
CHAPTER 14 USBU (UNIVERSAL SERIAL BUS UNIT)
User’s Manual U14579EJ2V0UM
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Table 14-3. ED Fields (2/2)
Field Name
R/W
Function
C
R/W
Toggle Carry
This bit is a data toggle carry bit. When a TD is retired, this bit is always written using the final data
toggle value (LSB of the T area) that was used by the retired TD. This field is not used by an
isochronous endpoint.
HeadP
R/W
TD Queue Head Pointer
Indicates the TD that is to be processed next by this endpointer.
NextED
R
NextED
Indicates the next ED (excluding 0x000 0000).
RFU
R
Reserved for Future Use
Reserved. Write 0 to these bits. 0 is returned after a read.
14.4.6 TD (Transfer Descriptor)
TDs (Transfer Descriptors) are used by the HC to indicate a buffer for the data that is transmitted to or received
from an endpoint. The two types of TDs are General and Isochronous. A GeneralTD is used by an interrupt, control, or
bulk endpointer. An IsochronousTD is used by an isochronous transfer.
Both a GeneralTD and an IsochronousTD can indicate a buffer from 0 to 8192 bytes. In addition, the data buffer
described by a single TD can be divided into two pages. This eliminates problems such as forcibly placing buffers so
they are physically contiguous and moving excess data.
When the HCD adds a TD, it links the new TD to the TD indicated by the TailP area and updates the TailP area so
that it points to the added TD. Therefore, the added TD must be added at the end of the TD queue. The HC processes
TDs asynchronously relative to processing by the host CPU. Therefore, when the TD queue must be switched to
another queue, the HC’s endpoint TD queue processing must be halted so the queue can be changed. The HCD halts
TD processing by setting (1) the K bit of the ED to be deleted.
14.4.7 GeneralTD format
A GeneralTD is a TD for control, bulk, or interrupt transfers. It is always located in system memory in units of 16
bytes.
Figure 14-10 shows the GeneralTD format. For details about each field, see Table 14-4.
Figure 14-10. GeneralTD Format
0
3
4
18 17
20
21
23
24
25
26
28 27
31
19
0
RFU
Next TD
BE
CBP
DP
R
DI
T
EC
CC
Dword0
Dword1
Dword2
Dword3