PIC24FJ256GB106-I Datasheet, PDF (200/328 Pages) Microchip Technology – 64/80/100-Pin, 16-Bit Flash Microcontrollers with USB On-The-Go (OTG)

PIC24FJ256GB110 FAMILY
17.1.1 BUFFER OWNERSHIP
Because the buffers and their BDs are shared between
the CPU and the USB module, a simple semaphore
mechanism is used to distinguish which is allowed to
update the BD and associated buffers in memory. This
is done by using the UOWN bit as a semaphore to
distinguish which is allowed to update the BD and
associated buffers in memory. UOWN is the only bit
that is shared between the two configurations of
BDnSTAT.
When UOWN is clear, the BD entry is “owned” by the
microcontroller core. When the UOWN bit is set, the BD
entry and the buffer memory are “owned” by the USB
peripheral. The core should not modify the BD or its
corresponding data buffer during this time. Note that
the microcontroller core can still read BDnSTAT while
the SIE owns the buffer and vice versa.
The buffer descriptors have a different meaning based
on the source of the register update. Register 17-1 and
Register 17-2 show the differences in BDnSTAT
depending on its current “ownership”.
When UOWN is set, the user can no longer depend on
the values that were written to the BDs. From this point,
the USB module updates the BDs as necessary, over-
writing the original BD values. The BDnSTAT register is
updated by the SIE with the token PID and the transfer
count is updated.
17.1.2 DMA INTERFACE
The USB OTG module uses a dedicated DMA to
access both the BDT and the endpoint data buffers.
Since part of the address space of the DMA is dedi-
cated to the Buffer Descriptors, a portion of the memory
connected to the DMA must comprise a contiguous
address space properly mapped for the access by the
module.
REGISTER 17-1: BDnSTAT: BUFFER DESCRIPTOR n STATUS REGISTER PROTOTYPE, USB
MODE (BD0STAT THROUGH BD63STAT)
R/W-x
UOWN
bit 15
R/W-x
DTS
R/W-x
PID3
R/W-x
PID2
R/W-x
PID1
R/W-x
PID0
R/W-x
BC9
R/W-x
BC8
bit 8
R/W-x
BC7
bit 7
R/W-x
BC6
R/W-x
BC5
R/W-x
BC4
R/W-x
BC3
R/W-x
BC2
R/W-x
BC1
R/W-x
BC0
bit 0
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared
x = Bit is unknown
bit 15
bit 14
bit 13-10
bit 9-0
UOWN: USB Own bit
1 = The USB module owns the BD and its corresponding buffer; the CPU must not modify the BD or
the buffer
DTS: Data Toggle Packet bit
1 = Data 1 packet
0 = Data 0 packet
PID3:PID0: Packet Identifier bits (written by the USB module)
In Device mode:
Represents the PID of the received token during the last transfer.
In Host mode:
Represents the last returned PID, or the transfer status indicator.
BC9:BC0: Byte Count
This represents the number of bytes to be transmitted or the maximum number of bytes to be received
during a transfer. Upon completion, the byte count is updated by the USB module with the actual
number of bytes transmitted or received.
DS39897B-page 198
Preliminary
© 2008 Microchip Technology Inc.