PIC18F4550ESOSQTP datasheet(170/430 Pages) MICROCHIP

170 / 430 page

PIC18F2455/2550/4455/4550

DS39632D-page 168

Preliminary

17.2.3

USB STATUS REGISTER (USTAT)

The USB Status register reports the transaction status

within the SIE. When the SIE issues a USB transfer

complete interrupt, USTAT should be read to determine

the status of the transfer. USTAT contains the transfer

endpoint number, direction and Ping-Pong Buffer

Pointer value (if used).

The USTAT register is actually a read window into a

four-byte status FIFO, maintained by the SIE. It allows

the microcontroller to process one transfer while the

SIE processes additional endpoints (Figure 17-4).

When the SIE completes using a buffer for reading or

writing data, it updates the USTAT register. If another

USB transfer is performed before a transaction

complete interrupt is serviced, the SIE will store the

status of the next transfer into the status FIFO.

Clearing the transfer complete flag bit, TRNIF, causes

the SIE to advance the FIFO. If the next data in the

FIFO holding register is valid, the SIE will immediately

reassert the interrupt. If no additional data is present,

TRNIF will remain clear; USTAT data will no longer be

reliable.

FIGURE 17-4:

USTAT FIFO

Note:

The data in the USB Status register is valid

only when the TRNIF interrupt flag is

asserted.

Note:

If an endpoint request is received while the

USTAT

FIFO

full,

the

SIE

will

automatically issue a NAK back to the

host.

Data Bus

USTAT from SIE

4-byte FIFO

for USTAT

Clearing TRNIF

Advances FIFO

USTAT: USB STATUS REGISTER

U-0

R-x

U-0

—

ENDP3

ENDP2

ENDP1

ENDP0

DIR

PPBI(1)

—

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

Unimplemented: Read as ‘0’

bit 6-3

ENDP3:ENDP0: Encoded Number of Last Endpoint Activity bits

(represents the number of the BDT updated by the last USB transfer)

1111

= Endpoint 15

1110

= Endpoint 14

....

0001

= Endpoint 1

0000

= Endpoint 0

bit 2

DIR: Last BD Direction Indicator bit

= The last transaction was an IN token

= The last transaction was an OUT or SETUP token

bit 1

PPBI: Ping-Pong BD Pointer Indicator bit(1)

= The last transaction was to the Odd BD bank

= The last transaction was to the Even BD bank

bit 0

Unimplemented: Read as ‘0’

Note 1:

This bit is only valid for endpoints with available Even and Odd BD registers.

PIC18F4550ESOSQTP Datasheet(PDF) 170 Page - Microchip Technology

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