Altera Corporation
2–63
October 2007
Stratix II GX Device Handbook, Volume 2
Stratix II GX Transceiver Architecture Overview
PPM Frequency Threshold Detector
The PPM frequency threshold detector senses whether the incoming
reference clock to the clock recovery unit (CRU) and the PLL VCO of the
CRU are within a prescribed PPM tolerance range. Valid parameters are
62.5, 100, 125, 200, 250, 300, 500, or 1,000 PPM. The default parameter, if
no assignments are made, is 1,000 PPM. The output of the PPM frequency
threshold detector is one of the variables that assert the
rx_freqlocked
signal. Refer to
“Automatic Lock Mode” on page 2–64
for more details
regarding the
rx_freqlocked
signal.
Receiver Bandwidth Type
The Stratix II GX receiver PLL in the CRU offers a programmable
bandwidth setting. The PLL bandwidth is the measure of its ability to
track the input data and jitter. The bandwidth is determined by the –3dB
frequency of the closed-loop gain of the PLL.
A higher bandwidth setting helps reject noise from the VCO and power
supplies. A low bandwidth setting filters out more high-frequency data
input jitter.
Valid receiver bandwidth settings are low, medium, or high. The –3dB
frequencies for these settings vary because of the non-linear nature and
data dependencies of the circuit. You can vary the bandwidth to adjust
and customize the performance on specific systems.
Clock Recovery Unit
The CRU (refer to
Figure 2–52
) in each Stratix II GX transceiver channel
recovers the clock from the serial data stream on
rx_datain
. You can set
the CRU to automatically or manually lock to the data phase and
frequency to match the bit transition to eliminate any clock-to-data skew
or to keep the receiver PLL locked to the reference clock (lock-to-data or
lock-to-reference mode). The CRU generates two clocks: a high-speed
clock that feeds the deserializer and a slow-speed clock that feeds the rest
of the receiver’s digital logic.