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LMH1982SQ Datasheet

  • LMH1982SQ

  • National Semiconductor [Multi-Rate Video Clock Generator wi...

  • 504.42KB

  • NSC

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LMH1982
TABLE 6. HD Clock Frequency Selection
HD_CLK (MHz)
74.25
74.25/1.001
148.5
148.5/1.001
HD_FREQ
Register 08h
00b
01b
10b
11b
PLL#
2
3
2
3
5.2 Programming The Output Timing Format
When the VCXO PLL is stable and locked to the input refer-
ence, the output clock and TOF pulse can be programmed to
be aligned to the desired reference frame. Before implement-
ing the output alignment sequence, the desired output timing
format (i.e. output clock and TOF pulse) needs to be specified
first. The reference and output timing formats must be fully
and correctly specified by programming the output format
registers 09h-12h. The output format registers define the fol-
lowing parameters:
鈥?Output Clock Reference
鈥?Output Frame Rate
鈥?Reference Frame Timing
鈥?Input-Output Frame Rate Ratio
鈥?Output Frame Line Offset
Once the output format registers are programmed according
to sections 5.2.1 to 5.2.5, the output alignment procedure can
then be implemented as described in section 5.3.
5.2.1 Output Clock Reference
The TOF pulse is derived from internal pixel, line, and frame
counters and can be made synchronous to either the SD_CLK
or HD_CLK, depending on the desired output format. The
output clock reference for timing the TOF pulse should be se-
lected by setting TOF_CLK (register 0Ch).
The TOF pulse output delay time (t
OD
) relative to the selected
clock will be deterministic and is specified in the Electrical
Characteristics table. For example,
Figure 3
and
Figure 4
show the typical t
OD
for 525i and 1080i50 output timing, re-
spectively. The width of the TOF pulse will be approximately
one line period of the programmed output format.
30052430
FIGURE 4. 1080i50 TOF Output Delay Time
Relative to 74.25 MHz
5.2.2 Output Frame Rate
The output format frame rate (TOF pulse rate) can be calcu-
lated as:
TOF rate = f
TOF_CLK
/ (TOF_PPL x TOF_LPFM)
Where:
f
TOF_CLK
= Clock reference frequency
TOF_PPL = Total pixels per output line
TOF_LPFM = Total lines per output frame
The output frame rate should be specified by programming
TOF_CLK, TOF_PPL (register 0Bh-0Ch) and TOF_LPFM
(0Dh-0Eh) based on the desired output format.
For example, if the output format is 625i, then:
TOF rate = 27 MHz / (1728 x 625) = 25 Hz frame rate
Where:
f
TOF_CLK
= 27 MHz (SD_FREQ = 0)
TOF_PPL = 1728
TOF_LPFM = 625
5.2.3 Reference Frame Rate
The reference frame timing is generated internally and used
for resetting the internal counters for output TOF generation.
The reference frame rate should be specified by programming
the total reference lines per frame to REF_LPFM (register
0Fh-10h).
5.2.4 Input-Output Frame Rate Ratio
The input-output frame rate ratio is also used for resetting the
internal counters for output TOF generation. The ratio is the
Input frame rate / Output frame rate, in which the numerator
and denominator values are reduced to lowest integer factors.
The numerator value of this reduced ratio should be pro-
grammed to TOF_RST (register 09h-0Ah), and the denomi-
nator value is discarded.
30052431
FIGURE 3. 525i TOF Output Delay Time
Relative to 27 MHz
For example, if the input reference is 525i with a frame rate
of 30/1.001 Hz, and the output format is 625i with a frame rate
of 25 Hz, then TOF_RST = 1200:
Frame rate ratio = (30 / 1.001) / 25 =
1200
/ 1001
5.2.5 Output Frame Line Offset
The output clock and TOF pulse can be aligned to any line of
the reference frame by specifying TOF_OFFSET (register
11h-12h) and subsequently programming the output align-
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