mt90503 Zarlink Semiconductor, mt90503 Datasheet - Page 52

• Manufacturer Part Number: mt90503
• Description: 2048vc Aal1 Sar
• Manufacturer: Zarlink Semiconductor

MT90503
Data Sheet
Figure 18 TDM Circular Buffers (Normal mode) shows that very few of the positions within the circular buffer are
invalid for the SAR pointers.
4.2.6.2
E1/T1 Multiframing (standard)
The TDM pointers change to respect the multiframing standards required by E1 and T1 operation. The pointer has
two halves. One half indicates the number of the multiframe that the TDM is writing or reading; the other half is the
number of the frame within the multiframe. In the T1 mode, this fraction goes from 0 to 23, while in the E1 mode it
ranges from 0 to 15. The same global standards of pointer definition still apply as the pointer still points to an invalid
byte.
The TDM_write_pnt has a global value, which is the value of the internal multiframe pointer. However, due to the
multiframing offset, the TDM_write_pnt can vary by almost an entire frame. Therefore, if the global
TDM_write_pointer points to 0.0 (Multiframe 0, frame 0) the individual TDM channels may be written anywhere from
0.0 to 0.23 in the T1 mode, and from 0.0 to 0.15 in the E1 mode. The TDM channels may be written almost an
entire frame ahead of the global pointer. The pointer sent to the TX_SAR is the same as the global
TDM_write_pointer.
The TDM_read_pnt is two multiframes ahead of the TDM_write_pnt. The need for this lead arises from the write
backs and from CAS. Since the TDM_write_pnt can be writing up to 15 frames ahead, a lead of at least one
multiframe must be given. In addition to this, the CAS must be considered. CAS is written during the multiframe of
the tx, which can be up to one multiframe ahead of the global pointer. When CAS is read from the rx side it is read
at the beginning of the multiframe. Therefore, to ensure that there is always a difference of at least one in the CAS
pointers, the delta between the TDM pointers is established at two multiframes.
Furthermore, the read pointer that is sent to the RX_SAR is almost one multiframe ahead. This is necessary
because CAS is sent at the end of the multiframe on the ATM side, and is read at the beginning of the multiframe on
the TDM side. Therefore, to ensure that the CAS of the multiframe is written by the RX_SAR before the TDM reads
it, the pointer passed to the RX_SAR is incremented by one multiframe minus one frame. Therefore, if the
TDM_read_pointer would have been 2.5 (two multiframes, five frames), the TDM_read_pnt passed to the RX_SAR
is 3.4 (three multiframes, four frames).
52
Zarlink Semiconductor Inc.