Datasheet

LMH1983
www.ti.com SNLS309G –APRIL 2010–REVISED JANUARY 2011
The alignment between the incoming F
IN
and the TOF1 output may be controlled in a number of ways. There are
three different alignment modes in which TOF1 may operate, selected via register 0x11h. The default, powerup
mode is for there to be no alignment, the other two options are to always force alignment to F
IN
, and to force
alignment to F
IN
when they are misaligned. Misalignment can be defined by the user via register 0x15h where a
window is defined which specifies the amount of mismatch that is permitted between F
IN
and TOF1 while still
considering them to be aligned. If the input reference signal has a significant amount of low frequency jitter or
wander, it may be possible for the relative alignment between TOF1 and F
IN
to move around since the TOF1
output will have its jitter and wander attenuated by the PLL1 loop filter. If the align always mode is selected, this
may lead to timing jumps on the output of CLKout1/TOF1 which would be undesirable.
Once the device decides that it needs to align TOF1 and F
IN
, there are two ways that it can be done. Crash lock
involves simply resetting the counter which keeps track of where the TOF1 output transition happens, and results
in an instantaneous shift of TOF1 to align with F
IN
. Drift lock involves using the second loop in PLL1, and
skewing the VCXO to make the frequency of CLKout1 either speed up or slow down, slowly pulling TOF1 and F
IN
into alignment. If a new reference is applied, which is not in alignment with TOF1, but the output is currently in
use, it may be better to slew TOF1 into alignment rather than to cause a major disruption in the timing with a
crash lock. The LMH1983 allows the user to select either crash lock, or drift lock under each of two different
conditions, controllable via register 0x11h. If the difference in phase between TOF1 and F
in
is small, and if the
difference between the two is large. Furthermore, if the difference is large, then the user has the opportunity to
tell the device to achieve alignment either via advancing or retarding the phase of PLL1 in order to achieve
alignment. Note that if the difference in alignment is large, to achieve alignment via drift lock may take a very
long time (10s of seconds), during which time the output clock will not be phase locked to the input H
IN
.
TOF2 and TOF3 TIMING
Like with TOF1, each of the second and third channels has a video format associated with it. The format is
determined by programming this into registers 0x07h and 0x08h respectively. Once the format is programmed,
and the TOF outputs are enabled, a TOF pulse is generated at the appropriate rate for each of the outputs.
There are four different alignment modes which may be selected for TOF2 and TOF3:
TOF2/TOF3 Alignment Modes
TOF2/TOF3 Alignment Mode Description
0 Auto Align when misaligned
1 One shot manual align
2 always align
3 never align
TOF2 and TOF3 are generally aligned with TOF1. The alignment status bit will only be set if the frame rates are
the same as one another. Another option for alignment is via software, where a but is written to the TOFX_INIT
bit. For example, the LSB of register 0x12h is the TOF2_INIT bit. Writing a one to this bit, while also setting
TOF2 alignment mode to anything other than 3, will cause TOF2 immediately reset its phase. This bit is a self
clearing bit, so reading it will always return a zero.
TOF4 ALIGNMENT MODES
The fourth channel of the LMH1983 is most often used to generate an audio clock. The default base audio clock
rate is 48 kHz, and this sample clock is synchronous in phase with the video frame only once every 5 frames for
29.97 and 30 Hz frame rate standards, or once every 10 frames for 60 Hz and 59.94 Hz systems. The LMH1983
can generate a TOF4 pulse which will happen at this rate, allowing audio frames to be synchronized with the
video frames.
TOF4 may be aligned either to TOF1, or to the F
IN
input. Additionally, there is an external INIT input which can
be used to set the TOF4 alignment.
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