Rainbow Electronics AT89C5122 - Manual (Page 41)

41
AT8xC5122/23
4202E–SCR–06/06
Clock Controller
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.
On-Chip Oscillator 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 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
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.
Quartz Specification The equivalent circuit of a crystal is represented on the figure below :
The Equivalent Serial Resistance R1 must be lower than 100 Ohm.
Feedback
Resistor
XTAL1 XTAL2
8 MHz
GND GND
Microcontroller
C1 C2
To internal
clock circuitry
L1 C1 R1
C0