PIC18F2450/4450
byte count, the data packet will be rejected and a NAK
handshake will be generated. When this happens, the
byte count will not be updated.
14.4.1.3
BDnSTAT Register (SIE Mode)
When the BD and its buffer are owned by the SIE, most
of the bits in BDnSTAT take on a different meaning. The
configuration is shown in Register 14-6. Once UOWN is
set, any data or control settings previously written there
by the user will be overwritten with data from the SIE.
The 10-bit byte count is distributed over two registers.
The lower 8 bits of the count reside in the BDnCNT
register. The upper two bits reside in BDnSTAT<1:0>.
This represents a valid byte range of 0 to 1023.
The BDnSTAT register is updated by the SIE with the
token Packet Identifier (PID) which is stored in
BDnSTAT<5:3>. The transfer count in the
corresponding BDnCNT register is updated. Values
that overflow the 8-bit register carry over to the two
most significant digits of the count, stored in
BDnSTAT<1:0>.
14.4.3
BD ADDRESS VALIDATION
The BD Address register pair contains the starting RAM
address location for the corresponding endpoint buffer.
For an endpoint starting location to be valid, it must fall
in the range of the USB RAM, 400h to 7FFh. No
mechanism is available in hardware to validate the BD
address.
14.4.2
BD BYTE COUNT
If the value of the BD address does not point to an
address in the USB RAM, or if it points to an address
within another endpoint’s buffer, data is likely to be lost
or overwritten. Similarly, overlapping a receive buffer
(OUT endpoint) with a BD location in use can yield
The byte count represents the total number of bytes
that will be transmitted during an IN transfer. After an IN
transfer, the SIE will return the number of bytes sent to
the host.
For an OUT transfer, the byte count represents the
maximum number of bytes that can be received and
stored in USB RAM. After an OUT transfer, the SIE will
return the actual number of bytes received. If the
number of bytes received exceeds the corresponding
unexpected
results.
When
developing
USB
applications, the user may want to consider the
inclusion of software-based address validation in their
code.
REGISTER 14-6: BDnSTAT: BUFFER DESCRIPTOR n STATUS REGISTER (BD0STAT THROUGH
BD63STAT), SIE MODE (DATA RETURNED BY THE SIDE TO THE
MICROCONTROLLER)
R/W-x
U-x
—
R/W-x
PID3
R/W-x
PID2
R/W-x
PID1
R/W-x
PID0
R/W-x
BC9
R/W-x
BC8
UOWN
bit 7
bit 0
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
bit 7
UOWN: USB Own bit
1= The SIE owns the BD and its corresponding buffer
Reserved: Not written by the SIE
bit 6
bit 5-2
PID3:PID0: Packet Identifier bits
The received token PID value of the last transfer (IN, OUT or SETUP transactions only).
BC9:BC8: Byte Count 9 and 8 bits
bit 1-0
These bits are updated by the SIE to reflect the actual number of bytes received on an OUT transfer
and the actual number of bytes transmitted on an IN transfer.
DS39760A-page 140
Advance Information
© 2006 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
