You can configure the CDR in either automatic lock mode or manual lock mode. By default, the Quartus® Prime software configures the CDR in automatic lock mode.
Automatic Lock Mode
In automatic lock mode, the CDR initially locks to the input reference clock (LTR mode). After the CDR locks to the input reference clock, the CDR locks to the incoming serial data (LTD mode) when the following conditions are met:
- The signal threshold detection circuitry indicates the presence of valid signal levels at the receiver input buffer when rx_std_signaldetect is enabled.
- The CDR output clock is within the configured ppm frequency threshold setting with respect to the input reference clock (frequency locked).
- The CDR output clock and the input reference clock are phase matched within approximately 0.08 unit interval (UI) (phase locked).
If the CDR does not stay locked to data because of frequency drift or severe amplitude attenuation, the CDR switches back to LTR mode.
Manual Lock Mode
The PPM detector and phase relationship detector reaction times can be too long for some applications that require faster CDR lock time. You can manually control the CDR to reduce its lock time using two optional input ports (rx_set_locktoref and rx_set_locktodata).
|rx_set_locktoref||rx_set_locktodata||CDR Lock Mode|
|1||0||Manual-RX CDR LTR|
|X||1||Manual-RX CDR LTD|
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