PIC18F2450 Datasheet, PDF (139/320 Pages) Microchip Technology – 28/40/44-Pin, High-Performance, 12 MIPS, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

14.4 Buffer Descriptors and the Buffer
Descriptor Table
The registers in Bank 4 are used specifically for end-
point buffer control in a structure known as the Buffer
Descriptor Table (BDT). This provides a flexible method
for users to construct and control endpoint buffers of
various lengths and configuration.
The BDT is composed of Buffer Descriptors (BD) which
are used to define and control the actual buffers in the
USB RAM space. Each BD, in turn, consists of four
registers, where n represents one of the 64 possible
BDs (range of 0 to 63):
• BDnSTAT: BD Status register
• BDnCNT: BD Byte Count register
• BDnADRL: BD Address Low register
• BDnADRH: BD Address High register
BDs always occur as a four-byte block in the sequence,
BDnSTAT:BDnCNT:BDnADRL:BDnADRH. The address
of BDnSTAT is always an offset of (4n – 1) (in
hexadecimal) from 400h, with n being the buffer
descriptor number.
Depending on the buffering configuration used
(Section 14.4.4 “Ping-Pong Buffering”), there are up
to 32, 33 or 64 sets of buffer descriptors. At a minimum,
the BDT must be at least 8 bytes long. This is because
the USB specification mandates that every device must
have Endpoint 0 with both input and output for initial
setup. Depending on the endpoint and buffering
configuration, the BDT can be as long as 256 bytes.
Although they can be thought of as Special Function
Registers, the Buffer Descriptor Status and Address
registers are not hardware mapped, as conventional
microcontroller SFRs in Bank 15 are. If the endpoint cor-
responding to a particular BD is not enabled, its registers
are not used. Instead of appearing as unimplemented
addresses, however, they appear as available RAM.
Only when an endpoint is enabled by setting the
UEPn<1> bit does the memory at those addresses
become functional as BD registers. As with any address
in the data memory space, the BD registers have an
indeterminate value on any device Reset.
A total of 256 bytes of address space in Bank 4 is
available for BDT and USB data RAM. In Ping-Pong
Buffer mode, all the 16 bidirectional endpoints can not
be implemented where BDT itself can be as long as
256 bytes. In the majority of USB applications, few
endpoints are required to be implemented. Hence, a
small portion of the 256 bytes will be used for BDT and
the rest can be used for USB data.
An example of a BD for a 16-byte buffer, starting at
480h, is shown in Figure 14-6. A particular set of BD
registers is only valid if the corresponding endpoint has
been enabled using the UEPn register. All BD registers
are available in USB RAM. The BD for each endpoint
should be set up prior to enabling the endpoint.
PIC18F2450/4450
14.4.1 BD STATUS AND CONFIGURATION
Buffer descriptors not only define the size of an
endpoint buffer, but also determine its configuration
and control. Most of the configuration is done with the
BD Status register, BDnSTAT. Each BD has its own
unique and correspondingly numbered BDnSTAT
register.
FIGURE 14-6:
EXAMPLE OF A BUFFER
DESCRIPTOR
Address Registers
Buffer
Descriptor
400h
401h
402h
403h
BD0STAT
BD0CNT
BD0ADRL
BD0ADRH
Contents
(xxh)
10h _{Size of Block}
80h
Starting
04h _Address
480h
Buffer
USB Data
Note:
48Fh
Memory regions not to scale.
Unlike other control registers, the bit configuration for
the BDnSTAT register is context sensitive. There are
two distinct configurations, depending on whether the
microcontroller or the USB module is modifying the BD
and buffer at a particular time. Only three bit definitions
are shared between the two.
14.4.1.1 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 (BDnSTAT<7>) 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. Prior to placing
ownership with the USB peripheral, the user can con-
figure the basic operation of the peripheral through the
BDnSTAT bits. During this time, the byte count and
buffer location registers can also be set.
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 137