AT89C5122 ATMEL [ATMEL Corporation], AT89C5122 Datasheet - Page 41

• Manufacturer Part Number: AT89C5122
• Description: MICROCONTROLLER WITH USB AND SMART CARD READER INTERFACES
• Manufacturer: ATMEL [ATMEL Corporation]

Clock Controller
On-Chip Oscillator
Quartz Specification
4202D–SCR–06/05
The clock controller is based on an on-chip oscillator feeding an on-chip Phase Lock
Loop (PLL). All the internal clocks to the CPU core and peripherals are generated by this
controller.
The on-chip oscillator is composed of a single-stage inverter and a parallel feedback
resistor. The XTAL1 and XTAL2 pins are respectively the input and the output of the
inverter, which can be configured with off-chip components as a Pierce oscillator (see
Figure 16).
The on-chip oscillator has been designed and optimized to work with an external 8 MHz
crystal and very few load capacitance. Then external load capacitors are not needed
given that :
The circuit works on its fundamental frequency at 8 MHz.
Figure 16. Oscillator Schematic
C1 and C2 represents the internal capacitance of the microcontroller and the stray
capacitance of the circuit board. It is recommended to implement the crystal as close as
possible from the microcontroller package.
The equivalent circuit of a crystal is represented on the figure below :
The Equivalent Serial Resistance R1 must be lower than 100 Ohm.
–
–
the internal capacitance of the microcontroller and the stray capacitance of
circuit board are enough to ensure a stable oscillation
a very high accuracy on the oscillation frequency is not needed
Microcontroller
XTAL1
C1
GND
Feedback
Resistor
8 MHz
L1
GND
C1
C0
To internal
clock circuitry
C2
XTAL2
R1
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