Multivibrator Circuits
 Bistable Multivibrator (Flip-Flops)
 Astable Multivibrator (Clocks or Oscillators)
 Schmitt-Trigger Inverter (7414)
 555 Timer
 Crystal Oscillator
 Monostable Multivibrator (One-Shots)
 Non-Retriggerable (74121)
 Retriggerable (74123)
 VHDL Coding
ECE 3450
M. A. Jupina, VU, 2014
Some Key Lecture Objectives
 To learn how timing signals are generated at low
frequencies (< 1 MHz) and high frequencies (>1
MHz).
 To learn how clock and pulse signals are
generated either through the use of ICs or VHDL
coding.
 To gain a better understanding of how to apply
timing signals in a digital design.
ECE 3450
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Schmitt-Trigger Inverter
(a) If input transition times are too long, a standard logic device-output
might oscillate or change erratically; (b) a logic device with a Schmitttrigger type of input will produce clean, fast output transitions.
ECE 3450
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Schmitt-Trigger Inverter Operation
VOUT
VOH
VOL
VTL
VTH
VIN
• As VIN increases, VOUT = VOH until VIN > VTH
then VOUT = VOL
• When VIN begins to decrease, VOUT = VOL until VIN < VTL
then VOUT = VOH
ECE 3450
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Schmitt-Trigger Oscillator
IIN
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Analysis of a Schmitt-Trigger Oscillator
VC(t)
VTH
VTL
t
VOH
VOUT(t)
VOL
TL
TH
t
t=0
t = 0'
Assume Iin = 0, thus IR(t) = IC(t)
Capacitor Charging
Capacitor Discharging
VOL
ECE 3450
VC(t)
VOH
VC(t)
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Analysis of a Schmitt-Trigger Oscillator
Continued
Capacitor Discharging
Capacitor Charging
I.C. VC (0)  VTH
I.C. VC (0)  VTL
VC (TL )  VTL
VC (TH )  VTH
VOL  VC (t )
dVC (t )

 C
R
dt
 t RC
VC (t )  VOL  (VOL  VTH )e
VTL  VOL  (VOL  VTH )e
V
TL  RC ln( VTHTL VOLOL )
V
TL RC
VOH  VC (t )
dVC (t )
C
R
dt
 t RC
VC (t )  VOH  (VOH  VTL )e
VTH  VOH  (VOH  VTL )e
TH  RC ln(
VOH VTL
VOH VTH
TH RC
)
f OSC  T1  TL 1TH , Duty Cycle  TTH
ECE 3450
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Example: Show how to use a 74LS14 Schmitt-trigger inverter to
produce an approximate square wave with a frequency of 10 KHz.
Solution:
ECE 3450
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PSPICE Simulation of a Schmitt-Trigger Oscillator (7414)
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555 Timer as an Astable Multivibrator
1 – Ground
2 – Trigger
3 – Output
4 – Reset (Set HIGH for normal operation)
ECE 3450
5 – FM Input (Tie to gnd via bypass cap)
6 – Threshold
7 – Discharge
8 – Voltage Supply (+5 to +15 V)
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Block Diagram of a 555 Timer
Configured as an Oscillator
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555 Timer Block Diagram Contents
• Resistive voltage divider (equal resistors) sets threshold voltages
for comparators
V1 = VTH = 2/3 VCC
V2 = VTL = 1/3 VCC
• Two Voltage Comparators
- For A1, if V+ > VTH
- For A2, if V- < VTL
then R =HIGH
then S = HIGH
• RS FF
- If S = HIGH, then FF is SET, Q = LOW, Q1 OFF, VOUT = HIGH
- If R = HIGH, then FF is RESET, Q = HIGH, Q1 ON, VOUT = LOW
• Transistor Q1 is used as a Switch
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Timing Diagram of a 555 Oscillator
VC(t)
VTH
1
2
3
VTL
t
VCC
VOUT(t)
TL
TH
t = 0 t = 0'
ECE 3450
t
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Operation of a 555 Oscillator
1)
Assume initially that the capacitor is discharged.
a) For A1, V+ = VC = 0V and for A2, V- = VC = 0V, so
R=LOW, S=HIGH, Q = LOW , Q1 OFF, VOUT = VCC
b) Now as the capacitor charges through RA & RB,
eventually VC > VTL so R=LOW & S=LOW.
FF does not change state.
RA
VCC
ECE 3450
RB
VC(t)
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Operation of a 555 Oscillator
Continued
2)
Once VC  VTH
a) R=HIGH, S=LOW, Q = HIGH ,Q1 ON, VOUT = 0
b) Capacitor is now discharging through RB and Q1 to
ground.
c) Meanwhile at FF, R=LOW & S=LOW since
VC < VTH.
RB
VC(t)
Q1
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M. A. Jupina, VU, 2014
Operation of a 555 Oscillator
Continued
3)
Once VC < VTL
a) R=LOW, S=HIGH, Q = LOW , Q1 OFF, VOUT = VCC
b) Capacitor is now charging through RA & RB again.
RA
VCC
ECE 3450
RB
VC(t)
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Analysis of a 555 Oscillator
Capacitor Discharging
Capacitor Charging
I.C. VC (0)  VTH  23 VCC
I.C. VC (0)  VTL
VC (TL )  VTL  13 VCC
VC (TH )  VTH
VCC  VC (t )
dVC (t )
C
RA  RB
dt
VC (t )
dVC (t )
C
RB
dt
VC (t )  VTH e
1
3

