Datasheet

Oscillator Characteristics
MC68HC908QB8 Data Sheet, Rev. 3
Freescale Semiconductor 215
18.11 Oscillator Characteristics
Characteristic Symbol Min Typ Max Unit
Internal oscillator frequency
(1)
ICFS1:ICFS0 = 00
ICFS1:ICFS0 = 01
ICFS1:ICFS0 = 10 (not allowed if V
DD
<2.7V)
1. Bus frequency, f
OP
, is oscillator frequency divided by 4.
f
INTCLK
—
—
—
4
8
12.8
—
—
—
MHz
Trim accuracy
(2)(3)
2. Factory trimmed to provided 12.8MHz accuracy requirement (± 5%, @25•C) for forced monitor mode communication. User
should trim in-circuit to obtain the most accurate internal oscillator frequency for his application.
3. Values are based on characterization results, not tested in production.
Δ
TRIM_ACC
— ± 0.4 — %
Deviation from trimmed Internal oscillator
(3)(4)
4, 8, 12.8MHz, V
DD
± 10%, 0 to 70°C
4, 8, 12.8MHz, V
DD
± 10%, –40 to 125°C
4. Deviation values assumes trimming in target application @25•C and midpoint of voltage range, for example 5.0 V for 5 V
± 10% operation.
Δ
INT_TRIM
—
—
± 2
—
—
± 5
%
External RC oscillator frequency, RCCLK
(1)(3)
f
RCCLK
2—10MHz
External clock reference frequencyy
(1)(5)(6)
V
DD
≥ 4.5V
V
DD
< 4.5V
5. No more than 10% duty cycle deviation from 50%.
6. When external oscillator clock is greater than 1MHz, ECFS1:ECFS0 must be 00 or 01
f
OSCXCLK
dc
dc
—32
16
MHz
RC oscillator external resistor
(3)
V
DD
= 5 V
V
DD
= 3 V
R
EXT
See Figure 18-7
See Figure 18-8
—
Crystal frequency, XTALCLK
(1)(7)(8)
ECFS1:ECFS0 = 00 ( V
DD
≥ 4.5 V)
ECFS1:ECFS0
= 00
ECFS1:ECFS0 = 01
ECFS1:ECFS0 = 10
7. Use fundamental mode only, do not use overtone crystals or overtone ceramic resonators
8. Due to variations in electrical properties of external components such as, ESR and Load Capacitance, operation above
16 MHz is not guaranteed for all crystals or ceramic resonators. Operation above 16 MHz requires that a Negative Resis-
tance Margin (NRM) characterization and component optimization be performed by the crystal or ceramic resonator vendor
for every different type of crystal or ceramic resonator which will be used. This characterization and optimization must be
performed at the extremes of voltage and temperature which will be applied to the microcontroller in the application. The
NRM must meet or exceed 10x the maximum ESR of the crystal or ceramic resonator for acceptable performance.
f
OSCXCLK
8
8
1
30
—
32
16
8
100
MHz
MHz
MHz
kHz
ECFS1:ECFS0 = 00
(9)
Feedback bias resistor
Crystal load capacitance
(10)
Crystal capacitors
(10)
9. Do not use damping resistor when ECFS1:ECFS0 = 00 or 10
10. Consult crystal vendor data sheet.
R
B
C
L
C
1
,C
2
—
—
—
1
20
(2 x C
L
) – 5pF
—
—
—
MΩ
pF
pF
ECFS1:ECFS0 = 01
(9)
Crystal series damping resistor
f
OSCXCLK
= 1 MHz
f
OSCXCLK
= 4 MHz
f
OSCXCLK
= 8 MHz
Feedback bias resistor
Crystal load capacitance
(10)
Crystal capacitors
(10)
R
S
R
B
C
L
C
1
,C
2
—
—
—
—
—
—
20
10
0
5
18
(2 x C
L
) –10 pF
—
—
—
—
—
—
kΩ
kΩ
kΩ
MΩ
pF
pF
AWU module internal RC oscillator frequency f
INTRC
— 32 — kHz