STM32F446RC datasheet(111/201 Pages) STMICROELECTRONICS

111 / 201 page

DocID027107 Rev 5

111/201

STM32F446xC/E

Electrical characteristics

174

fPLL_IN and fMod must be expressed in Hz.

As an example:

If fPLL_IN = 1 MHz, and fMOD = 1 kHz, the modulation depth (MODEPER) is given by

equation 1:

MODEPER

round 10

410

()

⁄

[]

250

Equation 2

Equation 2 allows to calculate the increment step (INCSTEP):

INCSTEP

round

–

() md PLLN

()

100

MODEPER

()

⁄

[]

fVCO_OUT must be expressed in MHz.

With a modulation depth (md) =

±2 % (4 % peak to peak), and PLLN = 240 (in MHz):

INCSTEP

round

–

() 2240

()

100

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

()

–

() PLLN

()

⁄

As a result:

mdquantized%

250

126

100

()

–

() 240

()

⁄

2.002%(peak)

Figure 29 and Figure 30 show the main PLL output clock waveforms in center spread and

down spread modes, where:

F0 is fPLL_OUT nominal.

Tmode is the modulation period.

md is the modulation depth.

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