RPI Questions (3D printer + GPIO output [not mesa])

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28 Jul 2021 20:41 #216148 by robertspark
Hi, I've got some general questions regarding the RPI and the use of LinuxCNC to directly drive step and dir signals via the GPIO port on the RPI.....   is it possible?


I stumbled across this RPI GPIO BOB / cape which seems like a nice neat solution for using the TMC2209 stepper motor drivers for a 3d printer application (I already use them with a bigtreetech SKR 1.4 turbo board..... but I was wondering about switching from marlin to linuxcnc.

www.pandapi3d.com/product-page/pandapi-2-8

www.trinamic.com/fileadmin/assets/Produc...9_Datasheet_V103.pdf

 

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28 Jul 2021 22:11 #216153 by rollfree
I don't think so.
RPi has too bad real-time capabilities to directly control DIR/STEP via GPIO with acceptable speed a accuracy.

On that referenced board the real-time operations is performed by dedicated MCU.
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28 Jul 2021 23:39 #216163 by robertspark
Hmm 

The FAO on this page seems to imply that it does use direct step and dir from the RPI and the schematic seems to indicate that too (although the MCU does seem to have IO's too for the step and dir...)

github.com/markniu/PandaPi

gitee.com/markyue/pandapi_wiki/raw/maste...atic_PandaPiV2.8.pdf

.... but I do hear you regarding the direct stepping and GPIO though   ( I use Mesa hardware on my other Linuxcnc mill + lathe + router + plasma.... but I was wondering about a compact solution to use the TMC2209's for my 3d printer..... keep looking...  [it's not essential, just something I wondered about if it was feasible])

Thanks for taking the time though

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29 Jul 2021 00:19 #216165 by rollfree
Yes you're right. According to the description, it really seems that they control it directly via GPIO. And HW can do both.

But after practical experience and measurement, I somehow can't believe that it would really work well.

Unless they used some magic trick there.
Interesting.

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30 Jul 2021 19:29 #216382 by Doogie
The Remora project puts small firmware on a standard LPC1768/1769 based 3D printer controller board( like SKR v1.3/v1.4/v1.4turbo) and it works well. I did have to drop my extruder microstepping down to 1/8th to get faster retraction response but other than that it's a viable option.

remora-docs.readthedocs.io/en/latest/

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01 Aug 2021 15:02 #216614 by andypugh

Hi, I've got some general questions regarding the RPI and the use of LinuxCNC to directly drive step and dir signals via the GPIO port on the RPI.....   is it possible?
 

It is definitely possible. Running the latency test on my Pi400 with a base thread of 100µs is returning a latency of about 50µs. 

So, if 10kHz step-rate is enough (and it often is) then it ought to work. 
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01 Aug 2021 15:40 #216619 by robertspark
thanks, awaiting an RPI 4B, to have a play on
will report back when I get it going

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