obviously I
> haven't a clue what the source impedance of a pi gpio pin is or what its
> swing is. The r-pi of course is running on 5 volts but I'd expect thats
> cut way down for power savings.
I would suspect that the output impedance of the RPIs signals is not lower
than 25-50 Ohms so 120 is probably too high.
Also note that when using series termination you must look at the signals at
the destination (the signals will look funky at intermediate points)
Also you need either a passive probe (easy to make with a 450 Ohm resistor
into a 50 Ohm coax/50 Ohm scope input) or a FET probe, a normal 10M probe is
pretty much useless for SI checking (way way too much input capacitance)
>
> I have a 2 gigahertz sampling scope, 1 ghz per trace but just looking at
> this stuff with a 10pf scope probe reminds me of Schroedingers famous
> cat.
>
>> Peter Wallace
>> Mesa Electronics
>>
>> (\__/)
>> (='.'=) This is Bunny. Copy and paste bunny into your
>> (")_(") signature to help him gain world domination.
>
>
> Cheers, Gene Heskett
> --
> "There are four boxes to be used in defense of liberty:
> soap, ballot, jury, and ammo. Please use in that order."
> -Ed Howdershelt (Author)
> Genes Web page <http://geneslinuxbox.net:6309/gene>
>
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Mesa Electronics
(\__/)
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