
Electrical characteristics ST72321xx-Auto
196/243 Doc ID 13829 Rev 1
19.5.3 Crystal and ceramic resonator oscillators
The ST7 internal clock can be supplied with four different crystal/ceramic resonator
oscillators. All the information given in this paragraph is based on characterization results
with specified typical external components. In the application, 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. Refer to the crystal/ceramic resonator
manufacturer for more details (such as frequency, package or accuracy).
Figure 79. Typical application with a crystal or ceramic resonator)
Table 116. Crystal and ceramic resonator oscillators
Symbol Parameter Conditions Min Typ Max Unit
f
OSC
Oscillator frequency
(1)
1. The oscillator selection can be optimized in terms of supply current using a high-quality resonator with
small R
S
value. Refer to crystal/ceramic resonator manufacturer for more details.
LP: Low power oscillator
MP: Medium power oscillator
MS: Medium speed oscillator
HS: High speed oscillator
1
>2
>4
>8
-
2
4
8
16
MHz
R
F
Feedback resistor
(2)
2. Data based on characterization results, not tested in production. The relatively low value of the RF resistor
offers a good protection against issues resulting from use in a humid environment, due to the induced
leakage and the bias condition change. However, it is recommended to take this point into account if the
microcontroller is used in tough humidity conditions.
-20-40k
C
L1
C
L2
Recommended load
capacitance versus
equivalent serial resistance
of the crystal or ceramic
resonator (R
S
)
(3)
3. For C
L1
and C
L2
it is recommended to use high-quality ceramic capacitors in the 5pF to 25pF range (typ.)
designed for high-frequency applications and selected to match the requirements of the crystal or
resonator. C
L1
and C
L2,
are usually the same size. The crystal manufacturer typically specifies a load
capacitance which is the series combination of C
L1
and C
L2
. PCB and MCU pin capacitance must be
included when sizing C
L1
and C
L2
(10pF can be used as a rough estimate of the combined pin and board
capacitance).
R
S
=200
R
S
=200
R
S
=200
R
S
=100
LP oscillator
MP oscillator
MS oscillator
HS oscillator
22
22
18
15
-
56
46
33
33
pF
i
2
OSC2 driving current V
DD
=5V, V
IN
=V
SS
LP oscillator
MP oscillator
MS oscillator
HS oscillator
-
80
160
310
610
150
250
460
910
µA
OSC2
OSC1
f
OSC
C
L1
C
L2
i
2
R
F
ST72XXX
RESONATOR
WHEN RESONATOR WITH
INTEGRATED CAPACITORS
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