SerialUSB.println() Buffer Limitation

Official Hardware Classic Zero
1

A call to SerialUSB.println() [or print()] seems to hang if you pass a buffer with a string length greater than 247. Additionally, the characters past the 240 index seem to get messed up during transmission.

Here is some example code which demonstrates this:

void setup() 
{
  while(!SerialUSB);
  char buffer1[512];

  //Strangely the limit seems to be 247 characters?
  //But it seems to start to fail after 240?

  //This works
  memset(buffer1, 'a', sizeof(buffer1));
  buffer1[247] = 0;
  SerialUSB.println("Attempting to print a 247 character buffer: ");
  SerialUSB.println(buffer1);

  //But this hangs
  memset(buffer1, 'a', sizeof(buffer1));
  buffer1[248] = 0;
  SerialUSB.println("Attempting to print a 248 character buffer: ");
  SerialUSB.println("Individual characters:");
  for (int i=0; i<248; i++)
    SerialUSB.print(buffer1[i]);
  SerialUSB.println();
  SerialUSB.println("In a single call to SerialUSB.println():");
  SerialUSB.println(buffer1);
}

void loop() 
{
}
2

very interesting, thank's for pointing that out.

maybe we get some "official" answer (regarding that issue) from the arduino-team sometime ...

I feel a slight decline in community support from the arduino side (regarding the ZERO) within the past weeks :confused:
and the documentation and libraries are obviously not up to date for the ZERO yet (?)
(even though the product has been announced for more than one year now...)

3

Maybe you should open an issue in GitHub

4

I believe this is the issue:

In samd21_device.c on line 38 the send buffer is declared with a size of 128:

__attribute__((__aligned__(4))) /*__attribute__((__section__(".bss_hram0")))*/ uint8_t udd_ep_in_cache_buffer[4][128];

Later in the send method it copies the data to be sent into that buffer without any size checks:

uint32_t UDD_Send(uint32_t ep, const void* data, uint32_t len)
{
	memcpy(&udd_ep_in_cache_buffer[ep], data, len);
...

This means that it will have trouble with anything over 128 bytes in length.