Crystal oscillator circuit in PCB Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the frequency of the crystal oscillator.

PCB Design

frequency = [1]  // Frequency in MHz

Drag options to blanks, or click blank then click option'

A5

B10

C16

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a frequency too high for the circuit design

Using a frequency not supported by the microcontroller

The frequency of the crystal oscillator is set to 5 MHz, which is a common value for many circuits.

2fill in blank

medium

Complete the code to set the load capacitance for the crystal oscillator.

PCB Design

load_capacitance = [1]  // Load capacitance in pF

Drag options to blanks, or click blank then click option'

A33

B10

C18

D22

Attempts:

3 left

💡 Hint

Common Mistakes

Using too low or too high capacitance causing frequency shift

Ignoring the datasheet recommendations

The load capacitance is set to 22 pF, which matches typical crystal specifications for stable oscillation.

3fill in blank

hard

Fix the error in the code to correctly connect the crystal pins.

PCB Design

connect(crystal_pin1, [1])
connect(crystal_pin2, oscillator_output)

Drag options to blanks, or click blank then click option'

AVcc

Breset_pin

Coscillator_input

Doscillator_output

Attempts:

3 left

💡 Hint

Common Mistakes

Connecting crystal pin to reset or Vcc instead of oscillator input

Reversing pin connections

The crystal pin 1 should connect to the oscillator input pin and pin 2 to the oscillator output pin of the microcontroller to function correctly.

4fill in blank

hard

Fill both blanks to complete the code for calculating the oscillation period and frequency.

PCB Design

period = 1 / [1]  // Period in microseconds
frequency = [2] * 1e6  // Frequency in Hz

Drag options to blanks, or click blank then click option'

Afrequency

Bperiod

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping period and frequency variables

Not converting MHz to Hz correctly

The period is calculated as the inverse of frequency, and frequency is converted from MHz to Hz by multiplying by 1 million.

5fill in blank

hard

Fill all three blanks to complete the code for setting up the oscillator circuit with capacitors and crystal.

PCB Design

c1 = [1]  // Capacitor 1 in pF
c2 = [2]  // Capacitor 2 in pF
connect(crystal, [3])

Drag options to blanks, or click blank then click option'

A22

Coscillator_pins

D18

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect capacitor values

Connecting crystal to wrong pins

Capacitors C1 and C2 are set to 22 pF and 18 pF respectively, and the crystal is connected to the oscillator pins for proper operation.

Practice

1. What is the main purpose of a crystal oscillator circuit in electronics?

easy

A. To amplify weak signals

B. To store electrical energy

C. To convert AC to DC power

D. To provide a precise and stable clock signal

Solution

1. Step 1: Understand the role of oscillators

   Oscillators generate repetitive signals used as timing references in circuits.

2. Step 2: Identify crystal oscillator function

   Crystal oscillators use a quartz crystal to produce a very stable and precise frequency.

3. Final Answer:

   To provide a precise and stable clock signal -> Option D

4. Quick Check:

   Crystal oscillator = precise clock signal [OK]

Hint: Crystal oscillators give stable timing signals [OK]

Common Mistakes:

• Confusing oscillator with amplifier
• Thinking it stores energy
• Mixing power conversion with timing

2. Which of the following is the correct way to connect the crystal in a typical Pierce oscillator circuit?

easy

A. Between the power supply and ground without capacitors

B. Directly in series with the power supply

C. Between the input and output of the inverter with capacitors to ground

D. In parallel with the load resistor only

Solution

1. Step 1: Recall Pierce oscillator configuration

   In a Pierce oscillator, the crystal is connected between the inverter input and output pins.

2. Step 2: Confirm capacitor placement

   Capacitors connect from these points to ground to form the feedback network.

3. Final Answer:

   Between the input and output of the inverter with capacitors to ground -> Option C

4. Quick Check:

   Pierce oscillator crystal connection = input-output with capacitors [OK]

Hint: Crystal goes between inverter input and output with caps to ground [OK]

Common Mistakes:

• Connecting crystal directly to power
• Omitting capacitors to ground
• Placing crystal only in parallel with load

3. Given a crystal oscillator circuit with a 10 MHz crystal and capacitors of 20 pF each, what is the expected output frequency?

medium

A. 5 MHz

B. 10 MHz

C. 20 MHz

D. Depends on capacitor values only

Solution

1. Step 1: Understand crystal frequency role

   The crystal sets the fundamental frequency of oscillation, here 10 MHz.

2. Step 2: Recognize capacitor effect

   Capacitors fine-tune frequency slightly but do not double or halve it.

3. Final Answer:

   10 MHz -> Option B

4. Quick Check:

   Crystal frequency = output frequency [OK]

Hint: Output frequency equals crystal frequency, capacitors adjust tuning [OK]

Common Mistakes:

• Assuming capacitors double frequency
• Confusing frequency division
• Ignoring crystal's dominant frequency

4. A crystal oscillator circuit is not oscillating. Which of the following is the most likely cause?

medium

A. Incorrect capacitor values causing no feedback

B. Crystal frequency too high for the circuit

C. Power supply voltage is too high

D. Using a resistor instead of a crystal

Solution

1. Step 1: Check feedback network

   Oscillation requires proper feedback; wrong capacitor values can break this loop.

2. Step 2: Evaluate other options

   Crystal frequency mismatch or power issues usually cause distortion, not no oscillation; resistor cannot replace crystal.

3. Final Answer:

   Incorrect capacitor values causing no feedback -> Option A

4. Quick Check:

   No oscillation often means feedback failure [OK]

Hint: No oscillation? Check capacitor feedback network first [OK]

Common Mistakes:

• Blaming crystal frequency without checking feedback
• Ignoring capacitor role
• Replacing crystal with resistor

5. You want to design a crystal oscillator circuit for a microcontroller that requires a 16 MHz clock. Which combination is best to ensure stable oscillation?

hard

A. Use a 16 MHz crystal with two 22 pF capacitors connected to ground

B. Use a 16 MHz crystal with no capacitors

C. Use a 8 MHz crystal with two 22 pF capacitors connected to ground

D. Use a 16 MHz crystal with one 100 pF capacitor in series

Solution

1. Step 1: Match crystal frequency to microcontroller clock

   The crystal frequency must be 16 MHz to match the required clock.

2. Step 2: Choose proper capacitor values and placement

   Two capacitors (~22 pF) connected from crystal terminals to ground stabilize oscillation.

3. Step 3: Evaluate other options

   No capacitors or wrong crystal frequency cause instability; one large capacitor in series is incorrect.

4. Final Answer:

   Use a 16 MHz crystal with two 22 pF capacitors connected to ground -> Option A

5. Quick Check:

   Correct crystal + proper capacitors = stable oscillation [OK]

Hint: Match crystal freq and use two small caps to ground [OK]

Common Mistakes:

• Skipping capacitors
• Using wrong crystal frequency
• Incorrect capacitor placement