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716
7.24 Link Enhancement Control Register
The link enhancement control register implements TI proprietary bits that are initialized by software or by a serial
EEPROM, if present. After these bits are set to 1, their functionality is enabled only if bit 22 (aPhyEnhanceEnable)
in the host controller control register at OHCI offset 50h/54h (see Section 8.16) is set to 1. See Table 720 for a
complete description of the register contents.
Bit
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
Name
Link enhancement control
Type
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Default
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Bit
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Name
Link enhancement control
Type
RW
R
RW
RW
R
RW
R
RW
RW
R
R
R
R
R
RW
R
Default
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
Register:
Offset:
Type:
Default:
Link enhancement control
F4h
Read/Write, Read-only
0000 1000h
Table 720. Link Enhancement Control Register Description
BIT
FIELD NAME
TYPE
DESCRIPTION
3116
RSVD
R
Reserved. Bits 3116 return 0s when read.
15
dis_at_pipeline
RW
Disable AT pipelining. When bit 15 is set to 1, out-of-order AT pipelining is disabled.
14
RSVD
R
Reserved.
1312
atx_thresh
RW
This field sets the initial AT threshold value, which is used until the AT FIFO is underrun. When the
PCI4510 device retries the packet, it uses a 2K-byte threshold, resulting in a store-and-forward
operation.
00 = Threshold ~ 2K bytes resulting in a store-and-forward operation
01 = Threshold ~ 1.7K bytes (default)
10 = Threshold ~ 1K bytes
11 = Threshold ~ 512 bytes
These bits fine-tune the asynchronous transmit threshold. For most applications the 1.7K-byte
threshold is optimal. Changing this value may increase or decrease the 1394 latency depending on
the average PCI bus latency.
Setting the AT threshold to 1.7K, 1K, or 512 bytes results in data being transmitted at these thresholds
or when an entire packet has been checked into the FIFO. If the packet to be transmitted is larger than
the AT threshold, then the remaining data must be received before the AT FIFO is emptied; otherwise,
an underrun condition occurs, resulting in a packet error at the receiving node. As a result, the link then
commences store-and-forward operation. Wait until it has the complete packet in the FIFO before
retransmitting it on the second attempt to ensure delivery.
An AT threshold of 2K results in store-and-forward operation, which means that asynchronous data
will not be transmitted until an end-of-packet token is received. Restated, setting the AT threshold to
2K results in only complete packets being transmitted.
Note that this device will always use store-and-forward when the asynchronous transmit retries
register at OHCI offset 08h (see Section 8.3) is cleared.
11
RSVD
R
Reserved. Bit 11 returns 0 when read.
10
enab_mpeg_ts
RW
Enable MPEG CIP timestamp enhancement. When bit 9 is set to 1, the enhancement is enabled for
MPEG CIP transmit streams (FMT = 20h).
9
RSVD
R
Reserved. Bit 9 returns 0 when read.
8
enab_dv_ts
RW
Enable DV CIP timestamp enhancement. When bit 8 is set to 1, the enhancement is enabled for DV
CIP transmit streams (FMT = 00h).
7
enab_unfair
RW
Enable asynchronous priority requests. OHCI-Lynx
compatible. Setting bit 7 to 1 enables the link to
respond to requests with priority arbitration. It is recommended that this bit be set to 1.