ST72521 STMICROELECTRONICS [STMicroelectronics], ST72521 Datasheet - Page 25

• Manufacturer Part Number: ST72521
• Description: 8-BIT MCU WITH NESTED INTERRUPTS, FLASH, 10-BIT ADC, FIVE TIMERS, SPI, SCI, I2C, CAN INTERFACE
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

6.2 MULTI-OSCILLATOR (MO)
The main clock of the ST7 can be generated by
three different source types coming from the multi-
oscillator block:
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
electrical characteristics section 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 con-
figuration, could generate an f
in excess of the allowed maximum (>16MHz.),
putting the ST7 in an unsafe/undefined state. The
product behaviour must therefore be considered
undefined when the OSC pins are left unconnect-
ed.
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.
Note: External clock source is not supported with
the PLL enabled.
Crystal/Ceramic Oscillators
This family of oscillators has the advantage of pro-
ducing 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
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 oscil-
lator pins in order to minimize output distortion and
start-up stabilization time. The loading capaci-
tance values must be adjusted according to the
selected oscillator.
an external source
4 crystal or ceramic resonator oscillators
an internal high frequency RC oscillator
Section 14.1 on page 199
Table
OSC
for more details on
clock frequency
4. Refer to the
These oscillators are not stopped during the
RESET phase to avoid losing time in the oscillator
start-up phase.
Internal RC Oscillator
This oscillator allows a low cost solution for the
main clock of the ST7 using only an internal resis-
tor and capacitor. Internal RC oscillator mode has
the drawback of a lower frequency accuracy and
should not be used in applications that require ac-
curate timing.
In this mode, the two oscillator pins have to be tied
to ground.
Table 4. ST7 Clock Sources
C
L1
EXTERNAL
Hardware Configuration
SOURCE
OSC1
OSC1
OSC1
CAPACITORS
LOAD
ST7
ST7
ST7
OSC2
OSC2
OSC2
C
ST72521
L2
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