RM0029 Datasheet, PDF (609/1740 Pages) STMicroelectronics – The primary objective of this document

RM0029
Frequency-modulated phase locked loop (FMPLL)
The following equations define how to calculate MODPERIOD and INCSTEP based on the
frequency of the feedback divider (f_fbk), the modulation frequency (f_mod) and the modulation
depth percentage (MD):
Equation 7
MODPERIOD = round(-₄----_×--^f--^f-^b_f--_m-^k---_o---_d-)
Equation 8
INCSTEP = round(₁--⁽--₀-²--₀-¹---⁵-_×---–--₅---¹--_×-⁾----_M^×-----^MO-----^DD-----_P-^×---_E-^E---_R--^M---_I--^FO----^D-_D---)
MODPERIOD and INCSTEP are subject to the following restriction:
Equation 9
(MODPERIOD × INCSTEP) < 2¹⁵
Because of the above rounding operations, the effective modulation depth applied to the
FMPLL is given by the following formula:
Equation 10
INCSTEP = round(^M------^O-----^D-----^P----^E----^R---₍--^I2--^O--₁---₅^D----–--^×--₁---^I-₎-^N---_×-^C----_E-^S---_M-^T----_F-^E---_D-^P-----^×-----¹---⁰----⁰-----^×----⁵--)
As an example, suppose the following configuration:
● Input frequency: 4 MHz
● Load divider (EMFD): 64
● Input divider: 1
● VCO frequency: 4 MHz × 64 = 256 MHz
● PLL output frequency: 256 MHz / ERFD
● Center spread (MODSEL = 0)
● Modulation frequency: 24 kHz
● Modulation depth: +/− 2.0 % (4% peak-to-peak)
● MODPERIOD = Round [(4 × 10⁶)/(4 × 24 × 10³)] = Round [41.66] = 42
● INCSTEP = Round [((2¹⁵ − 1) × 2 × 64) / (100 × 5 × 42)] = Round [199.722] = 200
● MODPERIOD × INCSTEP = 42 × 200 = 8400 (which is less than 2¹⁵)
● MD (quantized) = ((42 × 200 × 100 × 5) / ((2¹⁵ − 1) × 64) = 2.00278 %
In this example, the modulation depth error is 0.00278%.
The FM parameters can only be changed, and FM can only be enabled, when the PLL is
locked. Writing to the FMPLL_SYNFMMR while the PLL is unlocked has no effect.
Doc ID 15177 Rev 8
609/1740