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?