Download 555 Timer

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
+ is the pos. of the battery=9V
Resistive
divider
R
R
R
+3 V
comparator
FF=flip-flip, depicted as a DPDT switch.
+6 V
- is the neg. term. of the battery=0V
(Assumption: pin #3 is connected to 0V)
Comparator
comparator
We will assume that the
FF is in the “up” position when
the 555 timer IC is powered up.
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.
555 configured as a monostable
circuit
0V
Step 1: Ground pin 2,
• The output voltage of comparator A goes up
• Pin 7 will be disconnected from the ground.
0V
Step 2:
C4 will get charged through R4.
The voltage at pin 6 will go up.
You don’t need to memorize this. Just look
at switch drawn in the FF.
555 configured as a monostable
circuit
Step 3:
The voltage at pin 6 will take some time
to rise up to 9.
0V
Step 4:
Note that the voltage at pin 5 is fixed to
2/3 of 9 V. When the voltage at pin 6
exceeds the voltage at pin 5, the
output (pin 3) will be grounded and pin 7
will again be connected to ground.
(You don’t need to memorize this. Just
look at the switch in the flipflop.)
+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 the
Comparator to send a “down” signal to FF.
Step 2: The output voltage at (pin 3) goes up
and 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 voltag
at pin 5.
Step 6: The comparator sends a “high” signal
to the flip flop.
Step 7: The voltage of Pin 7 goes to ground an
The 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