Electrical characteristics
STM8S207xx, STM8S208xx
Table 32. HSE oscillator characteristics
Symbol
Parameter
Conditions
Min
Typ
Max
Unit
External high speed oscillator
frequency
fHSE
1
24
MHz
RF
Feedback resistor
220
kΩ
C(1)
Recommended load capacitance (2)
20
pF
C = 20 pF,
OSC = 24 MHz
6 (startup)
f
2 (stabilized)(3)
IDD(HSE) HSE oscillator power consumption
mA
C = 10 pF,
6 (startup)
fOSC = 24 MHz
1.5 (stabilized)(3)
gm
Oscillator transconductance
Startup time
5
mA/V
ms
(4)
tSU(HSE)
VDD is stabilized
1
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 Rm value.
Refer to crystal manufacturer for more details
3. Data based on characterization results, not tested in production.
4. tSU(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.
Figure 17. HSE oscillator circuit diagram
f
to core
HSE
R
m
R
F
C
O
L
m
C
L1
OSCIN
C
m
g
m
Resonator
Consumption
control
Resonator
STM8
OSCOUT
C
L2
HSE oscillator critical g formula
m
gmcrit = (2 × Π × fHSE)2 × Rm(2Co + C)2
R : Notional resistance (see crystal specification)
m
L : Notional inductance (see crystal specification)
m
C : Notional capacitance (see crystal specification)
m
Co: Shunt capacitance (see crystal specification)
C =C =C: Grounded external capacitance
L1
L2
g >> g
m
mcrit
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Doc ID 14733 Rev 9
STMICROELECTRONICS [ ST ]
