BS2824 Holtek Semiconductor Inc., BS2824 Datasheet - Page 28

• Manufacturer Part Number: BS2824
• Description: Touch Key Mcu
• Manufacturer: Holtek Semiconductor Inc.

BS2824/BS2854/BS2868
Touch Key MCU
Rev. 1.00
External Crystal/Resonator Oscillator - HXT
External RC Oscillator - ERC
The simple connection of a crystal across OSC1 and OSC2 will create the necessary phase shift and
feedback 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.
Using the ERC oscillator only requires that a resistor, with a value between 24k
connected between OSC1 and VDD, and a capacitor is connected between OSC 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 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.
Crystal Recommended Capacitor Values
Note: C1 and C2 values are for guidance only.
Crystal Frequency
12MHz
8MHz
4MHz
1MHz
Crystal/Resonator Oscillator - HXT
Crystal Oscillator C1 and C2 Values
External RC Oscillator - ERC
28
100pF
8pF
8pF
8pF
C1
100pF
10pF
10pF
10pF
C2
January 27, 2010
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