PIC16C54C-04/P【PDF 47/194 页】IC MCU OTP 512X12 18DIP

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PIC18F2450/4450

DS39760A-page 138

Advance Information

2006 Microchip Technology Inc.

When UOWN is set, the user can no longer depend on

the values that were written to the BDs. From this point,

the SIE updates the BDs as necessary, overwriting the

original BD values. The BDnSTAT register is updated

by the SIE with the token PID and the transfer count,

BDnCNT, is updated.

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.

14.4.1.2

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

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

parity 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 14-3.

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

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 14.4.2

“BD

Byte

Count”

for

information.

TABLE 14-3:

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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