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RELEASED
DATA SHEET
PM7366 FREEDM-8
ISSUE 4
PMC-1970930
FRAME ENGINE AND DATA LINK MANAGER
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA,INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
70
Links may also be unchannelised. Then, all data bytes on that link belong to one channel. The
TCAS performs a table look-up to identify the channel to which a data byte belongs using only the
outgoing link identity, as no time-slots are associated with unchannelised links. Link clocks are no
longer limited to T1 or E1 rates and may range up to 52 MHz for TCLK[2:0]. For TCLK[7:3] the
maximum clock rate is 10 MHz. The link clock is only active during bit times containing data to be
transmitted and inactive during bits that are to be ignored by the downstream devices, such as
framing and overhead bits. For the case of two unchannelised links, the maximum link rate
52 MHz for SYSCLK at 33 MHz. For the case of more numerous unchannelised links or a mixture
of channelised and unchannelised links, the total instantaneous link rate over all the links is limited
to 64 MHz.
9.8.1
Line Interface
There are two types of line interfaces in the TCAS; high-speed and low-speed interfaces. Three
identical high-speed interfaces are attached to the first three links, while five identical low-speed
interfaces are attached to the remaining links. Each line interface contains a bit counter, an 8-bit
shift register and a byte FIFO, that, together, perform parallel to serial conversion. For the high-
speed interfaces the FIFO is six bytes deep. For the low-speed interfaces, the FIFO is a single
byte holding register. Whenever the shift register is updated, a request for service is sent to the
priority encoder block. The request will eventually be serviced by the THDL block and the data is
written into the FIFO.
To support channelised links, each line interface block contains a time-slot counter and a clock
activity monitor. The time-slot counter is incremented each time the shift register is updated. The
clock activity monitor is a counter that increments at the system clock (SYSCLK) rate and is
cleared by a rising edge of the transmit clock (TCLK[n]). A framing bit (T1) or framing byte (E1) is
detected when the counter reaches a programmable threshold. At which point, the bit and time-
slot counters are initialised to indicate the next bit sampled is the most significant bit of the first
time-slot. For unchannelised links, the time-slot counter and the clock activity monitor are held
reset.
9.8.2
Priority Encoder
The priority encoder monitors the line interfaces for requests and synchronises them to the
SYSCLK timing domain. Requests are serviced on a fixed priority scheme where highest to
lowest priority is assigned from line interface TD[0] to line interface TD[7]. Thus, simultaneous
requests from line interface TD[m] will be serviced ahead of line interface TD[n], if m < n. The
priority encoder selects the request from the link with the highest priority for service. When there
are no pending requests, the priority encoder generates an idle cycle. In addition, once every
fourth SYSCLK cycle, the priority encoder inserts a null cycle where no requests are serviced.
This cycle is used by the channel assigner downstream for CBI accesses to the channel provision
RAM.