Datasheet

PIC18F47J53 FAMILY
DS39964B-page 392 Preliminary  2010 Microchip Technology Inc.
23.4.4 PING-PONG BUFFERING
An endpoint is defined to have a ping-pong buffer when
it has two sets of BD entries: one set for an Even
transfer and one set for an Odd transfer. This allows the
CPU to process one BD while the SIE is processing the
other BD. Double-buffering BDs in this way allows for
maximum throughput to/from the USB.
The USB module supports four modes of operation:
• No ping-pong support
• Ping-pong buffer support for OUT Endpoint 0 only
• Ping-pong buffer support for all endpoints
• Ping-pong buffer support for all other endpoints
except Endpoint 0
The ping-pong buffer settings are configured using the
PPB<1:0> bits in the UCFG register.
The USB module keeps track of the Ping-Pong Pointer
individually for each endpoint. All pointers are initially
reset to the Even BD when the module is enabled. After
the completion of a transaction (UOWN cleared by the
SIE), the pointer is toggled to the Odd BD. After the
completion of the next transaction, the pointer is
toggled back to the Even BD and so on.
The Even/Odd status of the last transaction is stored in
the PPBI bit of the USTAT register. The user can reset
all Ping-Pong Pointers to Even using the PPBRST bit.
Figure 23-6 shows the four different modes of
operation and how USB RAM is filled with the BDs.
BDs have a fixed relationship to a particular endpoint,
depending on the buffering configuration. Table 23-2
provides the mapping of BDs to endpoints. This
relationship also means that gaps may occur in the
BDT if endpoints are not enabled contiguously. This,
theoretically, means that the BDs for disabled
endpoints could be used as buffer space. In practice,
users should avoid using such spaces in the BDT
unless a method of validating BD addresses is
implemented.
FIGURE 23-6: BUFFER DESCRIPTOR TABLE MAPPING FOR BUFFERING MODES
EP1 IN Even
EP1 OUT Even
EP1 OUT Odd
EP1 IN Odd
Descriptor
Descriptor
Descriptor
Descriptor
EP1 IN
EP15 IN
EP1 OUT
EP0 OUT
PPB<1:0> = 00
EP0 IN
EP1 IN
No Ping-Pong
EP15 IN
EP0 IN
EP0 OUT Even
PPB<1:0> = 01
EP0 OUT Odd
EP1 OUT
Ping-Pong Buffer
EP15 IN Odd
EP0 IN Even
EP0 OUT Even
PPB<1:0> = 10
EP0 OUT Odd
EP0 IN Odd
Ping-Pong Buffers
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
D00h
DFFh DFFh DFFh
D00h D00h
D7Fh
D83h
Available
as
Data RAM
Available
as
Data RAM
Maximum Memory
Used: 128 bytes
Maximum BDs:
32 (BD0 to BD31)
Maximum Memory
Used: 132 bytes
Maximum BDs:
33 (BD0 to BD32)
Maximum Memory
Used: 256 bytes
Maximum BDs: 6
4 (BD0 to BD63)
Note: Memory area is not shown to scale.
Descriptor
Descriptor
Descriptor
Descriptor
Buffers
on EP0 OUT
on all EPs
EP1 IN Even
EP1 OUT Even
EP1 OUT Odd
EP1 IN Odd
Descriptor
Descriptor
Descriptor
Descriptor
EP15 IN Odd
EP0 OUT
PPB<1:0> = 11
EP0 IN
Ping-Pong Buffers
Descriptor
Descriptor
Descriptor
DFFh
D00h
Maximum Memory
Used: 248 bytes
Maximum BDs:
62 (BD0 to BD61)
on all other EPs
except EP0
Available
as
Data RAM
DF7h