On Tue, 16 May 2017, Gene Heskett wrote:
> Date: Tue, 16 May 2017 13:22:59 -0400
> From: Gene Heskett <ghesk...@shentel.net>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <emc-users@lists.sourceforge.net>
> To: emc-users@lists.sourceforge.net
> Subject: Re: [Emc-users] switching to a slower spi driver to see if it works,
>
> On Monday 15 May 2017 09:58:04 Gene Heskett wrote:
>
> I may have found it!!!
>
> Hooking up my scope, I discovered that the original query to get the
> board to ID itself, IS NOT MADE AT THE SAME CLOCK SPEED IT ACTUALLY
> OPERATES AT. THE INITIAL QUERY IS BEING MADE AT 50 MHz!
>
> I hung a scope probe on the clock, which due to its loading, changed the
> shape of the waveform and made it work. Its now running at 32 MHz, but
> I had captured the initial failure, and that was clocked at 50MHz.
>
> Now, I'll go remove my scope probe and see if it fails.
>
> Better yet, without the probe it fails, with the probe on the pi end of
> the 82 ohm terminating resistor I used in my home made adapter, it works
> 5 out of 5 times. Move the probe to the other end of that resistor and
> the failure rate is 5 out of 5 times. I took some pix of the single
> trace capture of this startup card query. Quite a bit of the voltage
> swing the pi is generating is being used up in the resistor, at least
> half a volt.
>
> Pix on my web page at
> <http://geneslinuxbox.net:6309/gene/lathe-stf/Sheldon_lathe-pix/Upstream-sclk.jpg>
> and
> <http://geneslinuxbox.net:6309/gene/lathe-stf/Sheldon_lathe-pix/Downstream-sclk.JPG>
>
> I note that if it succeeds, which it seems to do with my probe furnishing
> some initial loading, it then slows to 32MHz for the clock rate.
>
> And, when its hooked up with the OSHPark adapter and its 86.7 ohm
> resistors, there is not enough signal to tickle the 7i90 into responding
> at all.
>
> I'll go hook it up with one of those & try it again.
>
> 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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50 MHz will work if everything is terminated correctly and if the RSPI timing
is correct. (It may require using a SPI late data sample option if the RSPI
hardware has that option)
Correct series termination of RSPI outputs (130 Ohm total for 0.050" flat
cable) depends on the output impedance of the RSPI outputs so there's no
guarantee that the added 87 ohms is correct (this may also depend on RSPI
output drive strength settings if such things exist on the RSPI)
Peter Wallace
Mesa Electronics
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