HT46R46/C46/R47/C47/R48/R49
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.
System RC Oscillator
Using the external RC network as an oscillator requires
that a resistor, with a value between 24kW and 1MW, is
connected between OSC1 and ground, and a 470pF ca-
pacitor 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 pur-
poses. 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 effective oscillator configuration, the oscilla-
tion frequency can vary with VDD, temperature and pro-
cess variations on the device itself and is therefore not
suitable for applications where timing is critical or where
accurate oscillator frequencies are required. For the
value of the external resistor ROSC please refer to the Ap-
pendix section for typical RC Oscillator vs. Temperature
and VDD characteristics graphics.
System Clock Configurations
There are two methods of generating the system clock,
using an external crystal/ceramic oscillator or an exter-
nal RC network. The chosen method is selected through
the configuration options.
System Crystal/Ceramic Oscillator
For most crystal oscillator configurations, the simple
connection of a crystal across OSC1 and OSC2 will cre-
ate the necessary phase shift and feedback for oscilla-
tion. However, to ensure oscillation for certain lower
crystal frequencies and for all ceramic resonator appli-
cations, it is recommended that two small value capaci-
tors and a resistor, the values of which are shown in the
table, should be connected as shown in the diagram.
V
D
D
4
7
0
p
F
O
S
C
1
C
1
R
O
S
C
O
S
C
1
R
1
f
S
Y
S
/
4
N
M
O
S
O
p
e
n
D
r
a
i
n
O
S
C
2
O
S
C
2
C
2
RC Oscillator
Crystal/Ceramic Oscillator
Note: An internal capacitor together with the external re-
sistor, ROSC, are the components which determine the
frequency of the oscillator. The external capacitor
shown on the diagram does not influence the frequency
of oscillation. This external capacitor should be added to
improve oscillator stability if the open-drain OSC2 out-
put is utilised in the application circuit.
The table below shows the C1, C2 and R1 values for vari-
ous crystal/ceramic oscillating frequencies.
Crystal or Resonator
4MHz Crystal
C1, C2
0pF
R1
10kW
12kW
10kW
10kW
10kW
27kW
9.1kW
10kW
10kW
4MHz Resonator
10pF
0pF
Watchdog Timer Oscillator
3.58MHz Crystal
The WDT oscillator is a fully integrated free running RC
oscillator with a typical period of 65ms at 5V, requiring no
external components. It is selected via configuration op-
tion. If selected, when the device enters the Power Down
Mode, the system clock will stop running, however the
WDT oscillator will continue to run and keep the watch-
dog function active. However, as the WDT will consume a
certain amount of power when in the Power Down Mode,
for low power applications, it may be desirable to disable
the WDT oscillator by configuration option.
3.58MHz Resonator
2MHz Crystal and Resonator
1MHz Crystal
25pF
25pF
35pF
300pF
300pF
300pF
480kHz Resonator
455kHz Resonator
429kHz Resonator
The function of the resistor R1 is to ensure that the os-
cillator will switch off should low voltage conditions oc-
cur. Such a low voltage, as mentioned here, is one
which is less than the lowest value of the MCU operat-
ing voltage. Note however that if the LVR is enabled
then R1 can be removed.
Rev. 1.00
42
April 18, 2007
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
