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How does the Arduino handle serial buffer overflow?

How does the Arduino handle serial buffer overflow? Does it throw away the newest incoming data or the oldest? How many bytes can the buffer hold?

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For hardware serial ports you can see in [HardwareSerial.cpp][1] that the buffer size varies depending on the amount of RAM available on the particular AVR:

 #if (RAMEND < 1000)
 #define SERIAL_BUFFER_SIZE 16
 #else
 #define SERIAL_BUFFER_SIZE 64
 #endif

For a software serial port in [SoftwareSerial.h][2] the receiver buffer size `_SS_MAX_RX_BUFF` is defined as 64 bytes. In both cases it stops attempting to insert received data into the queue when it is full, so you could get a mix to old and new data depending on how you're retrieving data from the queue. 

Ideally it would be best to ensure the buffer always gets emptied in a prompt manner to avoid the buffer filling. Maybe take a look at timers and implementing a simple state machine if your problem is related to other code blocking the main loop.


 [1]: https://github.com/arduino/Arduino/blob/master/hardware/arduino/cores/arduino/HardwareSerial.cpp
 [2]: https://github.com/arduino/Arduino/blob/master/libraries/SoftwareSerial/SoftwareSerial.h

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1

I get the impression that if I transmit data to the Arduino and don't have an active "puller" of data on the Arduino side then if more data arrives than can fit in the buffer, it will be discarded. Can you confirm that? I had naively assumed that the transmitter would block until space available to hold the data became available.

Kolban
– Kolban

2015年06月19日 18:54:39 +00:00
Commented Jun 19, 2015 at 18:54
1

I just looked through all that code (under /usr/share/arduino/hardware/arduino/core/arduino/HardwareSer‌ial.cpp) and can confirm what you wrote here. The only thing I would add is that since the SRAM is 2K (RAMEND>1000) then that if statement will always use 64 rather than 16 on a Nano or an Uno. So if one would want to expand the size of the ring buffer that would be the place to change it

SDsolar
– SDsolar

2017年08月19日 06:10:38 +00:00
Commented Aug 19, 2017 at 6:10

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