Datasheet

Electrical characteristics STM8AF52/62xx, STM8AF51/61xx
66/110 Doc ID 14395 Rev 9
Figure 19. HSE oscillator circuit diagram
HSE oscillator critical g
m
formula
The crystal characteristics have to be checked with the following formula:
Equation 1
where g
mcrit
can be calculated with the crystal parameters as follows:
Equation 2
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
Table 34. HSE oscillator characteristics
Symbol Parameter Conditions Min Typ Max Unit
R
F
Feedback resistor — — 220 — kΩ
C
L1
/C
L2
(1)
Recommended load capacitance — — — 20 pF
g
m
Oscillator trans conductance — 5 — — mA/V
t
SU(HSE)
(2)
Startup time
V
DD
is
stabilized
—2.8 —ms
1. The oscillator needs two load capacitors, C
L1
and C
L2
, to act as load for the crystal. The total load capacitance (C
Load
) is
(C
L1
* C
L2
)/(C
L1
+ C
L2
). If C
L1
= C
L2
, C
load
= C
L1/2
. Some oscillators have built-in load capacitors, C
L1
and C
L2
.
2. This value is the startup time, measured from the moment it is enabled (by software) until a stabilized 24 MHz oscillation is
reached. It can vary with the crystal type that is used.
OSCOUT
OSCIN
f
HSE
to core
C
L1
C
L2
R
F
STM8A
Resonator
Current control
g
m
R
m
C
m
L
m
C
O
Resonator
g
m
g
mcrit
»
g
mcrit
2 Π× HSE
f
×()
2
R
m
× 2Co C+()
2
=