NEC UPD98413 User Manual page 113

(4) Clock Data Recovery
The receive clock is extracted by the clock recovery unit (CDR) from the receive data string input by each
port to RDIT/RDIC. After power is applied, the CDR of each port starts following up with the reference clock.
When the frequency of the recovered clock is within 500 ppm of the reference clock, the CDR switch the
following up to the receive data. If the correct receive data is input, a synchronization clock can be obtained
about T.B.D. ms after reset.
(5) Lock Detect
Each port CDR has unlock detection circuit that monitors whether the CDR follows up with a receive data
string and that the expected clock is extracted. This circuit monitors the frequency difference between the
clock extracted by the CDR and the reference clock. If the difference between the two clock frequencies
exceeds 500 ppm, the lock detection circuit judges that the CDR is in out of locked state (OOL status). This
status is reflected on the bits of the status register and can be used as an interrupt cause. If CDR enters the
OOL status, to prevent the receiver circuit from operating with an unstable clock, the clock that provided to
the receiver circuit (after DEMUX) be automatically changed to the TxPLL.
If the difference is within 500 ppm, the OOL status is cleared. When the OOL status is cleared, the
recovery clock is used as the receive clock again.
(6) CD (Carrier Detect)
Each port has Carrier Detect (CD) pin. When CD is low, µPD98413 assumes that the port is in loss of
signal status and the serial data on RDIT/RDIC will be internally forced to constant zero. And the clock that
provided to the receiver circuit be forcibly switched to the TxPLL. When a port is in loop-timing mode, it is
use the clock extracted by the CDR from the receive data as the transmit clock. In that case, if CDR enter
OOL status, the transmit clock is forcibly switched to the clock generated by TxPLL. The low level of CD
signal can be used as a condition for LOS detection.
CHAPTER 4 INTERFACES
PRELIMINARY
NEC confidential and Proprietary
113