GS4911B_09 Datasheet, PDF (57/119 Pages) Gennum Corporation – HD/SD/Graphics Clock and Timing Generator with GENLOCK

accomplished by programming the set of integers (A_Feedback_Divide,
A_Reference_Divide) in the equation:
-^A-----^_---^F---^e---^e---^d--^b----^a---^c--^k---^_---^D----ⁱ--^v---ⁱ-^d---^e--
A_Reference_Divide
=
n
×
----^f--^s----
f_out
where:
f_s = the fundamental audio sampling frequency
f_out = output video clock frequency
A_Feedback_Divide = numerator of the divide ratio (host register 3Bh-3Ch)
A_Reference_Divide = denominator of the divide ratio (host register 3Dh-3Eh)
n = an integer constant
The integer constant, n, will depend on the fundamental audio sampling frequency. It
will be one of the three values as defined in Table 3-5.
Table 3-5: Integer Constant Value
ASR_SEL[2:0]=100b
Enable_384fs = 0
Value of constant (n)
NO
X
3072
YES
YES
1024
YES
NO
1536
NOTES:
1. Enable_384fs corresponds to bit 5 of address 31h of the host interface. It is LOW by default.
2. ‘X’ signifies ‘don’t care.’ This bit will be ignored when determining n.
Before programming A_Feedback_Divide and A_Reference_Divide, the numerator and
denominator must be reduced to their lowest factors.
For example, to manually genlock a custom audio clock with a fundamental sampling
frequency of 42kHz to a 27MHz video clock, the following calculations are necessary:
n = 1024
∴_A-^A----_{__}---_RF----_ee---_fe-_e--^d-_r--^be---^an---^cc--^k-_e--^__---^DD----ⁱ-_i-^vv---ⁱⁱ-^d-_d--^e-_e- =
1024
×
-----4---2---0---0---0------
27000000
=
42---37---00---00---80-- ₌
1---7---9---2--
1125
Therefore, program A_Feedback_Divide = 1792 and A_Reference_Divide = 1125. Note
that n=1024 when programming a custom audio clock (see Section 3.9.2 on page 73).
GS4911B/GS4910B HD/SD/Graphics Clock and Timing
Generator with GENLOCK
Data Sheet
36655 - 5
June 2009
57 of 119