
MT90503
Data Sheet
72
Zarlink Semiconductor Inc.
AAL
Adaptation
Layer
+0/b8:b7
The AAL bits indicates the ATM format used to assemble the
cells in this structure. The AAL bits are decoded as follows:
00 = CBR AAL0
01 = AAL5-VTOA
10 = AAL1 without pointer
11 = AAL1 with pointer
These bits encode the size of the entire TX/RX circular
buffer that is to be written into. In T1 and E1 modes, a
certain portion of the space in the buffer is required to store
the CAS values; in the T1 mode 25% is required and in the
E1 mode 50% is required.
The eight MSBs of each word in the TX/RX Circular buffer
are used for TX data and the remaining 8 bits for RX data.
These bits are decoded as follows:
00 = 128 words
01 = 256 words
10 = 512 words
11 = 1024 words
Initialisation method for rx_sar_write_pnt
'0' = initialise rx_sar_write_pnt to nearest boundary: either
tdm_read_pnt + (Max used bytes in TX/RX Circular buffer) -
1 OR tdm_read_pnt + 1
'1' = initialise rx_sar_write_pnt to (Max used bytes in TX/RX
Circular buffer)/2
This initialisation method will be used when the first cell is
received and each time a slip (an overrun or underrun)
occurs.
The circuit emulation bits indicate the multiframing standard
used within the VC.
These bits are decoded as follows:
000 = No CAS, no multiframe
001 = Reserved
01x = Reserved
100 = T1 without CAS
101 = T1 with CAS
110 = E1 without CAS
111 = E1 with CAS
FASTCAS is enabled when FC is asserted high. If
FASTCAS is used, then the receive pointer used for regular
TDM bytes and for CAS bytes is not the same. CAS is
written one multiframe before TDM bytes in FASTCAS.
When FC is deasserted, regular multiframing is employed:
CAS is written in the same multiframe as TDM bytes.
Last Entry is the word offset from the 8 kB boundary
containing the structure to the last circular buffer pointer in
the structure.
When set, overrun and underrun slips will generate an
RX_SAR Error Report Structure (Figure 32) in the error
FIFO in control memory.
BS
TX/RX
Circular Buffer
Size
+0/b6:b5
IM
Initialisation
Method
+0/b4
CE
Circuit
Emulation
+0/b3:b1
FC
FASTCAS
Enable
+0/b0
Last Entry
Last Entry
+2/b15:b4
SE
Slip Error
Report Enable
+2/b3
Field
Name of Field
Byte Address
Offset/Bits Used
Description of Field
Table 23 - Description of the Fields for the RX_SAR Structure (continued)