ST72F321 STMICROELECTRONICS [STMicroelectronics], ST72F321 Datasheet - Page 24

• Manufacturer Part Number: ST72F321
• Description: 64/44-PIN 8-BIT MCU WITH 32 TO 60K FLASH/ROM, ADC, FIVE TIMERS, SPI, SCI, I2C INTERFACE
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

ST72321
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.
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
frequency ranges). In this mode of the multi-oscil-
lator, 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
start-up stabilization time. The loading capaci-
tance 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
start-up phase.
24/189
section 14.1 on page 175
an external source
4 crystal or ceramic resonator oscillators
an internal high frequency RC oscillator
for more details on the
Table
OSC
clock frequency
4. Refer to the
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
L2