I2C clock cannot go below 30MHz as prescaler is not calculated

[ladyada]

https://github.com/arduino/ArduinoCore-avr/blob/master/libraries/Wire/src/utility/twi.c#L139
doesn't calculate/change the TWSR prescaler, which means that we can't get below approx 16mhz / 255*2 = 30KHz
i'm happy to submit a PR, would look like this (TWSR is write-only on the two lowest bits)

  // calculate TWBR correctly
  uint8_t prescaler = 1;
  uint32_t atwbr = ((F_CPU / desiredclk) - 16) / 2;
  if (atwbr <= 255) {
    prescaler = 1;
    TWSR = 0x0;
  } else if (atwbr <= 1020) {
    atwbr /= 4;
    prescaler = 4;
    TWSR = 0x1;
  } else if (atwbr <= 4080) {
    atwbr /= 16;
    prescaler = 16;
    TWSR = 0x2;
  } else if (atwbr <= 16320) {
    atwbr /= 64;
    prescaler = 64;
    TWSR = 0x3;
  }
  TWBR = atwbr;

[RobTillaart]

@per1234
+1 for this issue. Important to connect to low speed devices e.g. AGS02MA
Such a solution would allow support for I2C bus speed from ~30 KHz down to 500 Hz (also think long wires)