GS4910B GENNUM [Gennum Corporation], GS4910B Datasheet - Page 72

• Manufacturer Part Number: GS4910B
• Description: HD/SD/Graphics Clock and Timing Generator with GENLOCK
• Manufacturer: GENNUM [Gennum Corporation]

3.9 Custom Clock Generation
3.9.1 Programming a Custom Video Clock
In addition to the device’s pre-programmed clock frequencies, the user may
generate a custom audio or video clock by programming designated registers in
the host interface.
Custom video clock generation is supported by both the GS4910B and GS4911B
and is described in
only supported by the GS4911B and is described in
The fundamental frequency of the video clock is defined by the output video format
initially set by VID_STD[5:0]. At any time, this fundamental frequency may be
modified to create a custom output video format.
The user may generate a video clock with any frequency between 13.5MHz and
165MHz. By programming the PCLK_Divide registers, the output PCLK may be as
low as 13.5/4 = 3.375MHz.
Generating a custom video clock will change the period of the video timing signals
presented to the TIMING_OUT pins; however, the pixels per line, lines per frame,
and other pixel and line-based timing signals will remain unchanged. To redefine
the pixel and line based timing parameters, registers 4Eh to 55h must be
reprogrammed as described in
The frequency of the custom video clock is determined using a ratio based on the
27MHz reference. Therefore, to program a custom clock, the user must calculate
and program the set of integers (N
where:
f
f
N
D
Before programming N
to their lowest factors. Two examples are given below:
Example 1: Programming an output video clock of 74.25MHz:
Therefore, program N
36655 - 2
∴
N
----- -
D
out
in
f
------- -
f
out
v
v
v
v
in
----- -
D
N
= 27MHz crystal reference
= numerator of the ratio (host register 20h-21h)
= denominator of the ratio (host register 22h-23h)
v
v
=
= desired output video clock frequency
=
=
f
------- -
f
out
in
74.25MHz
------------------------- -
7425
----------- -
2700
27MHz
April 2006
=
11
----- -
4
Section 3.9.1 on page
v
v
= 11 and D
and D
v
, the numerator and denominator must be reduced
Section 3.10 on page
v
v
, D
= 4.
v
) in the equation:
GS4911B/GS4910B Data Sheet
72. Custom audio clock generation is
Section 3.9.2 on page
74.
72 of 113
73.