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PIC18F2450/4450

DS39760A-page 136

Advance Information

2006 Microchip Technology Inc.

14.2.5

USB ADDRESS REGISTER

(UADDR)

The USB Address register contains the unique USB

address that the peripheral will decode when active.

UADDR is reset to 00h when a USB Reset is received,

indicated by URSTIF, or when a Reset is received from

the microcontroller. The USB address must be written

by the microcontroller during the USB setup phase

(enumeration) as part of the Microchip USB firmware

support.

14.2.6

USB FRAME NUMBER REGISTERS

(UFRMH:UFRML)

The Frame Number registers contain the 11-bit frame

number. The low-order byte is contained in UFRML,

while the three high-order bits are contained in

UFRMH. The register pair is updated with the current

frame number whenever a SOF token is received. For

the microcontroller, these registers are read-only. The

Frame

Number

primarily

used

for

isochronous transfers.

14.3

USB RAM

USB data moves between the microcontroller core and

the SIE through a memory space known as the USB

RAM. This is a special dual port memory that is

mapped into the normal data memory space in Bank 4

(400h to 4FFh) for a total of 256 bytes (Figure 14-5).

Some portion of Bank 4 (400h through 4FFh) is used

specifically for endpoint buffer control, while the

remaining portion is available for USB data. Depending

on the type of buffering being used, all but 8 bytes of

Bank 4 may also be available for use as USB buffer

space.

Although USB RAM is available to the microcontroller

as data memory, the sections that are being accessed

the

SIE

should

not

accessed

the

microcontroller. A semaphore mechanism is used to

determine the access to a particular buffer at any given

time. This is discussed in Section 14.4.1.1 “Buffer

Ownership”.

FIGURE 14-5:

IMPLEMENTATION OF

USB RAM IN DATA

MEMORY SPACE

400h

4FFh

7FFh

500h

USB Data or

Buffer Descriptors,

USB Data or User Data

User Data

Unused

SFRs

3FFh

000h

F80h

FFFh

Banks 0

Banks 5

Bank15

F00h

800h

to 14

to 1

Bank 4

Banks 2

to 3

Unused

1FFh

200h

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