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Topic: How to know when serial has finished sending (Read 2970 times) previous topic - next topic

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.

eried


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
My website: http://ried.cl

mattallen37

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.

mattallen37

#3
Aug 18, 2012, 03:37 am Last Edit: Aug 18, 2012, 04:01 am by mattallen37 Reason: 1
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".

mattallen37

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.

eried

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

Code: [Select]
#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);
 }
}
My website: http://ried.cl

el_supremo

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

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


Pete

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?

Nick Gammon

No library. That code stands on its own.
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

bperrybap


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

  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

eried


@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.
My website: http://ried.cl

mattallen37

@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.

GCone

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?

bperrybap


@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

mattallen37

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

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