Unable to write tmc spi 'stepper' register GLOBALSCALER

[Sineos]

Unable to write tmc spi ‘stepper’ register GLOBALSCALER

Background

The error “Unable to write tmc spi ‘stepper_…’ register…” indicates that your printer’s microcontroller unit (MCU) has failed to communicate with a specific TMC stepper motor driver. This communication happens over a bus known as the “Serial Peripheral Interface” (SPI).

Note:

• Klipper can identify which driver it failed to communicate with (e.g., stepper_x), but it cannot determine why the communication failed. The root cause is almost always a physical or configuration issue with power, wiring, or hardware settings that must be diagnosed and fixed manually.
• The reference to a specific TMC register like GLOBALSCALER, GCONF, or CHOPCONF is a “red herring.” The error is related to an SPI communication problem, not the function of that particular register. The error is triggered on the first register Klipper attempts to configure during initialization.

Causes and Potential Solutions

The klippy.log will confirm that this error occurred, but will not indicate the specific cause. A systematic, hands-on approach is required to identify and resolve the issue. Work through the following potential causes, ordered from most common to least common.

• No or Insufficient Motor Voltage (VM) to Drivers
  This is the single most common cause, especially on new builds or after wiring changes. The printer-board’s logic may be powered, but if the supply voltage jumper is set incorrectly or the motor power is not properly connected, the drivers will not receive power. This error will typically appear for all drivers at once.

  Important:

  • Many printer boards have multiple jumpers related to supply voltage. The most common are jumpers that select between USB power and main supply power for the board. For regular operation, only the main power supply may be used.
  • Some boards include additional power terminals specifically for the stepper drivers, along with hardware jumpers that select whether the drivers are powered from these dedicated terminals or from the board’s main supply. The jumper settings and wiring connections must match the intended configuration.

• Incorrect Printer-Board Jumper Settings for SPI Mode
  Most controller boards use physical jumpers under each driver to select its operational mode (Standalone, UART, or SPI). For SPI communication to work, these jumpers must be set correctly for SPI mode as specified in your board’s manual. Incorrectly placed jumpers are a very frequent cause of this error.

• Incorrect SPI Configuration in printer.cfg
  A mismatch between Klipper’s configuration and the board’s wiring will cause this error. There are two ways to configure SPI, and switching between them is an essential troubleshooting step.

  • Try Software SPI First: This method is often more reliable. In your [tmcXXXX ...] section, comment out the spi_bus: line and explicitly define the software SPI pins: spi_software_sclk_pin, spi_software_mosi_pin, and spi_software_miso_pin. The correct pin names are found in the board’s pinout diagram.

  • Verify Hardware SPI: If using hardware SPI (spi_bus:), ensure the correct bus name for the board (e.g., spi1, spi0a). If issues persist, try lowering the communication speed by adding spi_speed: 4000000 or lower. This should be only a diagnostic step, as it might just cover up the underlying root cause.

• SPI Bus Conflicts with Other Devices
  The SPI bus is often shared between multiple components, such as stepper drivers, an LCD display (via EXP ports), or an ADXL345 accelerometer. If another device on the same bus is physically connected but not configured in Klipper, it can interfere with communication to the drivers.
  Ensure every device on the shared SPI bus has a corresponding, correct configuration section in printer.cfg. As a test, physically disconnect all other SPI devices to see if the error resolves. If an on-bus device is unused, but cannot be disconnected, disable it by creating a [static_digital_output] section to set its Chip Select (CS) pin high.

• Faulty Wiring or Loose Connections
  A bad crimp, a loose plug, or a damaged wire in the motor cable can interrupt communication, often causing the error for a single axis. Visually inspect all cables and plugs for damage or improper seating (e.g., bent pins). Double-check the orientation and seating of the driver module. If only a single axis is affected, swapping the drivers and connectors (e.g., X with Y) is a good diagnostic step. If the error moves to the new axis, the issue is with that motor or its cable.

• Faulty Stepper Driver Module
  The driver module itself can be defective out of the box or may fail over time due to static discharge, overheating, or voltage spikes (such as unplugging motors while powered on). If the error is consistently tied to a specific driver regardless of socket, the module may be faulty. Swap the suspect driver module with a known-good one. If the error is resolved, the driver is likely at fault.

• Signal Integrity Issues (Electrical Noise - EMI/ESD)
  While usually not an issue for drivers seated directly on the board, avoid running high-current wires over or near the drivers to reduce interference risks. Static discharge when handling electronics can also cause permanent damage.
  In printers with long cable runs to external drivers, the SPI signal wires can act as antennas, picking up electromagnetic interference (EMI) from motors and heaters. This noise can corrupt the data and cause communication failures.
  If using long cables, use shielded or twisted-pair wiring and route the signal cables away from high-current motor and heater wires.

• Other Reasons
  These are less common but worth considering if all else fails:

  • Power Cycle: Fully shut down and power off the entire system. Wait to ensure all components are fully discharged before restarting (at least 60 seconds).

  • Thermal Issues: Make sure drivers are not overheating due to lack of ventilation or excessive current, as this can cause erratic errors.

  • Firmware Reflash: Reflash the MCU boards with a new firmware build, verifying the settings. This is especially helpful after Klipper updates, though typically not needed unless Klipper reports a protocol mismatch.

  • Remove Modifications/Extras: Temporarily remove any code modifications or extras, especially those affecting the drivers or TMC tuning.

  • Hardware Incompatibility: Rare incompatibilities between specific driver module versions and specific controller boards have been reported. These can manifest as persistent errors even with correct wiring and configuration, sometimes being resolved only by switching to a different brand of driver or a different controller board.

  • Defective Board: While rare, printer-board defects do happen.