LCD Type MCU
System Crystal/Ceramic Oscillator
For most crystal oscillator configurations, the simple connection of a crystal across OSC1 and
OSC2 will create the necessary phase shift and feedback for oscillation. However, to ensure oscil-
lation for certain lower crystal frequencies and for all ceramic resonator applications, it is recom-
mended that two small value capacitors and a resistor, the values of which are shown in the table,
should be connected as shown in the diagram.
C 1
O S C 1
R 1
O S C 2
C 2
Crystal/Ceramic Oscillator
The table below shows the C1, C2 and R1 values for various crystal/ceramic oscillating frequencies.
Crystal or Resonator
4MHz Crystal
4MHz Resonator
3.58MHz Crystal
3.58MHz Resonator
2MHz Crystal and Resonator
1MHz Crystal
480kHz Resonator
455kHz Resonator
429kHz Resonator
C1, C2
0pF
10pF
0pF
25pF
25pF
35pF
300pF
300pF
300pF
R1
10kW
12kW
10kW
10kW
10kW
27kW
9.1kW
10kW
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 is connected between
OSC1 and GND. For the HT49C30L, HT49C50L, HT49C70L devices, the value of this resistor
ranges from 560kW to 1MW, while for the remaining devices the value of resistor ranges from
24kW to 1MW. 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 oscilla-
tor configuration, 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 accu-
rate oscillator frequencies are required. For the value of the external resistor R
OSC
, refer to the Ap-
pendix section for typical RC Oscillator vs. Temperature and V
DD
characteristics graphics.
V
4 7 0 p F
O S C 1
R
O S C
D D
f
S
Y S
/4 N M O S O p e n D r a in
O S C 2
RC Oscillator
102
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