st72321j STMicroelectronics, st72321j Datasheet - Page 23

• Manufacturer Part Number: st72321j
• Description: 8-bit Mcu With Nested Interrupts, Flash, 10-bit Adc, 5 Timers, Spi, Sci, I2c Interface
• Manufacturer: STMicroelectronics

6.2 MULTI-OSCILLATOR (MO)
The main clock of the ST7 can be generated by
four 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.
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.
These oscillators are not stopped during the
RESET phase to avoid losing time in the oscillator
start-up phase.
External RC Oscillator
This oscillator allows a low cost solution for the
main clock of the ST7 using only an external resis-
tor and an external capacitor. The frequency of the
external RC oscillator (in the range of some MHz.)
is fixed by the resistor and the capacitor values.
Consequently in this MO mode, the accuracy of
the clock is directly linked to the accuracy of the
discrete components. The corresponding formula
is
Internal RC Oscillator
The internal RC oscillator mode is based on the
same principle as the external RC oscillator includ-
f
an external source
4 crystal or ceramic resonator oscillators
an external RC oscillator
an internal high frequency RC oscillator
Section 14.1 on page 169
OSC
=5/(R
EX
C
EX
)
.
Table
for more details on
5. Refer to the
ing the resistance and the capacitance of the de-
vice. This mode is the most cost effective one with
the drawback of a lower frequency accuracy. Its
frequency is in the range of several MHz.
In this mode, the two oscillator pins have to be tied
to ground.
Table 5. ST7 Clock Sources
R
C
L1
EX
Hardware Configuration
EXTERNAL
SOURCE
OSC1
OSC1
OSC1
OSC1
CAPACITORS
ST7
LOAD
ST7
ST7
ST7
OSC2
OSC2
OSC2
OSC2
C
ST72321J
C
EX
L2
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