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Topic: Stepper motor Micros stepping jumpers, do they need grounding to be low? (Read 222 times) previous topic - next topic

n00b

Hi, I'm building a PCB that uses a couple of A4988 Stepper motor drivers, and want to add jumpers for micro stepping.

From what I have read the A4988 has an internal pulldown resistor on PINs MS1 (100k), MS2 (50k), and MS3 (100k) 

Ergo, my jumpers would only need to have a connection from MS1/MS2/MS3 to VCC, and not ground. (jumper = High, No Jumper=Low)........Right?  :-[

Robin2

Are you using a bare Allegro A4988 microchip, or are you using an A4988 module such as those sold by Pololu?

If it's the bare microchip then this is an extract from the Allegro A4988 datasheet that I have
Quote
Microstep Select (MSx).
The microstep resolution is set by the voltage on logic inputs MSx, as shown in table 1. The MS1 and MS3 pins have a 100 kΩ pull-down resistance, and the MS2 pin has a 50 kΩ pull-down resistance.
which suggests that your jumpers need to pull the pins HIGH.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

n00b

Are you using a bare Allegro A4988 microchip, or are you using an A4988 module such as those sold by Pololu?

If it's the bare microchip then this is an extract from the Allegro A4988 datasheet that I havewhich suggests that your jumpers need to pull the pins HIGH.

...R
Sorry, should have made that clear, I'm using Pololu modules like this:



so thinking my jumper just needs to be like this, no need to tie them to ground due to the internal pulldown resistor?


MarkT

The chip has only weak pull-downs, so if there is any length of trace from the MSx pins, add external 4k7
pull-downs as well to avoid noise-pickup.
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n00b

The chip has only weak pull-downs, so if there is any length of trace from the MSx pins, add external 4k7
pull-downs as well to avoid noise-pickup.
Ahhh, thank you. I see on the RAMPS 1.4 board they have an extra 100K pull-down



Is this OK or do I need one on MS2 and MS3 too?

And one other question, if I want the jumpers to set multiple drivers, I just link the MSx pins?

MarkT

If it needs external pull downs, much stronger ones would be better.  Depends on trace lengths.
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n00b

If it needs external pull downs, much stronger ones would be better.  Depends on trace lengths.
about 4cm including the daisy chain to the second set of MS pins

Seen modules with and without external pulldown, and it totally works breadboard without external pull downs, but would rather go with good practice here, rather then "it worked so it's fine"  :)

MarkT

I'd go with 10k or 4k7 then.  Lower if the motor wiring runs near the MS traces/wires.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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