PIC18F4550ESOSQTP datasheet(174/430 Pages) MICROCHIP

174 / 430 page

PIC18F2455/2550/4455/4550

DS39632D-page 172

Preliminary

The BDnSTAT byte of the BDT should always be the

last byte updated when preparing to arm an endpoint.

The SIE will clear the UOWN bit when a transaction

has completed. The only exception to this is when KEN

is enabled and/or BSTALL is enabled.

No hardware mechanism exists to block access when

the UOWN bit is set. Thus, unexpected behavior can

occur if the microcontroller attempts to modify memory

when the SIE owns it. Similarly, reading such memory

may produce inaccurate data until the USB peripheral

returns ownership to the microcontroller.

17.4.1.2

BDnSTAT Register (CPU Mode)

When UOWN = 0, the microcontroller core owns the

BD. At this point, the other seven bits of the register

take on control functions.

The Keep Enable bit, KEN (BDnSTAT<5>), determines

if a BD stays enabled. If the bit is set, once the UOWN

bit is set, it will remain owned by the SIE independent

of the endpoint activity. This prevents the USTAT FIFO

from being updated, as well as the transaction

complete interrupt from being set for the endpoint. This

feature should only be enabled when the Streaming

Parallel Port is selected as the data I/O channel instead

of USB RAM.

The

Address

Increment

Disable

bit,

INCDIS

(BDnSTAT<4>), controls the SIE’s automatic address

increment function. Setting INCDIS disables the

auto-increment of the buffer address by the SIE for

each byte transmitted or received. This feature should

only be enabled when using the Streaming Parallel

Port, where each data byte is processed to or from the

same memory location.

The

Data

Toggle

Sync

Enable

bit,

DTSEN

(BDnSTAT<3>), controls data toggle parity checking.

Setting DTSEN enables data toggle synchronization by

the SIE. When enabled, it checks the data packet’s par-

ity against the value of DTS (BDnSTAT<6>). If a packet

arrives with an incorrect synchronization, the data will

essentially be ignored. It will not be written to the USB

RAM and the USB transfer complete interrupt flag will

not be set. The SIE will send an ACK token back to the

host to Acknowledge receipt, however. The effects of

the DTSEN bit on the SIE are summarized in

Table 17-3.

The Buffer Stall bit, BSTALL (BDnSTAT<2>), provides

support for control transfers, usually one-time stalls on

Endpoint

also

provides

support

for

the

SET_FEATURE/CLEAR_FEATURE commands speci-

fied in Chapter 9 of the USB specification; typically,

continuous STALLs to any endpoint other than the

default control endpoint.

The BSTALL bit enables buffer stalls. Setting BSTALL

causes the SIE to return a STALL token to the host if a

received token would use the BD in that location. The

EPSTALL bit in the corresponding UEPn control regis-

ter is set and a STALL interrupt is generated when a

STALL is issued to the host. The UOWN bit remains set

and the BDs are not changed unless a SETUP token is

received. In this case, the STALL condition is cleared

and the ownership of the BD is returned to the

microcontroller core.

The BD9:BD8 bits (BDnSTAT<1:0>) store the two most

significant digits of the SIE byte count; the lower 8 digits

are stored in the corresponding BDnCNT register. See

Section 17.4.2

“BD

Byte

Count”

for

information.

TABLE 17-3:

EFFECT OF DTSEN BIT ON ODD/EVEN (DATA0/DATA1) PACKET RECEPTION

OUT Packet

from Host

BDnSTAT Settings

Device Response after Receiving Packet

DTSEN

DTS

Handshake

UOWN

TRNIF

BDnSTAT and USTAT Status

DATA0

ACK

Updated

DATA1

ACK

Not Updated

DATA0

ACK

Updated

DATA1

ACK

Not Updated

Either

ACK

Updated

Either, with error

NAK

Not Updated

Legend: x = don’t care

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