Clock Generation 1/16/12. Required Materials Clock Generation Astable Mulivibrator Inverter-Based Oscillator 555 Timer Circuit.

Clock Generation

1/16/12

Required Materials

Clock Generation

• Astable Mulivibrator• Inverter-Based Oscillator• 555 Timer Circuit

Inverter-Based Oscillator

Delay Per Stage of the Inverter(273 pS) (395 ps)

Period: 395-273=122 pS, Delay per stage (TD): 234-214=20 pS. Period is 2NTD

Oscillation Frequency Calculation

• fosc=2.14 GHz

• Tosc=467 pS=2NTD

– TD=20 pS

– N=Tosc/(2TD)=467 pS/(2 20)=11.68

– N must be odd. – Choose N as 11.

11 Stage Oscillator

11 Stage Oscillator

Period: 1.501 nS-1.052 nS=0.449 nS, Freq=2.227 GHz

Astable Multivibrator Circuit

RC

555 configured as a monostable circuit

FF=flip-flip, depicted as a DPDT switch.

Resistive divider

comparator

+ is the pos. of the battery=9V

- is the neg. term. of the battery=0V

R R R

+6 V

+3 V

(Assumption: pin #3 is connected to 0V)

Comparator

comparator

Each comparator compares two inputs and delivers an output depending on whether the inputs are similar or different.

“UP” and “DOWN” remind you what each comparator does when it is activated.

We will assume that the FF is in the “up” position when

the 555 timer IC is powered up.

555 configured as a monostable circuit

Step 1: Ground pin 2, • The output voltage of comparator A goes up• Pin 7 will be disconnected from the ground.

Step 2: C4 will get charged through R4.The voltage at pin 6 will go up.

You don’t need to memorize this. Just lookat switch drawn in the FF.

0 V

0 V

555 configured as a monostable circuit

Step 3: The voltage at pin 6 will take some timeto rise up to 9.

Step 4: Note that the voltage at pin 5 is fixed to 2/3 of 9 V. When the voltage at pin 6exceeds the voltage at pin 5, the output (pin 3) will be grounded and pin 7will again be connected to ground.

(You don’t need to memorize this. Just look at the switch in the flipflop.)

0 V

+6V

Summary

Control the pulse duration

Sample Waveform

555 configured as an astable circuit

Explanation on the next slide.

Operations of astable timer

Let’s assume that C1 is initially grounded.Step 1: A low voltage at pin 2 causes theComparator to send a “down” signal to FF.Step 2: The output voltage at (pin 3) goes upand the pin 7 is disconnected from R2. (Again, look at the switch in the FF)Step 3: C1 gets charged up through R1+R2.Step 4: The voltage across C1 goes up.Step 5: The voltage at pin 6 exceeds the voltageat pin 5. Step 6: The comparator sends a “high” signalto the flip flop.Step 7: The voltage of Pin 7 goes to ground andThe voltage goes to ground.Step 8: C1 discharges through R2.Step 9: Go back to step 1.

Frequency of an Astable Oscillator

Summary of astable timer

Control the frequency of the astable timer

What if you need a clock?

Square Wave Oscillator Using 555 Timer IC

http://www.falstad.com/circuit/e-555square.html

A Closer Look at the Square Wave Generator

http://www.falstad.com/circuit/e-555int.html

Sawtooth Oscillator

http://www.falstad.com/circuit/e-555saw.html

Low Duty Cycle

http://www.falstad.com/circuit/e-555lowduty.html

Pinout Diagram