ST72344S4 datasheet(43/247 Pages) STMICROELECTRONICS

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ST72344xx, ST2345xx

Supply, reset and clock management

Doc ID 12321 Rev 6

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7.3

Multioscillator (MO)

The main clock of the ST7 can be generated by three different source types coming from the

multioscillator block:

●

an external source

●

4 crystal or ceramic resonator oscillators

●

an internal high-accuracy RC oscillator

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

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

Table 8. Refer to Section 13: Electrical characteristics for more details.

Caution:

The OSC1 and/or OSC2 pins must not be left unconnected. For the purposes of Failure

Mode and Effect Analysis, it should be noted that if the OSC1 and/or OSC2 pins are left

unconnected, the ST7 main oscillator may start and, in this configuration, could generate an

fOSC clock frequency in excess of the allowed maximum (>16 MHz), putting the ST7 in an

unsafe/undefined state. The product behavior must therefore be considered undefined when

the OSC pins are left unconnected.

7.3.1

External clock source

In this 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.

7.3.2

Crystal/ceramic oscillators

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

clock of the ST7. The selection within a list of 4 oscillators with different frequency ranges

has to be done by option byte in order to reduce consumption (refer to Section 15.1: Option

bytes 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

pins in order to minimize output distortion and startup stabilization time. The loading

capacitance values must be adjusted according to the selected oscillator.

These oscillators are not stopped during the reset phase to avoid losing time in the oscillator

startup phase.

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