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Multi-Service Access Device For Channelized Interfaces
Telecom Standard Product Data Sheet
Production
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-1990823, Issue 4
626
15.8.2 Transmit PL3 STPA and PTPA Behavior
The STPA signal gives the status of the FIFO which is being filled by the transmit PL3 interface.
STPA is used for “word transfer mode” in the PL3 specification, and it is recommended only for
use in single phy configurations. PTPA can also give this status, but it must be polled for the
desired channel. PTPA is used for “burst transfer mode” in the PL3 specification, and is
recommended for multi-phy configurations. The following discussion revolves around STPA, but
also applies to PTPA if the corresponding channel is polled (presented on the TADR pins).
When STPA is logic 1, it indicates that the FIFO has enough room to absorb a pre-determined
number of data transfers. Once the FIFO threshold has been exceeded, meaning that the FIFO
can no longer absorb the pre-determined number of data transfers, then STPA will transition to
logic 0.
The following diagrams show the behavior of STPA when the FIFO threshold is reached. It is
critical that the upstream device respond properly in order to avoid any FIFO overflows in the
TXSDQ.
There are two basic methods of transferring data across the PL3 interface: burst-transfer mode,
and word-transfer mode. Burst-transfer mode is similar to the UL3 interface. Where the UL3
interface will transfer one cell at a time, the PL3 interface will transfer a maximum number of
bytes or end at an end-of-packet indication in one burst, whichever comes first. Polling the TPA
signal will give the status of the FIFO for the next burst (with some latency) just like UL3’s TCA
indication. Setting TPAHOLD (register 0048H TXPHY Configuration) to logic 1 will configure
the S/UNI-MACH48 to burst-transfer mode.
Word-transfer mode has no such limitations. The upstream device is expected to watch the PTPA
signal to find when it can begin a transfer to a particular PHY channel. Once the transfer is
started, it must observe the STPA signal and pause when required to prevent oveflowing the
FIFO. Setting TPAHOLD to logic 0 will configure the S/UNI-MACH48 to word-transfer mode
or hybrid word/burst-transfer mode.
TPAHOLD = 0
Figure 40 shows the behavior of STPA when the TPAHOLD register bit (in register 0048H
TXPHY Configuration) is set to logic 0. This is the default operating mode. In this example,
the write to the FIFO which occurs at the start of cycle 7 crosses the TXSDQ’s BT[4:0] threshold.
STPA responds on cycle 12. There is a 5 clock cycle delay between the write and the response on
STPA.