ST72F321BAR6TCRE datasheet(45/247 Pages) STMICROELECTRONICS

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ST72321Bxxx-Auto

Supply, reset and clock management

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6.3

Phase locked loop

If the clock frequency input to the PLL is in the range 2 to 4 MHz, the PLL can be used to

multiply the frequency by two to obtain an fOSC2 of 4 to 8 MHz. The PLL is enabled by option

byte. If the PLL is disabled, then fOSC2 =fOSC/2.

Caution:

The PLL is not recommended for applications where timing accuracy is required (see

Section 19.5.5: PLL characteristics on page 202).

Figure 10.

PLL block diagram

6.4

Multi-oscillator (MO)

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

multi-oscillator block:

●

an external source

●

4 crystal or ceramic resonator oscillators

●

an internal high frequency 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 9. Refer to Chapter 19: 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.

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.

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 four oscillators with different frequency ranges

has to be done by option byte in order to reduce consumption (refer to Section 21.1.1: Flash

configuration on page 228 for more details on the frequency ranges). In this mode of the

PLL OPTION BIT

PLL x 2

fOSC2

/ 2

fOSC

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