Altera Corporation
2–65
October 2007
Stratix II GX Device Handbook, Volume 2
Stratix II GX Transceiver Architecture Overview
When the receiver PLL and CRU is in lock-to-reference mode, the PPM
detector, phase detector, and signal-detect circuits monitor the
relationship of the reference clock to the receiver PLL and VCO output. If
the frequency difference is within the prescribed PPM setting, the
amplitude is within the prescribed limits, and the phase is within 0.08 UI,
then the CRU switches to the lock-to-data mode.
In lock-to-data mode, the PLL uses a phase detector to keep the recovered
clock aligned properly with the data. If the PLL does not stay locked to
data because of problems such as frequency drift or severe amplitude
attenuation, the receiver PLL locks back to the reference clock of the CRU
to train the VCO. When the device is in lock-to-data mode
(
rx_freqlocked
is asserted), the CRU is trying to align with incoming
data and there is no phase relationship with the reference clock.
The
rx_freqlocked
signal indicates which mode the CRU is in (either
lock-to-data or lock-to-reference mode). In lock-to-data mode, the
rx_freqlocked
signal is high. In lock-to-reference mode, the
rx_freqlocked
signal is low.
In lock-to-data mode, the
rx_freqlocked
signal is asserted and the
rx_pll_locked
signal loses its significance because the
rx_pll_locked
signal indicates that the CRU has locked to the
reference clock. When the CRU is in lock-to-data mode, the phase of the
VCO may differ from the reference clock, which may deassert the
rx_pll_locked
signal. You should ignore the
rx_pll_locked
signal
when the
rx_freqlocked
signal is high.
In automatic lock mode, there are two conditions that force the CRU to
fall out of lock-to-data mode.
■
The serial data at the receiver input buffer is not within the
prescribed voltage signal loss threshold. This condition is ignored if
the Force signal detection option in the MegaWizard is enabled.
■
The CRU PLL is not within the prescribed PPM frequency threshold
setting (62.5, 100, 125, 200, 250, 300, 500, or 1,000 PPM) of the CRU
reference clock.
If the CRU falls out of lock-to-data mode, the
rx_freqlocked
signal is
deasserted. You can also deassert the
rx_freqlocked
signal by
asserting either
rx_analogreset
(powers down the receiver) or
gxb_powerdown
(powers down all four channels of the transceiver
block.
Содержание Stratix II GX
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