lsn 8 clock sources and circuits - university of dayton

Department of

Engineering Technology

LSN 8

Clock Sources and Circuits

ECT 224 Digital Computer Fundamentals

ECT 224 Digital Computer Fundamentals [email protected]

LSN 8 – Clock Sources

• Clock sources are devices with no inputs and one

output characterized by:

– Frequency

– Duty cycle

– Magnitude


LSN 8 – Multivibrator

• Circuit that changes between two digital levels on a continuous, free-running basis

– Bistable

• Triggered into one of two states by an external source and stays in that state until it is triggered in to the opposite state

– Monostable

• Provides a single output pulse of a specified time length when it is triggered from an external source

• One-shot

– Astable

• Free running oscillator that alternates between two digital levels at a specific frequency and duty cycle


LSN 8 – Schmitt Trigger

• Not a FF but shows a memory characteristic

• Accepts slow changing signals and produces a signal

that transitions quickly


LSN 8 – Monostable Multivibrator

• Changes from stable state to quasi-stable state for

a period of time determined by external

components (usually resistors and capacitors)



• Non-retriggerable devices will trigger and return to stable

state

• Retriggerable devices can be triggered while in the quasi-

stable state to begin another pulse



• IC versions

– 74xx121 single non-retriggerable monostable multivibrator

– 74xx221 dual non-retriggerable monostable multivibrator

– 74xx122 single retriggerable monostable multivibrator

– 74xx123 dual retriggerable monostable multivibrator


LSN 8 – Astable Multivibrator

• Schmitt-Trigger Oscillator

– The output state depends on the input level, and will change

only as the input crosses a pre-defined threshold.


LSN 8 – 555 Timer


LSN 8 – 555 Timer

• Monostable (One-Shot)

– Single output pulse with adjustable pulse-width

– Triggered by a negative pulse on the trigger pin

– Useful for extending pulses

• Astable Multivibrator

– Continuous pulse train with adjustable frequency and duty

cycle

– Frequency and duty cycle set by external component values

– Useful as a timer or clock


LSN 8 – 555 Timer (Astable)



• Frequency

F = 1.44 / (R1 + 2R2)C1

• Duty cycle

DC = (R1 + R2) / (R1 + 2R2)



• Application Considerations

– Resistor values R1 and R2 have a range of values that meet

current limitations of the device

1kΩ < R1 < 1MΩ

1kΩ < R2 < 1MΩ

– Capacitor value C1should be small as possible within

adequate range and non-polarized to help reduce the leakage

current

500pF < C1 < 10uF

– Typically not recommended when high accuracy is required

due to frequency drift


LSN 8 – Crystal Controlled Oscillators

• Uses the mechanical resonance of a physical crystal of

piezoelectric material with feedback to create a precise

frequency signal

• Increased accuracy and stability over RC oscillators

• IC or quartz crystal package


LSN 8 – Clock Circuits

• Voltage Controlled Oscillator (VCO)

– Uses a varactor diode as the principle tuning element

– The voltage controlled oscillator is tuned across its band by

applying a DC voltage to the varactor diode to vary the net

capacitance applied to the tuned circuit


LSN 8 – Clock Circuits

• Clock distribution

– Routing clock signal throughout circuit

• Clock skew

• Fan-out

• Frequency divider

– Useful in providing lower frequency clocks from a master

clock


LSN 8 – Clock Accuracy

• Measured in Parts Per Million

– One PPM (Part Per Million) is 0.0001% (1E-6)

– Calculator to determine time drift based upon PPM and

temperature

• http://www.maxim-ic.com/tools/calculators/index.cfm/calc_id/rtc

http://www.maxim-ic.com/tools/calculators/index.cfm/calc_id/rtc




LSN 8 – Homework

• Reading

– Chapter 7.5 – 7.6

• Assignment – HW10

– Create a clock circuit that can achieve a 32.768kHz signal (Must use standard component values)

• Use a 555 timer IC (ensure the 555 is within its safe operating range)

• Use a CMOS Schmitt-trigger oscillator (74HC14)

• Find a COTS high-accuracy clock sources (think voltage and temperature compensation) and submit data sheet with a brief comment on why you think it is accurate

• References

– http://www.epanorama.net/links/oscillator.html#crystal

– http://www.electronics-tutorials.com/oscillators/oscillators.htm

http://www.epanorama.net/links/oscillator.html

http://www.electronics-tutorials.com/oscillators/oscillators.htm



lsn 8 clock sources and circuits - university of dayton

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