GoogleCodeExporter
commented
9 years ago From "Noriaki Mitsunaga" on the developers list:
I have found that Arduino Leonard sometimes throws away received
characters through USB CDC. This will happen if you send many bytes
from PC at once or the main routine on Arduino does not read serial
buffer in time.
The issue is caused as follows. When Leonard receives data, it calls
Serial.accept from ISR(USB_GEN_vect) in USBCore.cpp;
while (USB_Available(CDC_RX)) // Handle received bytes (if any)
Serial.accept();
Serial.accept() is implemented in CDC.cpp;
void Serial_::accept(void)
{
ring_buffer *buffer = &cdc_rx_buffer;
int c = USB_Recv(CDC_RX);
int i = (unsigned int)(buffer->head+1) % SERIAL_BUFFER_SIZE;
if (i != buffer->tail) {
buffer->buffer[buffer->head] = c;
buffer->head = i;
}
}
So it throws away received data if the buffer overflows before main
routine handles them. It can be avoided if we change as follows;
o Modify return value of accept() (in USBAPI.h)
virtual bool accept(void);
^^^^
o Modify above part of USBCore.cpp as;
while (USB_Available(CDC_RX) && Serial.accept()) // Handle received
bytes (if any)
;
o Modify accept() in CDC.cpp as;
bool Serial_::accept(void)
{
ring_buffer *buffer = &cdc_rx_buffer;
int i = (unsigned int)(buffer->head+1) % SERIAL_BUFFER_SIZE;
// if we should be storing the received character into the location
// just before the tail (meaning that the head would advance to the
// current location of the tail), we're about to overflow the buffer
// and so we don't write the character or advance the head.
if (i != buffer->tail) {
int c = USB_Recv(CDC_RX);
if (c>=0) {
buffer->buffer[buffer->head] = c;
buffer->head = i;
}
return true;
}
return false;
}
It would not harm USB communication since PC (USB host) will postpone
sending data during USB device returns NAK. It might make delay
handling received data if interrupt routine is not called for a long
time after buffer becomes full. To handle that case we should modify
available, peek, and read in CDC.cpp so that they checks
USB_Available(CDC_RX).
How do you think?
// Noriaki MitsunagaOriginal comment by dmel...@gmail.com on 9 Aug 2012 at 12:46
- Added labels: Component-ATmega32U4
Original issue reported on code.google.com by
peter.va...@belgacom.neton 30 Jul 2012 at 9:51