GS4911B Datasheet, PDF (73/113 Pages) Gennum Corporation – HD/SD/Graphics Clock and Timing Generator with GENLOCK

GS4911B/GS4910B Data Sheet
Example 2: Programming an output video clock of 74.175824MHz (74.25/1.001):
f_--o---u---t
=
7----4---.-2---5- _MHz
-1---.-0---0---1--------------
f_in
27MHz
∴-^N----^v- = ^{7---4---2---5-----×}-----1---0---0---0- = ^2---5----0-
D_v 2700 × 1001 91
Therefore, program N_v = 250 and D_v = 91.
NOTE: The Nv and Dv values programmed in registers 20h-21h and 22h-23h are
not held until the custom video clock update bit (6) of register 16h is toggled.
3.9.2 Programming a Custom Audio Clock (GS4911B only)
The GS4911B’s audio clocks are derived from the fundamental audio sampling
frequency initially set by ASR_SEL[2:0]. At any time this fundamental sampling
frequency may be modified to create a custom output audio clock.
The user may generate any audio sampling frequency between 6.6kHz and 96kHz,
and therefore create a custom audio clock as high as 512*96kHz. When generating
a custom audio sampling frequency, ASR_SEL[2:0] must be set to 100b and bit 5
of register 31h (enable_384fs) must be kept LOW.
The fundamental sampling frequency is determined using a ratio based on the
27MHz reference. Therefore, to program a custom audio clock, the user must
calculate and program the set of integers (N_a, D_a) in the equation:
-^N----^a- = 1024 × --^f-^s--
D_a
f_in
where:
f_s = desired fundamental audio sampling frequency
f_in = 27MHz crystal reference
N_a = numerator of the ratio (host register 33h-34h)
D_a = denominator of the ratio (host register 35h-36h)
Before programming N_a and D_a, the numerator and denominator must be reduced
to their lowest factors.
For example, to program a fundamental audio sampling frequency of 42kHz:
-^N----^a- = 1024 × -----4---2---0---0---0------ = ^{4---3---0---0----8-} = ^{1---7----9---2-}
D_a
27000000 27000 1125
Therefore, program N_a = 1792 and D_a = 1125 and toggle the custom audio clock
update bit (6) of register 31h. Using registers 3Fh to 41h, the custom audio
sampling frequency generated may then be multiplied by a factor of 64, 128, 256,
or 512 before being presented to the ACLK pins.
NOTE: The AFS reset described in Section 3.7.2 on page 63 will always remain
active.
36655 - 2 April 2006
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