THS8083APZP datasheet(57/64 Pages) TI

57 / 64 page

A–1

Appendix A

PLL Formulas and Register Settings

If:

F(XTL) = frequency of external crystal or master clock connected to XTL1 input of THS8083A

F(VCO) = frequency of THS8083A–internal VCO

F(DTO) = frequency of THS8083A–internal DTO

F(DTOCLK) = frequency of externally available DTO clock output

F(HS) = frequency of HS input

CLKDIV = clock output divider setting

VCODIV = feedback divider in THS8083A–internal analog PLL loop

TERMCNT = feedback divider in THS8083A–internal digital PLL loop

DTO_INC = DTO increment (when NOM_INC is programmed, DTO_INC is initialized to NOM_INC)

Then:

F(VCO) = F(XTL) x VCODIV

F(DTO) = 32 x F(VCO) / DTO_INC

F(DTOCLK) = F(DTO) / CLKDIV

And, if PLL is locked:

F(DTOCLK) = TERMCNT * F(HS)

Summarizing:

DTO_INC = [32xF(XTL)xVCODIV] / [F(DTOCLK)xCLKDIV]

The formats of DTO_INC and NOM_INC:

Both are 33-bit values consisting of a 6-bit integer and a 27-bit fractional part. So, in hexadecimal notation,

the value is between 00.0000000hex and 3F.7FFFFFFhex. The decimal value of the increment is: <integer

part>.<fractional part interpreted as integer value>x2^(–27). This means, to arrive at the decimal value of

the increment:

Interpret the 6 MSBs as an integer value

Interpret the 27 LSBs expressed as a decimal integer value and multiply this by 2^(–27) to arrive at a

fractional value

Add 1 and 2.

Additional restrictions:

– NOM_INC should be within the range [16,32].

– CLKDIV should be chosen for different output clock frequency ranges as shown in register map

description for SEL_CLK.

Examples:

For generating the XGA@75Hz pixel clock of 78.75 MHz, with F(XTL) = 14.381818 MHz and

VCODIV=8:

NOM_INC = [32x14.31818x8]/[78.75x2] = 23.272724...

Since 23d = 17h and INT(0.272724

… x 2^27) = 36604438d = 22E8A16 hex, this can be written as

0x17.22E8A16 or as a contiguous 33 bit number: 010111010001011101000101000010110 = 0BA2E8A16

hex, i.e., the default setting for NOM_INC (see register map).

THS8083APZP Datasheet(PDF) 57 Page - Texas Instruments

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