STM32F405xx, STM32F407xx
Electrical characteristics
Equation 2
Equation 2 allows to calculate the increment step (INCSTEP):
INCSTEP = round[((215 – 1) × md × PLLN) ⁄ (100 × 5 × MODEPER)]
f
must be expressed in MHz.
VCO_OUT
With a modulation depth (md) = ±2 % (4 % peak to peak), and PLLN = 240 (in MHz):
INCSTEP = round[((215 – 1) × 2 × 240) ⁄ (100 × 5 × 250)] = 126md(quantitazed)%
An amplitude quantization error may be generated because the linear modulation profile is
obtained by taking the quantized values (rounded to the nearest integer) of MODPER and
INCSTEP. As a result, the achieved modulation depth is quantized. The percentage
quantized modulation depth is given by the following formula:
mdquantized% = (MODEPER × INCSTEP × 100 × 5) ⁄ ((215 – 1) × PLLN)
As a result:
mdquantized% = (250 × 126 × 100 × 5) ⁄ ((215 – 1) × 240) = 2.002%(peak)
Figure 35 and Figure 36 show the main PLL output clock waveforms in center spread and
down spread modes, where:
F0 is f
nominal.
PLL_OUT
T
is the modulation period.
mode
md is the modulation depth.
Figure 35. PLL output clock waveforms in center spread mode
Frequency (PLL_OUT)
md
F0
md
Time
2 x tmode
tmode
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