DS20006554A datasheet(16/94 Pages) MICROCHIP

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ZL30260-ZL30263

Data Sheet

DS20006554A

to correspond with FDIV having 24 fractional bits. Finally, in equation (6) the fdiv bits are shifted into the proper

position for the FDIV registers.

When a fractional scaling scenario involves multiplying an integer M times multiple scaling ratios N1 / D1 through

Nn / Dn, the equations above can still be used if the numerators are multiplied together to get N = N1 x N2

x … x Nn

and the denominators are multiplied together to get D = D1 x D2

x … x Dn.

The easiest way to calculate the exact values to write to the

APLL’s fractional output divider registers is to use the

ZL3026x evaluation software, available on the Microchip website. This software can be used even when no

evaluation board is attached to the computer.

5.5.6

Numerically Controlled Oscillator (NCO) Mode

5.5.6.1

Using the APLL’s Feedback Divider

System software can steer output frequencies with high resolution by manipulating the

APLL’s AFBDIV value. The

resolution can be better than 0.01ppb.

The nominal AFBDIV value, hereafter referred to as AFBDIV0, is the 0ppm nominal value for the desired device

configuration.

(Fractional frequency offset (FFO) is defined as (actual_frequency

– nominal_frequency) / nominal_frequency. FFO

is a unitless number but is typically expressed in parts per billion (ppb), parts per million (ppm) or percent.)

To control the NCO, system software first reads the AFBDIV0 value from the device. Even though the AFBDIV

follow, AFBDIV values should be treated as 42-bit unsigned integers.

To change the NCO frequency to a specific FFO (in ppm), system software calculates newAFBDIV (a 42-bit unsigned

integer) as follows:

newAFBDIV = round(AFBDIV0 * (1 + FFO/1e6))

System software then writes the newAFBDIV value directly to the AFBDIV registers.

Note that any subsequent frequency changes are calculated using the same equation from the original AFBDIV0

value and are not a function of the previous newAFBDIV value. The value of newAFBDIV should be kept within

±1000ppm of AFBDIV0 and within ±500ppm of the previous newAFBDIV value to avoid causing the APLL to lose

lock. If spread spectrum modulation is also in use, the total frequency change caused by spread spectrum modulation

and NCO control should be kept within ±5000ppm of AFBDIV0 to avoid causing the APLL to lose lock.

During NCO operation

using the APLL’s feedback divider, AFBREM should be set to 0, AFBDEN should be set to 1

and AFBBP should be set to 0.

5.5.6.2 Using the Fractional Output Divider

System software can steer output frequencies derived from the fractional output divider with high resolution by

manipulating the divider

’s FDIV value. The resolution can be better than 0.01ppb. In the case of fractional output

divider value >14.55192, the resolution is better than 1ppt.

The nominal FDIV value, hereafter referred to as FDIV0, is the 0ppm nominal value for the desired device

configuration.

(Fractional frequency offset (FFO) is defined as (actual_frequency

– nominal_frequency) / nominal_frequency. FFO

is a unitless number but is typically expressed in parts per billion (ppb), parts per million (ppm) or percent.)

To control the NCO, system software first reads the FDIV0 value from the device. Even though the FDIV register

description describes FDIV as having 4 integer bits and 36 fractional bits, for the NCO calculations that follow, FDIV

values should be treated as 40-bit unsigned integers.

To change the NCO frequency to a specific FFO (in ppm), system software calculates newFDIV (a 40-bit unsigned

integer) as follows:

newFDIV = round(FDIV0 / (1 + FFO/1e6))

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