HT46R064B/065B/066B
System Clock Configurations
frequency are minimised. As a resistance/frequency ref-
erence point, it can be noted that with an external 120K
resistor connected and with a 5V voltage power supply
and temperature of 25 degrees, the oscillator will have a
frequency of 4MHz within a tolerance of 2%. Here only
the OSC1 pin is used, which is shared with I/O pin PA6,
leaving pin PA5 free for use as a normal I/O pin.
There are five system oscillators. Three high speed os-
cillators and two low speed oscillators. The high speed
oscillators are the external crystal/ceramic oscillator -
HXT, the external - ERC, and the internal RC oscillator -
HIRC. The two low speed oscillator are the external
32768Hz oscillator - LXT and the internal 13kHz
(VDD=5V) oscillator - LIRC.
V
D
D
External Crystal/Resonator Oscillator - HXT
R
O
S
C
The simple connection of a crystal across OSC1 and
OSC2 will create the necessary phase shift and feed-
back for oscillation. However, for some crystals and
most resonator types, to ensure oscillation and accurate
frequency generation, it is 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 specification.
P
A
6
/
O
S
C
1
4
7
0
p
F
P
A
5
/
O
S
C
2
External RC Oscillator - ERC
Internal RC Oscillator - HIRC
Crystal Oscillator C1 and C2 Values
The internal RC oscillator is a fully integrated system os-
cillator requiring no external components. The internal
RC oscillator has three fixed frequencies of either
4MHz, 8MHz or 12MHz. Device trimming during the
manufacturing process and the inclusion of internal fre-
quency compensation circuits are used to ensure that
the influence of the power supply voltage, temperature
and process variations on the oscillation frequency are
minimised. As a result, at a power supply of either 3V or
5V and at a temperature of 25 degrees, the fixed oscilla-
tion frequency of 4MHz, 8MHz or 12MHz will have a tol-
erance within 2%. Note that if this internal system clock
option is selected, as it requires no external pins for its
operation, I/O pins PA5 and PA6 are free for use as nor-
mal I/O pins.
Crystal Frequency
12MHz
C1
8pF
C2
10pF
10pF
10pF
100pF
8MHz
8pF
4MHz
8pF
1MHz
100pF
Note: C1 and C2 values are for guidance only.
Crystal Recommended Capacitor Values
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.
Internal RC Oscillator - HIRC
Crystal/Resonator Oscillator - HXT
External 32768Hz Crystal Oscillator - LXT
External RC Oscillator - ERC
When the microcontroller enters the Idle/Sleep Mode,
the system clock is switched off to stop microcontroller
activity and to conserve power. However, in many
microcontroller applications it may be necessary to keep
the internal timers operational even when the
microcontroller is in the Power-down Mode. To do this,
another clock, independent of the system clock, must be
provided. To do this a configuration option exists to allow
a high speed oscillator to be used in conjunction with a a
low speed oscillator, known as the LXT oscillator. The
LXT oscillator is implemented using a 32768Hz crystal
connected to pins OSC1/OSC2. However, for some
Using the ERC oscillator only requires that a resistor,
with a value between 24kW and 1.5MW, is connected
between OSC1 and VDD, and a capacitor is connected
between OSC and ground, providing a low cost oscilla-
tor configuration. It is only the external resistor that de-
termines the oscillation frequency; the external
capacitor has no influence over the frequency and is
connected for stability purposes only. Device trimming
during the manufacturing process and the inclusion of
internal frequency compensation circuits are used to en-
sure that the influence of the power supply voltage, tem-
perature and process variations on the oscillation
Rev. 1.00
27
April 8, 2011
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
