2–64
Altera Corporation
Stratix II GX Device Handbook, Volume 2
October 2007
Receiver Modules
Figure 2–52. Clock Recovery Unit
Lock-to-Reference and Lock-to-Data Modes
The lock-to-reference and lock-to-data modes describe the receiver PLL
and the CRU. The receiver PLL is active in the lock-to-reference mode and
the CRU is active in the lock-to-data mode. The switch between the two
modes can be done automatically or manually.
Automatic Lock Mode
The CRU, by default, initially locks to the CRU reference clock
(lock-to-reference mode) until switching over to the incoming data
(lock-to-data mode). The switch to lock-to-data mode is indicated by the
assertion of the
rx_freqlocked
signal. The
rx_freqlocked
signal
only indicates the current mode of CRU (lock-to-data or
lock-to-reference). After switching into lock-to-data mode (assertion of
the
rx_freqlocked
signal), the CRU unit needs time to acquire phase
lock to the incoming data stream.
For automatic transition from the lock-to-reference mode to the
lock-to-data mode, the following conditions must be met:
■
The serial data at the receiver input buffer is within the prescribed
voltage signal loss threshold.
■
The CRU PLL is within the prescribed PPM frequency threshold
setting (62.5, 100, 125, 200, 250, 300 , 500, or 1,000 PPM) of the CRU
reference clock.
■
The reference clock and CRU PLL output are phase matched (phases
are within approximately 0.08 UI).
rx_cruclk
CP+LF
Up
Down
VCO
/m
rx_datain
High-Speed Recovered Clock
Low-Speed Recovered Clock
Down
Up
rx_locktorefclk
rx_locktodata
rx_freqlocked
Clock Recovery Unit Control
PFD
÷
L
÷
2
÷
N
÷
1, 2, 4
Inactive Circuits
Active Circuits
rx_pll_locked