HT56R66 Holtek Semiconductor Inc., HT56R66 Datasheet - Page 78

• Manufacturer Part Number: HT56R66
• Description: Tinypower Tm A/d Type With Lcd 8-bit Otp Mcu
• Manufacturer: Holtek Semiconductor Inc.

Oscillator
Various oscillator options offer the user a wide range of
functions according to their various application require-
ments. Five 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 Clock Configurations
There are five methods of generating the system clock,
two high oscillators, two low oscillators and an externally
supplied clock. The two high oscillators are the external
crystal/ceramic oscillator and the external RC network.
The two low oscillators are the fully integrated 32K_INT
oscillator and the external 32768Hz oscillator. Selecting
whether the low or high oscillator is used as the system
oscillator is implemented using the HLCLK bit in the
CLKMOD register. The source clock for the high and low
oscillators is chosen via configuration options. The fre-
quency of the slow oscillator is also determined using
the SLOWC0~SLOWC2 bits in the CLKMOD register.
System Crystal/Ceramic Oscillator
After selecting the external crystal configuration option,
the simple connection of a crystal across OSC1 and
OSC2, is normally all that is required to create the nec-
essary phase shift and feedback for oscillation, without
requiring external capacitors. However, for some crystal
types and frequencies, to ensure oscillation, it may be
necessary to add two small value capacitors, C1 and
C2. Using a ceramic resonator will usually require two
small value capacitors, C1 and C2, to be connected as
shown for oscillation to occur. The values of C1 and C2
should be selected in consultation with the crystal or
resonator manufacturer s specification. In most applica-
tions, resistor R
plications where the LVR function is not used, R
be necessary to ensure the oscillator stops running
when VDD falls below its operating range. The internal
oscillator circuit contains a filter circuit to reduce the
possibility of erratic operation due to noise on the oscil-
lator pins. An additional configuration option must be
setup to configure the device according to whether the
oscillator frequency is high, defined as equal to or above
1MHz, or low, which is defined as below 1 MHz.
More information regarding oscillator applications is lo-
cated on the Holtek website.
Rev. 1.10
Crystal/Ceramic Oscillator
P1
is not required, however for those ap-
P1
may
78
External System RC Oscillator
After selecting the correct configuration option, using
the external system RC oscillator requires that a resis-
tor, with a value between 47k
nected between OSC1 and VDD, and a 470pF capacitor
is connected to ground. Although this is a cost effective
oscillator configuration, the oscillation frequency can
vary with VDD, temperature and process variations and
is therefore not suitable for applications where timing is
critical or where accurate oscillator frequencies are re-
quired. For the value of the external resistor R
to the Appendix section for typical RC Oscillator vs.
Temperature and VDD characteristics graphics.
Note that an internal capacitor together with the external
resistor, R
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 internal oscil-
lator circuit contains a filter circuit to reduce the
possibility of erratic operation due to noise on the oscil-
lator pins.
Crystal Frequency
12MHz
8MHz
4MHz
1MHz
455kHz (see Note 2)
Note: 1. C1 and C2 values are for guidance only.
Crystal Recommended Capacitor Values
Crystal Oscillator C1 and C2 Values
2. XTAL mode configuration option: 455kHz.
3. R
OSC
P1
, are the components which determine the
=5M ~10M is recommended.
RC Oscillator
HT56R66/HT56R666
10pF
C1
and 1.5M , is con-
September 8, 2009
10pF
C2
OSC
refer