STM32F205RC datasheet(89/177 Pages) STMICROELECTRONICS

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STM32F20xxx

Electrical characteristics

Doc ID 15818 Rev 9

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5.3.11

PLL spread spectrum clock generation (SSCG) characteristics

The spread spectrum clock generation (SSCG) feature allows to reduce electromagnetic

interferences (see Table 40: EMI characteristics). It is available only on the main PLL.

Equation 1

The frequency modulation period (MODEPER) is given by the equation below:

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

Equation 2

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

fVCO_OUT must be expressed in MHz.

With a modulation depth (md) =

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

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:

As a result:

Table 34.

SSCG parameters constraint

Symbol

Parameter

Min

Typ

Max(1)

Unit

fMod

Modulation frequency

KHz

Peak modulation depth

0.25

MODEPER * INCSTEP

215

−1-

Guaranteed by design, not tested in production.

MODEPER

round f

PLL_IN

Mod

()

⁄

[]

MODEPER

round 10

410

()

⁄

[]

250

INCSTEP

round

–

() md PLLN

() 100 5

MODEPER

()

⁄

[]

INCSTEP

round

–

() 2 240

() 100 5

250

()

⁄

[]

126md(quantitazed)%

quantized%

MODEPER

INCSTEP

100

()

–

() PLLN

()

⁄

quantized%

250

126

100

()

–

() 240

()

⁄

2.0002%(peak)

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