External Rc Oscillator - Erc; Internal High Speed Rc Oscillator - Hirc - Holtek HT66F20 Manual

HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
External RC Oscillator – ERC
Using the ERC oscillator only requires that a resistor, with a value between 56kΩ and 2.4MΩ is
connected between OSC1 and VDD, and a capacitor is connected between OSC1 and ground,
providing a low cost oscillator configuration. It is only the external resistor that determines 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 ensure that the influence of the power
supply voltage, temperature and process variations on the oscillation frequency are minimised. As a
resistance/frequency reference point, it can be noted that with an external 120kΩ resistor connected
and with a 5V voltage power supply and temperature of 25˚C degrees, the oscillator will have a
frequency of 8MHz within a tolerance of 2%. Here only the OSC1 pin is used, which is shared with
I/O pin PB1, leaving pin PB2 free for use as a normal I/O pin.
For oscillator stability and to minimise the effects of noise and crosstalk, it is important to locate the
capacitor and resistoras close to the MCU as possible.
Internal High Speed RC Oscillator – HIRC
The internal RC oscillator is a fully integrated system oscillator 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 frequency 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˚C degrees, the fixed oscillation frequency of 4MHz, 8MHz or 12MHz
will have a tolerance within 2%. Note that if this internal system clock option is selected, as it
requires no external pins for its operation, I/O pins PB1 and PB2 are free for use as normal I/O pins.
Rev. 2.50
V
D D
R
O S C
O S C 1
2 0 p F
External RC Oscillator – ERC
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