THS8083A95 Datasheet, PDF (57/66 Pages) Texas Instruments – TRIPLE 8-BIT, 95MSPS, 3.3V VIDEO AND GRAPHICS

Appendix A
PLL Formulas and Register Settings
If:
F(XTL) = frequency of external crystal or master clock connected to XTL1 input of THS8083A95
F(VCO) = frequency of THS8083A95−internal VCO
F(DTO) = frequency of THS8083A95−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 THS8083A95−internal analog PLL loop
TERMCNT = feedback divider in THS8083A95−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:
1. Interpret the 6 MSBs as an integer value
2. Interpret the 27 LSBs expressed as a decimal integer value and multiply this by 2^(−27) to arrive at a
fractional value
3. Add 1 and 2.
Additional restrictions:
− NOM_INC must be within the range [16,32]. During an unlocked condition of the PLL (e.g., during video
format changes), the user needs to take care that the instantaneous DTO increment does not go outside
this valid range. This can be achieved by monitoring the SYNC_DETECT register bit (for no video input)
and the LOCK pin (for unlock) and disabling the PFD (DISABLE_PFD register bit) when no video input or
unlock is detected. This will keep the DTO operting at the nominal frequency. This update must be done
within a few PFD updates (i.e., within a few video line periods) to avoid the DTO from drifting outside its
valid range. When drifting outside this range, the DTO output might become unstable or absent. Since the
I²C interface requires a valid DTO clock, I²C communication errors will occur (no I²C ACK) when a stable
DTO clock is lost. When this happens, a hardware reset (/RESET pin) will be needed to recover.
− CLKDIV must be chosen for different output clock frequency ranges as shown in register map description
for SEL_CLK.
A−1