The 8T49N28X UFT family of devices contain a Write DCO mode that allows the user to offset the VCO frequency from its normal operating
frequency using digital register settings. The following procedure shows how to program the WDCO mode.
Calculate the WDCO Value
The WDCO granularity for the IDT8T49N281/IDT8T49N282/IDT8T49N2813 devices is calculated as follows:
1) Calculate the WDCO least-significant-bit (lsb) value:
WDCOlsb ppm offset = 1/( fvco/fxtal *2^21)*1e6
fvco= VCO frequency
fxtal= Crystal frequency taking into account the doubler
Example: VCO= 3000MHz, XTAL=40MHz, Crystal doubler enabled
WDCOlsb(ppm) = 1/ (3000/(40*2)*2^21)*1e6= 0.0127ppm
Note: A higher VCO frequency will yield more granularity but a smaller WDCO range.
2) Divide the target ppm by the WDCO lsb value and then round to the nearest integer in order to obtain the WDCO setting:
WDOC setting= round (ppm target / WDCOlsb)
Example: 25ppm target offset
25ppm /.0127ppm = 1966.08 → 1966