Microchip Technology DM240015 Data Sheet

PIC24FJ128GC010 FAMILY

DS30009312B-page 270

 2012-2013 Microchip Technology Inc.

19.2

USB Buffer Descriptors and the
BDT

Endpoint buffer control is handled through a structure
called the Buffer Descriptor Table (BDT). This provides
a flexible method for users to construct and control
endpoint buffers of various lengths and configurations.
The BDT can be located in any available 512-byte,
aligned block of data RAM. The BDT Pointer
(U1BDTP1) contains the upper address byte of the
BDT and sets the location of the BDT in RAM. The user
must set this pointer to indicate the table’s location.
The BDT is composed of Buffer Descriptors (BDs)
which are used to define and control the actual buffers
in the USB RAM space. Each BD consists of two 16-bit,
“soft” (non-fixed-address) registers, BDnSTAT and
BDnADR, where n represents one of the 64 possible
BDs (range of 0 to 63). BDnSTAT is the status register
for BDn, while BDnADR specifies the starting address
for the buffer associated with BDn.

Depending on the endpoint buffering configuration
used, there are up to 64 sets of Buffer Descriptors, for
a total of 256 bytes. At a minimum, the BDT must be at
least 8 bytes long. This is because the “USB 2.0 OTG
Specification”

mandates that every device must have

Endpoint 0 with both input and output for initial setup.
Endpoint mapping in the BDT is dependent on three
variables:
• Endpoint number (0 to 15)
• Endpoint direction (RX or TX)
• Ping-pong settings (U1CNFG1<1:0>)

Figure 19-7

illustrates how these variables are used to

map endpoints in the BDT.
In Host mode, only Endpoint 0 Buffer Descriptors are
used. All transfers utilize the Endpoint 0 Buffer Descrip-
tor and Endpoint Control register (U1EP0). For received
packets, the attached device’s source endpoint is
indicated by the value of ENDPT<3:0> in the USB status
register (U1STAT<7:4>). For transmitted packets, the
attached device’s destination endpoint is indicated by
the value written to the Token register (U1TOK).

FIGURE 19-7:

BDT MAPPING FOR ENDPOINT BUFFERING MODES

Note:

Since BDnADR is a 16-bit register, only
the first 64 Kbytes of RAM can be
accessed by the USB module.

EP1 TX Even

EP1 RX Even

EP1 RX Odd

EP1 TX Odd

Descriptor

EP1 TX

EP15 TX

EP1 RX

EP0 RX

PPB<1:0> = 00

EP0 TX

EP1 TX

No Ping-Pong

EP15 TX

EP0 TX

EP0 RX Even

PPB<1:0> = 01

EP0 RX Odd

EP1 RX

Ping-Pong Buffer

EP15 TX Odd

EP0 TX Even

EP0 RX Even

PPB<1:0> = 10

EP0 RX Odd

EP0 TX Odd

Ping-Pong Buffers

Descriptor

Note:

Memory area is not shown to scale.

Descriptor

Buffers

on EP0 OUT

on All EPs

EP1 TX Even

EP1 RX Even

EP1 RX Odd

EP1 TX Odd

Descriptor

EP15 TX Odd

EP0 RX

PPB<1:0> = 11

EP0 TX

Ping-Pong Buffers

Descriptor

on All Other EPs

Except EP0

Total BDT Space:

128 Bytes

132 Bytes

256 Bytes

248 Bytes