Holtek HT48R10A-1 - System RC Oscillator; Internal System RC Oscillator

The table below shows the C1, C2 and R1 values for various crystal/ceramic oscillating frequencies.
Crystal or Resonator C1, C2 R1
4MHz Crystal 0pF
10kW
4MHz Resonator 10pF
12kW
3.58MHz Crystal 0pF
10kW
3.58MHz Resonator 25pF
10kW
2MHz Crystal and Resonator 25pF
10kW
1MHz Crystal 35pF
27kW
480kHz Resonator 300pF
9.1kW
455kHz Resonator 300pF
10kW
429kHz Resonator 300pF
10kW
The function of the resistor R1 is to ensure that the oscillator 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 operating
voltage. Note however that if the LVR is enabled then R1 can be removed.
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 VDD, and a 470pF capacitor is connected to ground.
The generated system clock divided by 4 will be provided on OSC2 as an output which can be
used for external synchronization purposes. Although this is a cost effective oscillator configura
-
tion, the oscillation frequency can vary with VDD, temperature and process variations on the chip
itself and is therefore not suitable for applications where timing is critical or where accurate oscilla
-
tor frequencies are required. For the value of the external resistor R
OSC
please refer to the Appen-
dix section for typical RC Oscillator vs. Temperature and V
DD
characteristics graphics.
Internal System RC Oscillator
In addition to the external crystal/resonator or external RC system clock configurations, the de
-
vices also contain an internal RC system clock. This oscillator is integral to the microcontroller and
requires no external components. The frequency of this internal RC oscillator can be selected by
configuration option to have a typical value at 5V of either 3.2MHz, 1.6MHz, 800kHz or 400kHz.
The 1.6MHz, 800kHz and 400kHz frequencies are internally generated by sequentially dividing
the base 3.2MHz frequency by two. Note that if this internal system clock option is selected then
with the exception of the HT48R70A-1/HT48C70-1 and HT48RU80/HT48CU80 devices, the
84
I/O Type MCU
O S C 1
O S C 2
f
S Y S
/ 4 N M O S O p e n D r a i n
4 7 0 p F
V
D D
R
O S C
RC Oscillator