PIC18F4550ESOSQTP datasheet(172/430 Pages) MICROCHIP

172 / 430 page

PIC18F2455/2550/4455/4550

DS39632D-page 170

Preliminary

17.2.5

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.

17.2.6

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.

17.3

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 Banks 4

through 7 (400h to 7FFh) for a total of 1 Kbyte

(Figure 17-5).

Bank 4 (400h through 4FFh) is used specifically for

endpoint buffer control, while Banks 5 through 7 are

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 17.4.1.1 “Buffer

Ownership”.

FIGURE 17-5:

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

F60h

FFFh

Banks 0

Banks 4

Bank15

(USB RAM)

F00h

Banks 8

800h

to 14

to 3

to 7

PIC18F4550ESOSQTP Datasheet(PDF) 172 Page - Microchip Technology

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