LMH1982SQ National Semiconductor Corporation, LMH1982SQ Datasheet - Page 14

• Manufacturer Part Number: LMH1982SQ
• Manufacturer: National Semiconductor Corporation

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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:
•
•
•
•
•
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
clock will be deterministic and is specified in the Electrical
Characteristics table. For example, Figure 3 and Figure 4
show the typical t
spectively. The width of the TOF pulse will be approximately
one line period of the programmed output format.
HD_CLK (MHz)
Output Clock Reference
Output Frame Rate
Reference Frame Timing
Input-Output Frame Rate Ratio
Output Frame Line Offset
74.25/1.001
148.5/1.001
74.25
148.5
TABLE 6. HD Clock Frequency Selection
FIGURE 3. 525i TOF Output Delay Time
OD
Relative to 27 MHz
for 525i and 1080i50 output timing, re-
Register 08h
HD_FREQ
00b
01b
10b
11b
OD
) relative to the selected
PLL#
30052431
2
3
2
3
14
5.2.2 Output Frame Rate
The output format frame rate (TOF pulse rate) can be calcu-
lated as:
Where:
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:
Where:
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.
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:
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-
TOF rate = f
f
TOF_PPL = Total pixels per output line
TOF_LPFM = Total lines per output frame
TOF rate = 27 MHz / (1728 x 625) = 25 Hz frame rate
f
TOF_PPL = 1728
TOF_LPFM = 625
Frame rate ratio = (30 / 1.001) / 25 = 1200 / 1001
TOF_CLK
TOF_CLK
FIGURE 4. 1080i50 TOF Output Delay Time
= Clock reference frequency
= 27 MHz (SD_FREQ = 0)
TOF_CLK
Relative to 74.25 MHz
/ (TOF_PPL x TOF_LPFM)
30052430