Microchip Technology AC164139 Data Sheet
2010 Microchip Technology Inc.
DS39969B-page 245
PIC24FJ256DA210 FAMILY
BDs have a fixed relationship to a particular endpoint,
depending on the buffering configuration. Table 18-2
provides the mapping of BDs to endpoints. This rela-
tionship also means that gaps may occur in the BDT if
endpoints are not enabled contiguously. This, theoreti-
cally, 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.
depending on the buffering configuration. Table 18-2
provides the mapping of BDs to endpoints. This rela-
tionship also means that gaps may occur in the BDT if
endpoints are not enabled contiguously. This, theoreti-
cally, 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.
18.2.1
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
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 18-1 and
Register 18-2 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.
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 18-1 and
Register 18-2 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.
18.2.2
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.
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.
TABLE 18-2:
ASSIGNMENT OF BUFFER DESCRIPTORS FOR THE DIFFERENT
BUFFERING MODES
BUFFERING MODES
Endpoint
BDs Assigned to Endpoint
Mode 0
(No Ping-Pong)
Mode 1
(Ping-Pong on EP0 OUT)
Mode 2
(Ping-Pong on all EPs)
Mode 3
(Ping-Pong on all other EPs,
except EP0)
Out
In
Out
In
Out
In
Out
In
0
0
1
0 (E), 1 (O)
2
0 (E), 1 (O)
2 (E), 3 (O)
0
1
1
2
3
3
4
4 (E), 5 (O)
6 (E), 7 (O)
2 (E), 3 (O)
4 (E), 5 (O)
2
4
5
5
6
8 (E), 9 (O)
10 (E), 11 (O)
6 (E), 7 (O)
8 (E), 9 (O)
3
6
7
7
8
12 (E), 13 (O)
14 (E), 15 (O) 10 (E), 11 (O) 12 (E), 13 (O)
4
8
9
9
10
16 (E), 17 (O)
18 (E), 19 (O) 14 (E), 15 (O) 16 (E), 17 (O)
5
10
11
11
12
20 (E), 21 (O)
22 (E), 23 (O) 18 (E), 19 (O) 20 (E), 21 (O)
6
12
13
13
14
24 (E), 25 (O)
26 (E), 27 (O) 22 (E), 23 (O) 24 (E), 25 (O)
7
14
15
15
16
28 (E), 29 (O)
30 (E), 31 (O) 26 (E), 27 (O) 28 (E), 29 (O)
8
16
17
17
18
32 (E), 33 (O)
34 (E), 35 (O) 30 (E), 31 (O) 32 (E), 33 (O)
9
18
19
19
20
36 (E), 37 (O)
38 (E), 39 (O) 34 (E), 35 (O) 36 (E), 37 (O)
10
20
21
21
22
40 (E), 41 (O)
42 (E), 43 (O) 38 (E), 39 (O) 40 (E), 41 (O)
11
22
23
23
24
44 (E), 45 (O)
46 (E), 47 (O) 42 (E), 43 (O) 44 (E), 45 (O)
12
24
25
25
26
48 (E), 49 (O)
50 (E), 51 (O) 46 (E), 47 (O) 48 (E), 49 (O)
13
26
27
27
28
52 (E), 53 (O)
54 (E), 55 (O) 50 (E), 51 (O) 52 (E), 53 (O)
14
28
29
29
30
56 (E), 57 (O)
58 (E), 59 (O) 54 (E), 55 (O) 56 (E), 57 (O)
15
30
31
31
32
60 (E), 61 (O)
62 (E), 63 (O) 58 (E), 59 (O) 60 (E), 61 (O)
Legend:
(E) = Even transaction buffer, (O) = Odd transaction buffer