HT46R46/C46/R47/C47/R48A/C48A/R49
Rev. 1.41
42
December 30, 2008
Oscillator
Various oscillator options offer the user a wide range of
functions according to their various application require-
ments. Two types of system clocks can be selected
while various clock source options for the Watchdog
Timer are provided for maximum flexibility. All oscillator
options are selected through the configuration options.
The two methods of generating the system clock are:
·
External crystal/resonator oscillator
·
External
RC oscillator
One of these two methods must be selected using the
configuration options.
More information regarding the oscillator is located in
Application Note HA0075E on the Holtek website.
External Crystal/Resonator Oscillator
The simple connection of a crystal across OSC1 and
OSC2 will create the necessary phase shift and feed-
back for oscillation, and will normally not require exter-
nal capacitors. However, for some crystals and most
resonator types, to ensure oscillation and accurate fre-
quency generation, it may be necessary to add two
small value external capacitors, C1 and C2. The exact
values of C1 and C2 should be selected in consultation
with the crystal or resonator manufacturer
¢
s specifica-
tion. The external parallel feedback resistor, Rp, is nor-
mally not required but in some cases may be needed to
assist with oscillation start up.
Internal Ca, Cb, Rf Typical Values @ 5V, 25
°
C
Ca
Cb
Rf
11~13pF
13~15pF
470k
W
Oscillator Internal Component Values
Crystal Oscillator C1 and C2 Values
Crystal Frequency
C1
C2
CL
8MHz
TBD
TBD
TBD
4MHz
TBD
TBD
TBD
1MHz
TBD
TBD
TBD
Note:
1. C1 and C2 values are for guidance only.
2. CL is the crystal manufacturer specified
load capacitor value.
Crystal Recommended Capacitor Values
Resonator C1 and C2 Values
Resonator Frequency
C1
C2
3.58MHz
TBD
TBD
1MHz
TBD
TBD
455kHz
TBD
TBD
Note:
C1 and C2 values are for guidance only.
Resonator Recommended Capacitor Values
External RC Oscillator
Using the external system RC oscillator requires that a
resistor, with a value between 24k
W
and 1M
W
, is con-
nected between OSC1 and ground, and a capacitor is
connected to VDD. The generated system clock divided
by 4 will be provided on OSC2 as an output which can
be used for external synchronization purposes. Note
that as the OSC2 output is an NMOS open-drain type, a
pull high resistor should be connected if it to be used to
monitor the internal frequency. Although this is a cost ef-
fective oscillator configuration, the oscillation frequency
can vary with VDD, temperature and process variations
and is therefore not suitable for applications where tim-
ing is critical or where accurate oscillator frequencies
are required.
For the value of the external resistor R
OSC
refer to the Holtek website for typical RC Oscillator vs.
Temperature and VDD characteristics graphics.
Note
that it is the only microcontroller internal circuitry to-
gether with the external resistor, that determine the fre-
quency of the oscillator. The external capacitor shown
on the diagram does not influence the frequency of os-
cillation.
O S C 1
O S C 2
R f
T o i n t e r n a l
c i r c u i t s
I n t e r n a l
O s c i l l a t o r
C i r c u i t
C 1
C 2
C a
C b
R p
N o t e : 1 . R p i s n o r m a l l y n o t r e q u i r e d .
2 . A l t h o u g h n o t s h o w n O S C 1 / O S C 2 p i n s h a v e a p a r a s i t i c
c a p a c i t a n c e o f a r o u n d 7 p F .
Crystal/Resonator Oscillator
O S C 1
4 7 0 p F
V
D D
R
O S C
O S C 2
f
S Y S
/ 4 N M O S O p e n D r a i n
RC Oscillator
