Limited writeMicroseconds() resolution when using Servo Library

Official Hardware Nano Family Nano Every
1

I have some code that I have run successfully on an Uno, and I am now trying to migrate it to the Nano Every. The issue that I am running into at the moment is that when using the Nano Every board, I can only position my servo at increments of 4us. When using the Uno board, I am able to set the position at 1us increments. Here are the code snippets that call writeMicroseconds().

void loop() {

while (!Serial.available()) {
      delay(100);
    }
    cmdString = Serial.readString();
    int n = cmdString.length();
    char char_array[n + 1];
    strcpy(char_array, cmdString.c_str());
    int steps = atoi(char_array);
    moveServoRelative(tmpServo, steps);
    Serial.print("MOVE : Moved current axis by ");
    Serial.print(steps);
    Serial.print(" microseconds to ");
    Serial.print(tmpServo.readMicroseconds());
    Serial.println(" microseconds.");

}

void moveServoRelative(Servo servo, int steps) {
  int currentPos = servo.readMicroseconds();
  Serial.print("Current Position setting: ");
  Serial.println(currentPos);
  int newPos = currentPos + steps;
  Serial.print("New Position setting: ");
  Serial.println(newPos);
  servo.writeMicroseconds(newPos);
}

With the Nano Every, a sequence of commands attempting to move the servo position by 1us up to 6us yeilds this result:

Current Position setting: 1445
New Position setting: 1446
MOVE : Moved current axis by 1 microseconds to 1445 microseconds.

Current Position setting: 1445
New Position setting: 1447
MOVE : Moved current axis by 2 microseconds to 1445 microseconds.

Current Position setting: 1445
New Position setting: 1448
MOVE : Moved current axis by 3 microseconds to 1445 microseconds.

Current Position setting: 1445
New Position setting: 1449
MOVE : Moved current axis by 4 microseconds to 1449 microseconds.

Current Position setting: 1449
New Position setting: 1454
MOVE : Moved current axis by 5 microseconds to 1453 microseconds.

Current Position setting: 1453
New Position setting: 1459
MOVE : Moved current axis by 6 microseconds to 1457 microseconds.

Looking through the Servo library, I haven't been able to figure out where this rounding is happening, and I'm hoping someone with more expertise can direct me to the root cause of this issue.

2

I have not looked at the Servo code extensively, but there is a difference in how microseconds are converted to ticks within the libraries (presumably ticks are timing intervals used within the Servo library). Not sure why the calculations are handled differently, clockCyclesPerMicrosecond should be the same for the UNO and Nano Every since both run at 16MHz, it may be that the timer is running at a different frequency.

There is a bug in the megaavr Servo library that does not handle a 20MHz clock frequency properly, but that would not affect anything if you are using the default 16MHz clock on the Nano Every. Note that the description of the Nano Every in the Arduino Store, and the packaging, incorrectly state the clock is 20MHz, it runs at 16MHz unless you specifically change it in the boards.txt file.

Code from avr Servo.cpp:

#define usToTicks(_us)    (( clockCyclesPerMicrosecond()* _us) / 8)     // converts microseconds to tick (assumes prescale of 8)  // 12 Aug 2009
#define ticksToUs(_ticks) (( (unsigned)_ticks * 8)/ clockCyclesPerMicrosecond() ) // converts from ticks back to microseconds

Code from megaavr Servo.cpp: (used for atmega4809 based boards)

#define usToTicks(_us)    ((clockCyclesPerMicrosecond() / 16 * _us) / 4)                 // converts microseconds to tick
#define ticksToUs(_ticks) (((unsigned) _ticks * 16) / (clockCyclesPerMicrosecond() / 4))   // converts from ticks back to microseconds