# "Unable to read TMC UART 'extruder' register IFCNT" - Klipper and Robin Nano

I'm trying to use software UART for TMC2209 steppers on a BigTreeTech v1.2 and apply that to my Robin Nano v.1.1 using Klipper firmware.

I declared a different pin for every driver and I edit my printer.cfg in this way:

[tmc2209 stepper_x]
uart_pin: PA10
run_current:  0.75
hold_current: 0.4
stealthchop_threshold: 9999
interpolate: true

[tmc2209 stepper_y]
uart_pin: PA9
run_current:  0.75
hold_current: 0.4
stealthchop_threshold: 9999
interpolate: true

[tmc2209 stepper_z]
uart_pin: PC7
run_current:  0.75
hold_current: 0.4
stealthchop_threshold: 1
interpolate: true

[tmc2209 extruder]
uart_pin: PC13
run_current: 0.3
stealthchop_threshold: 1
interpolate: true


I watched this video

But I have this error:

Unable to read TMC UART 'extruder' register IFCNT


Why is this happening?

• That means that the pins are not correct or that a connection is missing. Some TMC stepper boards require soldering to connect UART from the chip to one of the two pins shown in the enlargement. Have you done it?
– FarO
Jun 10, 2021 at 13:22
• @FarO That sounds like the beginning of an answer? Please write an answer if you're sure about this. Indeed this message tells that the pins aren't connected correctly, and for 2208's I'm sure this can be a soldering issue, is that also applicable to 2209's? Please elaborate in your answer.
– 0scar
Jun 10, 2021 at 21:52

I had the same issue. In general, that means that the pins you specified doesn't work.

"I declared a different pin for every driver": you don't need just "different pins", you need the CORRECT pins. Check if you specified the correct ones: below the SKR the pins are labelled.

And then check that the TMC module has the UART pin(s) (there are two) are connected to the chip! the TMC modules often are sold with three empty pads on the lower side of the PCB. The central one goes to the chip, then each other pad goes to one pin. You need to bridge with solder the central one and the pad for the pin you want to use.

In the following image you can (barely) see that the central pad near the red line is bridged with the upper one.

Also, for the extruder the stealthchop threshold should be 0, not 1! you don't want stealthchop for the extruder.

• yes i solved....the correct pin for extruder is PA8 on robin nano v1.1 Jun 11, 2021 at 16:35
• you can accept the answer if you think it was useful
– FarO
Jul 12, 2021 at 20:09