How to know when serial has finished sending

How can I tell (from the user-program) when a serial stream has finished being transmitted?

I want something like

uint8_t Serial.sending();

that returns true if it's still sending, or false if it has completed (nothing in the send queue). Otherwise it could return the number of bytes in the send queue, it doesn't really matter either way.

mattallen37:
How can I tell (from the user-program) when a serial stream has finished being transmitted?

I want something like

uint8_t Serial.sending();

that returns true if it's still sending, or false if it has completed (nothing in the send queue). Otherwise it could return the number of bytes in the send queue, it doesn't really matter either way.

You have to add something to each transmission at the end or via timeout, like if nothing was received in "n" milis -> Sending is false

I mean, how can I tell when it has finished transmitting from itself. I want it to send an array of bytes, and I don't want the program to continue until the last bit has been transmitted.

Nevermind, I just realized that "flush" does what I want.

OT, but doesn't it seem that "flush" is mis-termed? Flush normally means "remove any data in the rx buffers", and not "wait until the tx queue is empty".

Okay, so I guess I do need a different solution after all. flush seems to return as soon as the last byte has started to send, not when it has finished sending.

I can use flush and then delay for 1ms though, since is takes much less than 1ms to finish transmitting the last byte at 115200 baud.

Yes, you need another approach. I built an example for you:

#define MAXDATA 50
#define TIMEOUT 10
void setup()
{
  // This code will only run once, after each powerup or reset of the board
  Serial.begin(9600);
}

long timeoutOver;
char data[MAXDATA];
  
void loop()
{
  // This code will loops consecutively
  boolean somethingReceived = false;
  int pos = 0;
  
  if(Serial.available())
  {
    somethingReceived = true;
    do
    {
      while(Serial.available())
      {
        data[min(pos,MAXDATA-1)] = Serial.read();
        timeoutOver = millis()+TIMEOUT; pos++;
      }
    }
    while(millis()<timeoutOver);
    data[pos] = '\0';
  }
  
  if(somethingReceived)
  {
    // Do something
    Serial.print("Received: ");
    Serial.println(data);
  }
}

Nick Gammon posted code which checks the hardware to make sure the last byte has gone:

  Serial.flush ();
  // wait for transmit buffer to empty
  while ((UCSR0A & _BV (TXC0)) == 0)
    {}

Pete

@eried thanks but I don't need help with receiving data.

@el_supremo Thanks. What library do I need to #include to get access to that functionality?

No library. That code stands on its own.

el_supremo:
Nick Gammon posted code which checks the hardware to make sure the last byte has gone:

  Serial.flush ();

// wait for transmit buffer to empty
  while ((UCSR0A & _BV (TXC0)) == 0)
    {}




Pete

In my testing that was not good enough. It worked when only 1 character was sent but it failed
if there was still a character in the TX data register when the polling started.
It is very unfortunate that the bit isn't a "BUSY" bit. It is merely a transition bit that clears when the last bit
of a character leaves the shift register. This means that if multiple characters have been sent,
the bit also has to be set appropriately when the character is fed into the USART data register otherwise
you end up seeing the bit clear for the 2nd to last character since there are two characters in the USART.
1 in the shift register and one in the data register.

It took a patch in HardwareSerial.cpp to be able to set the bit appropriately to ensure that there
were no race conditions.
I had to modify the UDRE0 ISR routine and the flush() routine to make flush() work correctly and reliably
in my testing.

--- bill

mattallen37:
@eried thanks but I don't need help with receiving data.

@el_supremo Thanks. What library do I need to #include to get access to that functionality?

The code I posted is just a simple way to wait for the "full" data (timeout 10 ms), not just receive data.

@Nick Gammon, the compiler complains that some of the variables are undeclared (as if maybe I need to #include something).

@bperrybap, shouldn't the flush at the beginning wait till there is only one byte left to send? At that point, there should only be 1 byte, so shouldn't it work fine? If not, can you please post the changes you made to the HardwareSerial library?

@eried the code you posted is a way to wait until an entire string has been received not sent.

This may sound like a ridiculous question from a noob, because it is, but - when you read a byte from the serial receive buffer, is it removed from the buffer automatically at that time?

mattallen37:
@bperrybap, shouldn't the flush at the beginning wait till there is only one byte left to send? At that point, there should only be 1 byte, so shouldn't it work fine? If not, can you please post the changes you made to the HardwareSerial library?

flush() currently waits until there are no characters in the s/w queue,
but there are still up to two characters still in the USART remaining to be transmitted.
One in the transmit shift register that is currently being shifted and transmitted
out and one in the transmit data buffer register (TXB).

I don't like the way Atmel implemented their TXCn bit.
It marks a transition and isn't a state. The only automatic clearing of this
bit is by execution of a TXCI interrupt, which not normally an interrupt
that is used or needed for interrupt driven transmission.
(UDR interrupts are used to allow for the double buffering to ensure back to back transmission)
This makes TXCn status kind of a pain to use.
s/w has to clear it for the hardware to set it
but it has do it in a way that doesn't create a race condition.

A robust flush() solution has to handle all conditions when called.

  • A character in the transmit shift register and no character in TXB.
  • A character in TXB and in the transmit shift register
  • No characters in TXB or the shift register.

What I found is that some potential solutions didn't handle all of the
above situations or couldn't handle them more than once.

--- bill

@GCone for questions that are OT, it's best to start your own thread. However, the answer is yes (unless you just peek).

A robust flush() solution has to handle all conditions when called.

  • A character in the transmit shift register and no character in TXB.
  • A character in TXB and in the transmit shift register
  • No characters in TXB or the shift register.

What I found is that some potential solutions didn't handle all of the
above situations or couldn't handle them more than once.

Well many of us use to complain that the serial transmitting function was a simple blocking command so I guess the lesson is to be very careful what you ask for as you may get it. :wink:

@bperrybap so there isn't a solution? You said "I had to modify the UDRE0 ISR routine and the flush() routine to make flush() work correctly and reliably
in my testing.", so how can I do the same?

@retrolefty if it's going to send serial in the background, there needs to be a way to know when it has finished. If there isn't a solution that supports the current implementation, then perhaps I will bit-bang the output.

mattallen37:
@retrolefty if it's going to send serial in the background, there needs to be a way to know when it has finished. If there isn't a solution that supports the current implementation, then perhaps I will bit-bang the output.

And perhaps that will work for you. I think what Bill was trying to convey is that one really needs to understand the AVR hardware functions at the basic register level (serial hardware in this case) and even then be careful of what is possible at this level via what one's actual real world application requirements are. Sometimes there is no 'perfect' match or solution and the best compromise possible is what one has to live with. I don't know it that is the case with this issue or not, but I've worked with serial communications sense the mid 60s and it is amazing how unsimple and error prone asynchronous communications can be, especially when dealing with mixed vendors implementation, both in software and hardware.

Lefty

Most processors do not implement this function. Therefore things like changing the transmit / receive flags on RS485 bi directional buffers is difficult. The common solution is to look for the buffer empty flag and then implement a delay such that when it times out your buffer will be empty. This of course depends on the baud rate you are using.