PIC18F4550ESOSQTP datasheet(176/430 Pages) MICROCHIP

176 / 430 page

PIC18F2455/2550/4455/4550

DS39632D-page 174

Preliminary

17.4.1.3

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 17-6. Once UOWN

is set, any data or control settings previously written

there by the user will be overwritten with data from the

SIE.

The 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 correspond-

ing 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>.

17.4.2

BD BYTE COUNT

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

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.

The 10-bit byte count is distributed over two registers.

The lower 8 bits of the count reside in the BDnCNT

This represents a valid byte range of 0 to 1023.

17.4.3

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.

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

unexpected

results.

When

developing

USB

applications, the user may want to consider the

inclusion of software-based address validation in their

code.

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

UOWN

—

PID3

PID2

PID1

PID0

BC9

BC8

bit 7

bit 0

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

bit 7

UOWN: USB Own bit

= The SIE owns the BD and its corresponding buffer

bit 6

Reserved: Not written by the SIE

bit 5-2

PID3:PID0: Packet Identifier bits

The received token PID value of the last transfer (IN, OUT or SETUP transactions only).

bit 1-0

BC9:BC8: Byte Count 9 and 8 bits

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.

PIC18F4550ESOSQTP Datasheet(PDF) 176 Page - Microchip Technology

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