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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

3.2 EMISSION

3.2.1 Internal PLL

Some ST Microcontrollers have an embedded programmable PLL Clock Generator allowing

the usage of standard 3 to 5 MHz crystals to obtain a large range of internal frequencies (up

to 24MHz). By these means, ST Microcontroller can operate with cheaper, medium frequency

crystals, while still providing a high frequency internal clock for maximum system perform-

ance. The high clock frequency source is contained inside the chip and does not go through

the PCB (Printed Circuit Board) tracks and external components. This reduces the potential

noise emission of the application.

The use of PLL network also filters CPU clock against external sporadic disturbances

(glitches).

3.2.2 Global low power approach

3.2.2.1 Low powered oscillator

The oscillator is an major source of noise. To reduce this noise emission, the current driven by

the oscillator is limited.

The main clock of some of ST Microcontrollers can be generated by four different source types

coming from the multi-oscillator block (MO). This allows the designer to easily select the best

trade-off in terms of cost, performance and noise emission. The clock sources are listed below

in order from the most noisy to the least noisy:

s

an external source

s

crystal or ceramic resonator oscillators

s

an internal high frequency RC oscillator

Each oscillator is optimized for a given frequency range in terms of consumption and is se-

lectable through the option byte. The associated hardware configurations are shown in Table

11. Refer to the electrical characteristics section of the datasheet for more details in each

case.

External Clock Source

In external clock mode, a clock signal (square, sinus or triangle) with ~50% duty cycle has to

drive the OSC1 pin while the OSC2 pin is tied to ground.

Crystal/Ceramic Oscillators

This family of oscillators has the advantage of producing a very accurate rate on the main

clock of the microcontroller. The selection within a list of 5 oscillators with different frequency

ranges has to be done by option byte in order to reduce consumption (refer to the Microcon-

troller datasheet for more details on the frequency ranges). In this mode of the multi-oscillator,

the resonator and the load capacitors have to be placed as close as possible to the oscillator