EL4585C
Horizontal Genlock, 8 F
SC
be regular during this time. Once the near lock
condition is attained, charge pump output should
be very close to its lock-on value, and placing the
device into normal mode should result in a nor-
mal lock very quickly. Fast lock mode is intended
to be used where H-sync becomes irregular, until
a stable signal is again obtained.
Description Of Operation Ð Contd.
External Divide
DIV SEL (pin 8) controls the use of the internal
divider. When high, the internal divider is en-
abled and EXT DIV (pin 13) outputs the CLK
out divided by 2N. This is the signal to which the
horizontal sync input will lock. When divide se-
lect is low, the internal divider output is disabled,
and external divide becomes an input from an ex-
ternal divider, so that a divisor other than one of
the 8 pre-programmed internal divisors can be
used.
Coast Mode
Coast mode is enabled by pulling COAST (pin 9)
high (above )/3*Vcc). In coast mode the internal
phase detector is disabled and filter out remains
in high impedance mode to keep filter out volt-
age and VCO frequency as constant a possible.
VCO frequency will drift as charge leaks from the
filter capacitor, and the voltage changes the VCO
operating point. Coast mode is intended to be
used when noise or signal degradation result in
loss of horizontal sync for many cycles. The
phase detector will not attempt to adjust to the
resultant loss of signal so that when horizontal
sync returns, sync lock can be re-established
quickly. However, if much VCO drift has oc-
curred, it may take as long to re-lock as when
restarting.
Normal Mode
Normal mode is enabled by pulling COAST (pin
d
9) low (below (/3*Vcc). If H-SYNC and CLK 2N
have any phase or frequency difference, an error
signal is generated and sent to the charge pump.
The charge pump will either force current into or
out of the filter capacitor in an attempt to modu-
late the VCO frequency. Modulation will contin-
d
ue until the phase and frequency of CLK 2N
exactly match the H-sync input. When the phase
and frequency match (with some offset in phase
that is a function of the VCO characteristics), the
error signal goes to zero, lock detect no longer
pulses high, and the charge pump enters a high
impedance state. The clock is now locked to the
H-sync input. As long as phase and frequency
differences remain small, the PLL can adjust the
VCO to remain locked and lock detect remains
low.
Lock Detect
Lock detect (pin 12) will go low when lock is es-
tablished. Any DC current path from charge
pump out will skew EXT DIV relative to H-
SYNC in, tending to offset or add to the 110nS
internal delay, depending on which way the extra
current is flowing. This offset is called static
phase error, and is always present in any PLL
system. If, when the part stabilizes in a locked
mode, lock detect is not low, adding or subtract-
Fast Lock Mode
Fast Lock mode is enabled by either allowing
coast to float, or pulling it to mid supply (be-
tween (/3 and )/3*Vcc). In this mode, lock is
achieved much faster than in normal mode, but
the clock divisor is modified on the fly to achieve
this. If the phase detector detects an error of
enough magnitude, the clock is either inhibited
or reset to attempt a ‘‘fast lock’’ of the signals.
Forcing the clock to be synchronized to the
H-sync input this way allows a lock in approxi-
mately 2 H-cycles, but the clock spacing will not
ing from the loop filter series resistor R will
2
change this static phase error to allow LDET to
go low while in lock. The goal is to put the rising
edge of EXT DIV in sync with the falling edge of
a
H-SYNC
110nS. (See timing diagrams.) In-
creasing R decreases phase error, while decreas-
2
ing R increases phase error. (Phase error is posi-
2
tive when EXT DIV lags H-SYNC.) The resist-
ance needed will depend on VCO design or VCXO
module selection.
7
ELANTEC [ ELANTEC SEMICONDUCTOR ]
