Data Sheet ADN2804
Rev. D | Page 17 of 24
LOCK DETECTOR OPERATION
The lock detector on the ADN2804 has three modes of
operation: normal mode, REFCLK mode, and static LOL mode.
Normal Mode
In normal mode, the ADN2804 is a CDR that locks onto a
622 Mbps data rate without the use of a reference clock as an
acquisition aid. In this mode, the lock detector monitors the
frequency difference between the VCO and the input data
frequency and deasserts the loss of lock signal, which appears
on Pin 16, LOL, when the VCO is within 250 ppm of the data
frequency. This enables the D/PLL, which pulls the VCO
frequency in the remaining amount and acquires phase lock.
Once locked, if the input frequency error exceeds 1000 ppm
(0.1%), the loss-of-lock signal is reasserted and control returns
to the frequency loop, which begins a new frequency
acquisition. The LOL pin remains asserted until the VCO locks
onto a valid input data stream to within 250 ppm frequency
error. This hysteresis is shown in Figure 20.
LOL
0–250 250 1000 f
VCO
ERROR
(ppm)
–1000
1
5801-020
Figure 20. Transfer Function of LOL
LOL Detector Operation Using a Reference Clock
In REFCLK mode, a reference clock is used as an acquisition aid
to lock the ADN2804 VCO. Lock-to-reference mode is enabled
by setting CTRLA[0] to 1. The user also needs to write to the
CTRLA[7, 6] and CTRLA[5:2] bits to set the reference
frequency range and the divide ratio of the data rate with
respect to the reference frequency. For more details, see the
Reference Clock (Optional) section. In this mode, the lock
detector monitors the difference in frequency between the
divided down VCO and the divided down reference clock. The
loss-of-lock signal, which appears on Pin 16, LOL, is deasserted
when the VCO is within 250 ppm of the desired frequency. This
enables the D/PLL, which pulls the VCO frequency in the
remaining amount with respect to the input data and acquires
phase lock. Once locked, if the input frequency error exceeds
1000 ppm (0.1%), the loss-of-lock signal is reasserted and
control returns to the frequency loop, which reacquires with
respect to the reference clock. The LOL pin remains asserted
until the VCO frequency is within 250 ppm of the desired
frequency. This hysteresis is shown in Figure 20.
Static LOL Mode
The ADN2804 implements a static LOL feature that indicates if
a loss-of-lock condition has ever occurred. This feature remains
asserted, even if the ADN2804 regains lock, until the static LOL
bit is manually reset. The I
2
C register bit, MISC[4], is the static
LOL bit. If there is ever an occurrence of a loss-of-lock condition,
this bit is internally asserted to logic high. The MISC[4] bit remains
high even after the ADN2804 has reacquired lock to a new data
rate. This bit can be reset by writing a 1 followed by 0 to I
2
C
Register Bit CTRLB[6]. Once reset, the MISC[4] bit remains
deasserted until another loss-of-lock condition occurs.
Writing a 1 to I
2
C Register Bit CTRLB[7] causes the LOL pin,
Pin 16, to become a static LOL indicator. In this mode, the LOL
pin mirrors the contents of the MISC[4] bit and has the
functionality described in the previous paragraph. The CTRLB[7]
bit defaults to 0. In this mode, the LOL pin operates in the
normal operating mode, that is, it is asserted only when the
ADN2804 is in acquisition mode and deasserts when the
ADN2804 has reacquired lock.
SQUELCH MODES
Two modes for the SQUELCH pin are available with the
ADN2804: squelch data outputs and clock outputs mode and
squelch data outputs or clock outputs mode. Squelch data outputs
and clock outputs mode is selected when CTRLC[1] is 0 (default
mode). In this mode, when the SQUELCH input, Pin 27, is driven
to a TTL high state, both the data outputs (DATAOUTN and
DATAOUTP) and the clock outputs (CLKOUTN and CLKOUTP)
are set to the zero state to suppress downstream processing. If
the squelch function is not required, Pin 27 should be tied to VEE.
Squelch data outputs or clock outputs mode is selected when
CTRLC[1] is 1. In this mode, when the SQUELCH input is
driven to a high state, the DATAOUTN and DATAOUTP pins
are squelched. When the SQUELCH input is driven to a low
state, the CLKOUTN and CLKOUTP pins are squelched. This is
especially useful in repeater applications, where the recovered
clock may not be needed.
I
2
C INTERFACE
The ADN2804 supports a 2-wire, I
2
C-compatible serial bus
driving multiple peripherals. Two inputs, serial data (SDA) and
serial clock (SCK), carry information to and from any device
connected to the bus. Each slave device is recognized by a
unique address. The ADN2804 has two possible 7-bit slave
addresses for both read and write operations. The MSB of the
7-bit slave address is factory programmed to 1. B5 of the slave
address is set by Pin 19, SADDR5. Slave Address Bits [4:0] are
defaulted to all 0s. The slave address consists of the seven MSBs
of an 8-bit word. The LSB of the word either sets a read or write
operation (see Figure 7). Logic 1 corresponds to a read operation,
while Logic 0 corresponds to a write operation.
