The problem is that no code can trust the data in any buffer
discriptor without the buffer descriptor's uown bit being cleared (the
only trusted part is the uown bit in the status register). This is
from the PIC's datasheet. I use the 18F14K50, but there are many pics
with similar BD structures and uown bits.
For transmissions TO the host, the status uown bits are tested and
work. For receptions FROM the host, the status uown bits are not
checked first. The result is an intermittent extra data received.
The fix is to read the uown status bit in the BD to see if it is
cleared, before reading the address and byte-count from the BD, then
call the usb_ep_data_out_callback to retrieve the data from the
endpoint only when cleared.
I made a fix that is not elegant, but is simple to understand and
works to repair the problem. This actually works in my real hardware.
The lines I added -- *
broken snipet:
if defined(usb_ep_data_out_callback) == true then
var byte idx = end_point * 8
var word addr
var byte bcnt
var byte bt_addr[2] at addr
bt_addr[0] = usb_bd_array[ idx + 2 ]
bt_addr[1] = usb_bd_array[ idx + 3 ]
bcnt = usb_bd_array[ idx + 1 ]
usb_ep_data_out_callback( end_point, addr, bcnt )
else
usb_prime_epx_out( end_point, 0x08 )
end if
FIXED SNIPPET:
if defined(usb_ep_data_out_callback) == true then
var byte idx = end_point * 8
var byte _statBD = usb_bd_array[idx] --
*
var bit statBD at _statBD : 7 -- *
if statBD == 0 then -- *
var word addr
var byte bcnt
var byte bt_addr[2] at addr
bt_addr[0] = usb_bd_array[ idx + 2 ]
bt_addr[1] = usb_bd_array[ idx + 3 ]
bcnt = usb_bd_array[ idx + 1 ]
usb_ep_data_out_callback( end_point, addr, bcnt )
end if -- *
else
usb_prime_epx_out( end_point, 0x08 )
end if
Special thanks to Matt for reviewing.
ivan.jal33
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