### NSC LMH1983

```National Semiconductor
Application Note 2108
Mark Sauerwald
December 15, 2010
Introduction
The LMH1983 is a highly integrated programmable audio / video (A/V) clock generator intended for broadcast and professional
applications. The programmable features allow for the generation of many different clocks and control signals. This application
note describes how to generate a 44.1 kHz audio clock using PLL4.
To generate a 44.1 kHz based clock with the LMH1983, make the following writes to the PLL4 Advance Control registers:
0x35 (Charge Pump Current): 0F'h
0x36 (R Counter): 7D'h
0x37 ( N Counter MSB): 03'h
0x38 ( N Counter LSB): 10'h
0x39 ( VCO Frequency): 66'h
To calculate these numbers you need to match the frequencies coming in to the phase comparator — the first of these frequencies is 27 MHz / R. The second frequency is 44.1 kHz x 512 x 4 x 5 x 3 / (N*8) (assuming that the IS125 bit and the
PLL_DIV registers have not been changed from their default vaules.)
Setting these two equal to each other gives:
27 MHz * 8 * N = 44.1 kHz x 512 x 4 x 3 x R
Break 27 MHz and 44.1 kHz down into their prime factors:
27 MHz = 26 * 33 * 56
44.1 kHz = 22 * 32 * 52 * 72
therefore:
29 * 33 * 56 * N = 213 * 33 * 53 * 72 x R
simplifying:
53 * N = 24 * 72 * R
Therefore the following values were selected:
Generating 44.1 kHz Based Clocks with the LMH1983
Generating 44.1 kHz Based
Clocks with the LMH1983
53 = 125 = 7D'h for R
24 * 72 = 784 = 0310'h for N
Since the frequency coming into the charge pump is about one half the default value, increase the charge pump current to
about twice the default value to compensate, then adjust the VCO center frequency to main stable lock.
AN-2108
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Generating 44.1 kHz Based Clocks with the LMH1983
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AN-2108
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