Freescale Semiconductor MCF5329 Reference Manual page 470

Universal Serial Bus Interface – On-The-Go Module
Upon discovery of a transmit (OUT/SETUP) packet in the data structures, the host controller checks to
ensure T remains before the end of the (micro)frame. If so, it pre-fills the TX FIFO. If at anytime during
p
the pre-fill operation the time remaining the (micro)frame is less than T
at a later time. Although this is not an error condition and the module eventually recovers, a mark is made
in the scheduler health counter to mark the occurrence of a back-off event. When a back-off event is
detected, the partial packet fetched may need to be discarded from the latency buffer to make room for
periodic traffic beginning after the next SOF. Too many back-off events can waste bandwidth and power
on the system bus and should be minimized (not necessarily eliminated). The TSCHHEALTH (T
parameter described below can minimize back-offs.
Address: 0xFC0B_0164 (TXFILLTUNING)
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
R 0 0 0 0 0 0 0 0 0 0
W
Reset 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Figure 21-25. Transmit FIFO Tuning Controls (TXFILLTUNING)
Field
31–22 Reserved, must be cleared.
21–16 FIFO burst threshold. Controls the number of data bursts that are posted to the TX latency FIFO in host mode
TXFIFOTHRES before the packet begins on the bus. The minimum value is 2 and this value should be as low as possible to
maximize USB performance. Systems with unpredictable latency and/or insufficient bandwidth can use a
higher value where the FIFO may underrun because the data transferred from the latency FIFO to USB
occurs before it can replenish from system memory.
This value is ignored if the USBMODE[SDIS] bit is set. When the USBMODE[SDIS] bit is set, the host
controller behaves as if TXFIFOTHRES is set to its maximum value.
15–13 Reserved, must be cleared.
21-28
TXFIFOTHRES
Table 21-28. TXFILLTUNING Field Descriptions
MCF5329 Reference Manual, Rev 3
, packet attempt ceases and tries
s
9 8
0 0 0
TXSCHHEALTH
Description
)
ff
Access: User read/write
7 6 5 4 3 2 1 0
TXSCHOH
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