With Arduino Uno Rev3, I am trying to maintain a delicate timing while handling data transmission. I want to send 6 bytes at a time, which takes around ~44 us when I time Serial.write(). The time it takes from Arduino Serial to computer is irrelevant in my case. I only mind the run time of the function.

My idea is sending not all 6 bytes but a few at a time, so that the total sending time overhead would be distributed between time-sensitive tasks. Note that I will never be exceeding the buffer size, so the Serial.write() will never wait for actually transmission to be completed.

const uint8_t MSG_LEN = 6;
uint8_t snd_buffer[MSG_LEN];
uint32_t us_now = 0L;
uint8_t snd_byte_cnt = MSG_LEN;
uint8_t dummy_var = 0;

void setup() {
  Serial.begin(115200);
  delay(1000);
}

void loop() {
  us_now = micros();

  operation_dummy();
  
  com_dummy();

  //the aim is to use no delay functions
}

void com_dummy(){ //ex communication
  if (snd_byte_cnt < MSG_LEN){ //bytes need to be sent
    Serial.write(snd_buffer[snd_byte_cnt]); //send the target byte
    snd_byte_cnt++; //move to the next byte ind
  } //else nothing to be sent
}

void operation_dummy() { //ex operation
  dummy_var++; //do something (time sensitive in reality)
  if (snd_byte_cnt == MSG_LEN){ //if no data pending to be sent
    * (uint32_t *) &snd_buffer[1] = us_now; //modify buffer between 1st and 5th bytes (both inclusive)
    snd_buffer[MSG_LEN - 1] = dummy_var; 
    snd_byte_cnt = 0; //trigger start sending
  } //else wait for next cycle to send
}

My question is, how many bytes should be used with Serial.write() at a time?

I've heard the Serial library processes 16 bits (2 bytes) at a time. What happens if I try to send a single byte? Does it wait a certain time for the second byte and if doesn't arrive then it pushes a single byte? (Edit: this turned out to be a misunderstanding and only the FIFO pointer is 16 bits, not the data to be sent. * Source: https://forum.arduino.cc/t/serial-write-time/233360/23 )*

Would never calling a sleep function and using micros() OR the baudrate affect my time keeping? I think micros() and actually transmission uses interrupts. So different baudrates may delay micros() calls differently since both need to disable interrupts.