Clock Generators: RC, Crystal, and 555 Timer

• Taktgeneratoren generate clock signals

KORE (Core Concepts)

• RC clock generators with integrated circuits can be drawn and dimensioned using data sheets
• Explain the structure of a clock generator with a crystal and discuss the EMC problems (short lines, small radiation surfaces, limited currents)
• Describe the influence of quartz on the stability of the resonant circuit
• Describe the structure and functioning of quartz
• Draw and describe the equivalent circuit diagram of the quartz crystal

Application of clock generators

• Clock generators are used to generate rectangular voltages
• These are used as clock signals for CPUs, MCUs, FPGAs (Field-Programmable Gate Arrays), DSPs (Digital Signal Processors), DACs and ADCs.

xx555 Timer

• The xx555 timer IC, including variants such as NE555, MC1455, LM555, KA555, and SN72555, is described
• Pin configuration includes:
  • GND (Ground)
  • TRIG (Trigger)
  • OUT (Output)
  • RESET
  • CONT (Control)
  • THRES (Threshold)
  • DISCH (Discharge)
  • VCC (Supply Voltage)
• TRIG: The output signal OUT rises to the level of the operating voltage (Vcc) when the value at the trigger input falls below 1/3 of the operating voltage.
• OUT: The output is switched by the IC either to the level of the operating voltage Vcc or to ground (GND).
• RESET: Interrupts the NE555's timing interval when connected to ground (GND).
• CONT: Affects the internal voltage divider.
• THRES: Ends the timing interval if the value is greater than 2/3 Vcc.
• DISCH: Open-collector output for switching tasks.

Internal Structure and Functionality of the xx555

• The internal circuit diagram of the xx555 timer shows connections between TRIG, GND, THRES, CONT, RESET, DISCH and OUT

Typical Structure of an Astable xx555 Circuit

• A diagram illustrates a typical astable xx555 circuit setup, including components such as RA, RB, C, and RL, with example values provided
• The capacitor voltage and output voltage waveforms are shown

External RC Oscillator with PIC-MCUs

• External RC oscillators are used with PIC-MCUs

Clock Generator with Quartz Crystal, Quartz Oscillator

• Inverter U1 provides a phase shift of 180°
• Schmitt trigger U2 acts as an amplifier stage to obtain a rectangular voltage output
• R1 ensures linear operation of U1
• R2 limits the peak current from U1
• X: Quartz crystal
• C1 & C2: Load or pull capacitors to adjust the phase shift of the feedback network to 180°
• Other components should not be placed nearby.
• Use short and wide conductor tracks as specified by the quartz manufacturer
• Maintain a large distance from other conductor tracks
• The structure of the conductor tracks should be as symmetrical as possible

Quartz Crystal Oscillators in MCUs

• In MCU quartz crystal oscillators, short, symmetrical lines are important

Construction Types of Quartz Crystals

• Various physical forms of quartz crystals range in size and mounting style

Structure and Function of Quartz Crystals

• Constructed from silicon dioxide (SiO2), also known as rock crystal.
• Crystals are artificially grown and sawn crystallographically into wafers.
• Mechanical deformation of the wafer generates electrical charges on the surface due to the piezoelectric effect
• The piezoelectric effect is reversible
• Quartz is operated using the inverse piezoelectric effect by applying an alternating voltage
• Positive feedback maintains signal oscillation.

Equivalent Circuit Diagram of the Quartz Crystal

• An equivalent circuit model of a quartz crystal includes elements such as Rs, Ls, Cp, and Cs, representing the crystal's electrical characteristics.

Equivalent Circuit Diagram of the Quartz Crystal

• The equivalent circuit of a crystal includes L1, C1, R1 which represent the actual crystal in a series resonant circuit
• Co represents the parasitic parallel capacitance
• Electrodes (connections) are part of the circuit
• The circuit includes a housing
• L₁ represents the oscillating mass of the quartz
• C₁ represents the elasticity constants of the quartz
• R₁ represents losses
  • Internal friction
  • Mechanical losses in the mounting system
  • Acoustical losses.

Properties and Stability of a Quartz Resonant Circuit

• High quality factor (Q-factor) of approximately 25000 is achieved
  • The quality factor is a dimensionless quantity describing damping and resonance sharpness in an oscillating system.
• Very small error limits of the oscillation frequency in the range of 0.001% - 10 ppm
  • ppm means parts per million, corresponding to a factor of 10^-6 or one millionth
• Long-term stability is better than 0.0001%
• The temperature coefficient is very small
• Low phase noise (frequency domain) or low jitter (time domain) is observed.

Integrated Quartz Oscillators

• Integrated quartz oscillators combine crystal and circuitry

Special Forms: TCXO, OCXO, VCXO

• TCXO (Temperature Compensated Crystal Oscillator)
  • Temperature compensated oscillators
  • Compensated with a temperature sensor and control circuit.
• OCXO (Oven Controlled Crystal Oscillator)
  • Oven controlled crystal oscillators
  • Temperature compensation uses heating and a control circuit.
• VCXO (Voltage Controlled Crystal Oscillator)
  • Voltage controlled oscillators
  • Voltage-dependent oscillator for frequency modulation or for PLL (phase-locked loop).

MEMS Oscillators

• MEMS oscillators use a resonator made of polysilicon and are integrated on a semiconductor die
• They have smaller dimensions
• MEMS oscillators have a lower power consumption

Various Clock Generators in C8051F330

• A diagram illustrates different clock generator options and settings in the C8051F330

Phase-Locked Loop (PLL)

• A control circuit utilizes a voltage-controlled oscillator (VCO) to generate a new oscillation that is synchronized with a reference signal

Methods for Multiplying a Frequency, e.g. doubling a frequency

• Bridge rectifier (2f, low frequency)
• XOR gate (2f, digital, some 100 MHz)
• Harmonic overtones through overdrive and subsequent
• PLL with frequency divider in the feedback