Datasheet

STM8S207xx, STM8S208xx Electrical characteristics
Doc ID 14733 Rev 12 65/103
Figure 17. HSE oscillator circuit diagram
HSE oscillator critical g
m
formula
R
m
: Notional resistance (see crystal specification)
L
m
: Notional inductance (see crystal specification)
C
m
: Notional capacitance (see crystal specification)
Co: Shunt capacitance (see crystal specification)
C
L1
=C
L2
=C: Grounded external capacitance
g
m
>> g
mcrit
Table 32. HSE oscillator characteristics
Symbol Parameter Conditions Min Typ Max Unit
f
HSE
External high speed oscillator
frequency
124MHz
R
F
Feedback resistor 220 kΩ
C
(1)
Recommended load capacitance
(2)
20 pF
I
DD(HSE)
HSE oscillator power consumption
C = 20 pF,
f
OSC
= 24 MHz
6 (startup)
2 (stabilized)
(3)
mA
C = 10 pF,
f
OSC
= 24 MHz
6 (startup)
1.5 (stabilized)
(3)
g
m
Oscillator transconductance 5 mA/V
t
SU(HSE)
(4)
Startup time V
DD
is stabilized 1 ms
1. C is approximately equivalent to 2 x crystal Cload.
2. The oscillator selection can be optimized in terms of supply current using a high quality resonator with small R
m
value.
Refer to crystal manufacturer for more details
3. Data based on characterization results, not tested in production.
4. t
SU(HSE)
is the start-up time measured from the moment it is enabled (by software) to a stabilized 24 MHz oscillation is
reached. This value is measured for a standard crystal resonator and it can vary significantly with the crystal manufacturer.
OSCOUT
OSCIN
f
HSE
to core
C
L1
C
L2
R
F
STM8
Resonator
Consumption
control
g
m
R
m
C
m
L
m
C
O
Resonator
g
mcrit
2 Π× f
HSE
×()
2
R
m
× 2Co C+()
2
=