A4988 driver with carrier question: decay mode


I’m a relative newbie in electronics.

Here are my questions pertaining to decay mode selection for
A4988 driver pololu.com/product/1183
Allegro documentation pololu.com/file/download/a49 … e_id=0J450

On page 7, the Allegro documentation says:
“By pulling the ROSC pin to ground, mixed decay is set to be active 100% of the time, for both rising and falling currents, and prevents missed steps as shown in figure 2”

My question:
The schematic for the carrier board shows ROSC (pin 13) grounded via R4 (10k).
Am I correctly understanding that this board is hardwired to automatic decay mode selection (slow decay periods are present)?
And that Toff = ROSC/825 implies a fixed Toff of 12 uS?
In order to achieve mixed decay 100% of the time (no exclusively slow decay cycles) and avoid missed steps described in figure 2, the suggested approach is to wire OSC directly to ground bypassing R4 completely?
Is it possible to do this mod ‘in the field’ with a regular soldering iron?

Thank you for the response!



Yes, we have the A4988 carrier set up to operate in automatic Mixed decay mode with a fixed off time of around 12 us. Connecting the ROSC pin directly to ground will change it to Mixed decay mode with an off-time of 30 us when microstepping. You can make this modification by removing that resistor and bridging across the pads.

Please note, the A4988 carrier you link to is the A4988 stepper motor driver carrier with voltage regulators, and the resistor connected to the ROSC pin is R5 in the schematic. For the A4988 carrier without voltage regulators, the resistor connected to the ROSC pin is R4.

- Jeremy

Thank-you for replying, Jeremy.

Being a noob, can you quickly tell me how to identify R5 on the A4988 carrier with voltage regulators?

I can’t seem to find any diagram depicting which resistor is the one I’m looking for on the board.

Thanks in advance!


I have circled R5 in the picture below.

- Jeremy