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MT90520
Data Sheet
104
Zarlink Semiconductor Inc.
Figure 43 - Block Diagram of the Digital PLL Module
Modes of Operation (PLL_INPUT_SEL bits)
The PLL has four major modes of operation which are configured on a per-port basis via the PLL_INPUT_SEL bits
in the Clocking Configuration Registers. The four modes are, briefly, as follows:
Line-clocking mode: Synchronize to a 1.544 MHz, 2.048 MHz, or 4.096 MHz clock input to the chip on the
port’s corresponding STiCLK input line. Generate an outgoing clock with the appropriate frequency. The
outgoing service clock (1.544 MHz, 2.048 MHz, or 4.096 MHz) is phase-locked to the input clock. This mode
of operation may be used to “clean up” a jittery input clock. In line-clocking mode, the maximum locking time
is about 15 seconds.
SRTS mode: Synchronize to the incoming RTS nibbles (time stamps). Generate a 1.544 MHz, 2.048 MHz, or
4.096 MHz clock. The proper service output clock is fed into the per-port SRTS generator (outlined in
Section 4.7.2.4, “Transmit SRTS Circuit Sub-module,” on page 99) to generate a local RTS value that is
compared with subsequent received RTS values. In SRTS mode, the maximum locking time is
approximately 100 seconds.
Network mode: Similar to line-clocking mode, but the input clock is an 8 kHz clock (coming either directly
from the PHY_CLK pin or from a divided down version of a 19.44 MHz clock input at the PHY_CLK pin). The
8 kHz clock is sourced from the Clock Management module, as explained in Section 4.7.2.3, “Network Clock
Divider Circuit,” on page 97. The output clock (1.544 MHz, 2.048 MHz, or 4.096 MHz) is synchronous and
phase-locked to the 8 kHz input clock. In network mode, the maximum locking time is about 40 seconds.
Adaptive mode: Synchronize to the incoming cell stream by monitoring the fill level of a VC’s Reassembly
Circular Buffers. The PLL receives a phaseword from the UDT RX_SAR or SDT RX_SAR. A clock rate of
approximately 1.544 MHz, 2.048 MHz, or 4.096 MHz is generated, depending upon the PLL_FREQ_SEL
bits.
Note that the UDT RX_SAR and SDT RX_SAR Reassembly Control Structures must be configured appropriately if
either Adaptive or SRTS Clock Recovery methods are to be used. For more details, refer to the UDT RX_SAR and
SDT RX_SAR module descriptions in Section 4.6.1, “UDT RX_SAR and SDT RX_SAR Modules” earlier in this
document.
SRTS
Generator
*
* - outside PLL module
fnxi
Phase
Detector
Loop
Filter
DCO
Divider
RTS out
output
clock
frequency
offset
Filter implemented in
software (via CPU)
DCO_DIFF
Phaseword
Clock
Received
RTS
2:1
Mux
8 kHz_PLL
Network mode