ST72104G, ST72215G, ST72216G, ST72254G
7.3 MULTI-OSCILLATOR (MO)
The main clock of the ST7 can be generated by
four different source types coming from the multi-
oscillator block:
Table 3. ST7 Clock Sources
Hardware Configuration
■ an external source
■ 4 crystal or ceramic resonator oscillators
■ an external RC oscillator
ST7
OSC1
OSC2
■ 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
configuration are shown in Table 3. Refer to the
electrical characteristics section for more details.
EXTERNAL
SOURCE
External Clock Source
ST7
OSC1
OSC2
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. In this
mode of the multi-oscillator, the resonator and the
load capacitors have to be placed as close as pos-
sible to the oscillator pins in order to minimize out-
put distortion and start-up stabilization time. The
loading capacitance values must be adjusted ac-
cording to the selected oscillator.
C
C
L2
L1
LOAD
CAPACITORS
ST7
OSC1
OSC2
R
C
EX
EX
These oscillators are not stopped during the
RESET phase to avoid losing time in the oscillator
start-up phase.
External RC Oscillator
ST7
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.
OSC1
OSC2
Internal RC Oscillator
The internal RC oscillator mode is based on the
same principle as the external RC oscillator includ-
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.
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ETC [ ETC ]
