Access to TinyG step/dir for larger drivers?

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  • March 7, 2012 at 8:59 pm #1094
    epicycloid
    Member

    I am just about to order a TinyG but I have two things I can’t seem to find answers to by reading/searching…

    1) I’m wondering if there is access to the step, direction and enable signals on the board that might be used to run other larger (higher current) drivers instead of the on-board DRV8811’s, e.g. a Geckodrive G540? Or their 7A drivers like the G201X, or G203V.

    I see mention of the signals, at the port level in the hardware doc, but I don’t know if they are exposed at pads on the board? I see 8 tiny pads at one end of the driver chips, but no silkscreen indicating what they might be. I didn’t see any indication of connectors or other tie points in the schematics for step, dir or enable.

    2) A related/next question is about where/how microstepping is determined for TinyG… I see the two microstep bits for the DRV8811, and see the modes it supports. But the TinyG docs imply that only 1/8th step mode is used. If one were to be able to get step/dir signals, most/many of the Geckodrives use 1/10th microsteps, and other drivers use smaller than 1/8th microsteps, e.g. 1/16th steps. Would it be fairly easy to change the microstep size to something other than 1/8th steps?

    And BTW, thanks for working so hard on such a cool idea!

    March 7, 2012 at 9:43 pm #1095
    happytriger2000
    Member

    I use a voltmeter to track the puls and dir pins then solder a wire onto those pads and link it to the driver.

    March 8, 2012 at 9:34 am #1096
    alden
    Member

    The step and direction pins are not broken out separately as there are not enough IO pins to do this for the way the board is configured. However, if you want to hack the board or the C code this is what I’d recommend.

    – You can grab the TinyG schematics here:
    https://github.com/synthetos/TinyG/blob/master/hardware/board131v6_schematics_pg1.pdf

    – The TI chip pinout is here:
    http://www.ti.com/lit/ds/symlink/drv8811.pdf

    – Repurpose the GPIO2 port (J8 on the schematic) as outputs instead of inputs. You lose some homing inputs, but actually you only need one pin to trigger homing, so you get 7 that can be used for outputs. Bear in mind that these are 3.3v outputs and are not isolated (opto or otherwise) so you need to be careful with them. Alternately you could just take the existing bits that go to the steppers off the board as Freddy has. These are all mapped in system.h. Look at stepper.c to see how they are used.

    Microstepping is an interesting question. Microsteps is a motor parameter that is set in the default profiles for the settingsXXXXXX.h files. The microstep value can be changed from the serial port using the $1mi command (where ‘1’ is the motor number 1-4). This command does 2 things: (1) it sets the _set_motor_steps_per_unit() internal system parameter and (2) it sets the IO bits for the 8811s. You don’t care about (2) but you do care about (1). I have not tested this, but I see no reason you could not enter 10 or 16 which should set (1) correctly for step generation. You may lose some maximum step rate as the steps/sec X microsteps cannot exceed 50,000 (Hz). My examination of the code points out that even though the (2) operation would fail, the system won’t complain, so perhaps I need an error message here, and possibly blocking the (1) action from occurring!

    If you are doing any software hacking I’d wait a few days for version 0.93 to appear on the github site.

    – Alden

    March 8, 2012 at 12:03 pm #1097
    epicycloid
    Member

    Thank you very much Alden. That gives me a much better notion of what I’d need to change. I had read through system.h and stepper.c, but hadn’t dived into the settings header files. Now I see where each machine’s defaults are setup and it makes more sense.

    I think I’ll go down the “plain vanilla” path first, and make sure I have a more generic working system, before branching out into the wilderness.

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