Electrical characteristics
ST72321Bxxx-Auto
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 75.
Typical application with a crystal or ceramic resonator
Table 115.
Crystal and ceramic resonator oscillators
Symbol
Parameter
Conditions
Min
Typ Max Unit
fOSC
Oscillator frequency(1)
1.
The oscillator selection can be optimized in terms of supply current using a high-quality resonator with
small RS 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
RF
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
-
40
k
CL1
CL2
Recommended load
capacitance versus
equivalent serial resistance of
the crystal or ceramic
resonator (RS)
(3)
3.
For CL1 and CL2 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. CL1 and CL2, are usually the same size. The crystal manufacturer typically specifies a load
capacitance which is the series combination of CL1 and CL2. PCB and MCU pin capacitance must be
included when sizing CL1 and CL2 (10pF can be used as a rough estimate of the combined pin and board
capacitance).
RS =200
RS =100
LP oscillator
MP oscillator
MS oscillator
HS oscillator
22
18
15
-
56
46
33
pF
i2
OSC2 driving current
VDD =5V, VIN =VSS
LP oscillator
MP oscillator
MS oscillator
HS oscillator
-
80
160
310
610
150
250
460
910
A
OSC2
OSC1
fOSC
CL1
CL2
i2
RF
ST72XXX
RESONATOR
WHEN RESONATOR WITH
INTEGRATED CAPACITORS
VDD/2
Ref
LOGIC
FEEDBACK
LOOP
LINEAR
AMPLIFIER