TMC2209 returns wrong microsteps from the REGISTRY when it is set to 4

[korianv]

I have noticed that when setting microsteps to 4, the TMC2209 does not return the correct value of 4 when reading from the Register Map, but 256, although the driver is, indeed, in 4 microsteps mode.

This only happens when the interpolation is disabled [driver.intpol(false);]. With it enabled, the returned value is correct.

Here is a code to set all 9 possible microsteps using the UART:

uint16_t MicroStep[9] = {0, 2, 4, 8, 16, 32, 64, 128, 256};
  for (int16_t i = 0; i < 9; i++)
  {
    Serial.print("i=");
    Serial.println(i);
    Serial.print("set microsteps=");
    Serial.println(MicroStep[i]);
    driver.microsteps(MicroStep[i]);
    Serial.print("get microsteps=");
    Serial.println(driver.microsteps());
    if (i == 0)
      driver.VACTUAL(839);
    else
      driver.VACTUAL(839 * MicroStep[i]);
    Serial.print("get VACTUAL=");
    Serial.println(driver.VACTUAL());
    Serial.println();
    delay(4000);
  }

The output from serial is:

i=0 set microsteps=0 get microsteps=0 get VACTUAL=839

i=1 set microsteps=2 get microsteps=2 get VACTUAL=1678

i=2 set microsteps=4 get microsteps=256 <<<<<<------------------ WRONG! get VACTUAL=3356

i=3 set microsteps=8 get microsteps=8 get VACTUAL=6712

i=4 set microsteps=16 get microsteps=16 get VACTUAL=13424

i=5 set microsteps=32 get microsteps=32 get VACTUAL=26848

i=6 set microsteps=64 get microsteps=64 get VACTUAL=53696

i=7 set microsteps=128 get microsteps=128 get VACTUAL=107392

i=8 set microsteps=256 get microsteps=256 get VACTUAL=214784

The motor is always spinning at exactly 3 RPM, so internally the driver TMC2209 is running with 4 microsteps, but it returns a wrong value when requesting the status for microsteps.

The library's code seems correct. Does the driver itself has a bug?