UPSD3422 datasheet(167/293 Pages) STMICROELECTRONICS

167 / 293 page

uPSD34xx

USB interface

167/293

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USB FIFO Valid Size (USIZE)

The Endpoint selected by the USB Endpoint Select Register (see Table 82 on

page 165) determines the direction and FIFO that is controlled by the USB FIFO Valid

Size (see Table 84). The USB FIFO Valid Size Register indicates the number of bytes

loaded into the IN FIFO that the SIE is to send in a Data packet for an Endpoint IN case

and indicates the number of bytes received for an Endpoint OUT case.

Table 84.

USB FIFO valid size (USIZE 0F2h, reset value 00h)

0BSY

R/W

FIFO Busy Status

– Endpoint IN Case

Once the FIFO has been loaded and armed (USIZE written with the

number of bytes to send), the BSY Bit is set and remains set until the

SIE has transmitted the data in the FIFO. The CPU should only access

the FIFO when BSY = 0.

– Endpoint OUT Case

While the SIE is receiving data and storing it in the FIFO (BSY = 1), it

should not be accessed by the CPU. Once the OUT transaction is

complete (BSY=0), the CPU may read the contents of the FIFO. The

BSY Bit will remain cleared until another OUT transaction is received.

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

–

SIZE[6:0]

Bit

Symbol

R/W

Definition

–

Reserved

6:0

SIZE

R/W

– Endpoint IN Case

The CPU writes the USIZE register with the number of bytes it

loaded into the IN endpoint FIFO for transmission with the next

IN transaction. Once the USIZE register has been written, the

FIFO becomes ready for transmission.

– Endpoint OUT Case

The CPU reads the USIZE register to determine how many

bytes were received in the data packet during the last OUT

transaction. This tells the CPU how many valid bytes to read

from the FIFO.

Note: Since the FIFOs are 64 bytes in length, the maximum

value for SIZE is 64 (40h).

Bit

Symbol

R/W

Definition

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