Buffer Descriptors and Connection Tables
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
37-17
0x00
(cont.)
13
CDIS
Channel disable status. Set by the
STOP
RECEIVE
command; cleared by the
RESTART
RECEIVE
command.
0 The channel is enabled to receive cells.
1 The channel is disabled. Cells addressed to this channel are discarded.
Note that the user should not modify this bit directly; that is, the
STOP
RECEIVE
and
RESTART
RECEIVE
commands should be used to control the CDIS bit.
However, CDIS should be cleared during initialization.
14–15
—
Reserved, should be cleared.
0x02
0–15
WCOUNT
Wrap count. Counts the number of BD table wraps. The CP increments
WCOUNT on every BD table wrap and approximates the total cell count,
where:
WCOUNT*table_size < cell count < (1)*table_size
This field should be cleared during initialization.
0x04
0–15
PTP_TX_CH
Point-to-point transmit channel number. The receive channel switches to this
transmit channel. The PTP mechanism activates the specified channel number
and directly inserts it into the APC scheduling table when IAQ=0.
For example, if channel #1 in SCC1 is being switched to channel #32 in the
second level APC of UTOPIA (SCC4) and AVCO is set, PTP_TX_CH should
be programmed to 32, APC_LEVEL should be pointing to the second APC
priority level of SCC4 and SCC# should be programmed to 4.
0x06
0
ITQ
Insert to Transmit Queue
0 Insertion to the Transmit Queue is done by the APC
1 Automatically inserts TX_CHANNEL# into SCC#/PHY# Transmit Queue.
There is no need for APC scheduling for this channel (No need for Tx
Activate Channel command).
1
PPD
PPD enable. Partial packet discard, also known as early packet discard or
frame discard, is a more efficient way to handle congestion for AAL5 channels.
Instead of individual cells, whole user frames (except the last cell, indicated in
the PTI field) are discarded as soon as one cell is dropped (not transferred) due
to congestion. The ATM controller becomes congested when the channel’s BD
table or the transmit queues are full (BUSY exception).
0 Incoming cells are dropped individually based on the immediate congestion
status.
1 Enable PPD. PTP RCT[FHNT] reflects the congestion status of the channel;
that is, it is set as the frame is discarded and cleared again as the last cell is
received.
The user should set this bit to switch the cells of AAL5 channels.
2–3
ATR
Address translation
00 = No translation. The original (incoming) cell header is copied to the buffer.
01 = Address translation on VPI only. Replace the original VPI with the address
translation header’s VPI. The GFC, VCI, PTI and CLP are preserved (copied
from the original cell header).
10 =Address translation header replaces the original GFC, VPI and VCI in the
cell header. The PTI and CLP are preserved (copied from the original).
11 = Address translation on VPI and GFC only. The address translation
header’s VPI and GFC replaces the VPI and GFC in original cell header. The
VCI, PTI and CLP are preserved (copied from the original).
Table 37-4. PTP RCT Field Descriptions (continued)
CT Offset
Bits
Name
Description
Содержание PowerQUICC MPC870
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