Datasheet

PIC18F87J50 FAMILY
DS39775C-page 318 © 2009 Microchip Technology Inc.
22.2.5 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.
22.2.6 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 registers are primarily used for
isochronous transfers. The contents of the UFRMH and
UFRML registers are only valid when the 48 MHz SIE
clock is active (i.e., contents are inaccurate when
SUSPND (UCON<1>) bit = 1).
22.3 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 access memory that is
mapped into the normal data memory space in Banks
0 through 15 (60h to F3Fh) for a total of 3.9 Kbyte
(Figure 22-4).
Bank 4 (400h through 4FFh) is used specifically for
endpoint buffer control, while Banks 0 through Bank3
and Banks 5 through Bank15 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
by the SIE should not be accessed by the
microcontroller. A semaphore mechanism is used to
determine the access to a particular buffer at any given
time. This is discussed in Section 22.4.1.1 “Buffer
Ownership”.
FIGURE 22-4: IMPLEMENTATION OF
USB RAM IN DATA
MEMORY SPACE
400h
4FFh
500h
USB Data or
Buffer Descriptors,
USB Data or User Data
User Data
USB Data or
SFRs
3FFh
000h
F60h
FFFh
Banks 0
(USB RAM)
F5Fh
to 15
Access Ram
060h
05Fh
F40h
F3Fh
F00h
User Data