AT85C5122D-RDRUM datasheet(39/208 Pages) ATMEL

39 / 208 page

AT83R5122, AT8xC5122/23

4202F–SCR–07/2008

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 15).

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 15.

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 with a tolerance of

+/- 2500 ppm only.

Feedback

Resistor

XTAL1

XTAL2

8 MHz

GND

Microcontroller

To internal

clock circuitry

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