VCC  32 VCC e
t
VC (t )  VCC  (VCC  VTL )e
RBC
TL RBC
2
3
TL  RB C ln(2)  0.69 RB C
f OSC  T1 
ECE 3450
1
0.69 C ( RA  2 RB )

t
VCC  VCC  (VCC  13 VCC )e
( R A  RB ) C
TH ( RA  RB ) C
TH  0.69( RA  RB )C
, Duty Cycle  TTH 
RA  RB
RA  2 RB
 0.5 or 50%
M. A. Jupina, VU, 2014
Example: Design a 555 Oscillator to produce an approximate
square-wave at 40 KHz. Let C > 470 pF.
Examples
One Possible F=40KHz; T=25µs; t1=t2=12.5µs
Solution:
For a square-wave RA<<RB; Let RA=1K and RB=10K
t1=0.693(RB)(C); 12.5µs=0.693(10K)(C); C=1800pF
T=0.693(RA+2RB)C: T=0.693(1K+20K)1800pF
T=26.2µs; F=1/T; F=38KHz (almost square-wave).
Example: A 555 oscillator can be combined with a J-K FF to
produce a 50% duty-cycle signal. Modify the above
circuit to achieve a 50% duty-cycle, 40 KHz signal.
One Possible Reduce by half the 1800pF. This will create a T=13.1µs or F=76.35 KHz
Solution:
(almost square-wave). Now, take the output of the 555 Timer and connect
it to the CLK input of a J-K FF wired in the toggle mode (J and K inputs
connected to +5V). The result at the Q output of the J-K FF is a perfect
38.17 KHz square-wave.
ECE 3450
M. A. Jupina, VU, 2014
Problem 10.24 - The 555 Programmable Timer Chip
5V
4
8
Ra
Clock
(output)
3
2
555
Timer
7
Rb
6
C1
1
5
0.01m F
(a) For 50% duty cycle and 500 KHz frequency, C1 = 100 pF, then Ra = 1 kΩ and Rb = 14 kΩ
(b) For 75% duty cycle and 500 KHz frequency, C1 = 1000 pF, then Ra = 1.42 kΩ and Rb = 0.71 kΩ
ECE 3450
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PSPICE Simulation of a 555 Oscillator
ECE 3450
R1=R2, duty cycle = 66% and f =16 KHz
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Crystal Oscillator
• An oscillator circuit that uses a piezoelectric crystal.
• Piezoelectric materials support an exchange of energy
between mechanical compression and applied electric
field.
• If a potential is applied between the electrodes, forces
will be exerted on the bound charges within the crystal.
Deformations take place within the crystal and an
electromechanical system is formed which will vibrate
when properly excited.
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Crystal Oscillator, Continued
• The oscillation frequency (10 KHz – 80 MHz) is very
precisely determined by the physical dimensions of the
crystal.
• High Q (1000 – 10,000) resonators are usually built using
quartz single crystal or ceramic materials.
• These resonators are extremely stable (ppm’s) with respect
to time and temperature.
• To generate clock frequencies into the GHz range, DPLL
(Digital Phase Lock Loops) are used for frequency
multiplication.
XTAL
OSC
ECE 3450
fo
DPLL
N•fo
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Crystal Oscillator Circuit Example
crystal oscillator symbol
and equivalent circuit
Pierce oscillator circuit
Steady-state oscillation occurs
at the frequency where
Yi  YN  0
equivalent circuit of Pierce oscillator
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Example: 50 MHz Oscillator on the
Altera DE2 Board
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Clock Divider
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Non-Retriggerable One-Shot (OS)
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Comparison of Non-Retriggerable and
Retriggerable OS responses for tPULSE = 2 ms
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Example: Refer to the waveforms in (a) on previous page.
Change the OS pulse duration to 0.5 ms and determine the Q
output for both types of OS. Then repeat using a pulse duration of
1.5 ms.
Example
Solution:
ECE 3450
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74121 Non-Retriggerable OS
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Example: (a) Show how a 74121 can be connected to produce a
negative-going pulse with a 5 ms duration whenever
A1 or A2 is connected to a negative-going trigger.
(b) Modify the circuit so that when a signal “G” goes low
it can be used to disable the 74121.
Example
Solution:
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74123 Dual One Shot PSPICE Simulation
tw = 0.33 RT Cext
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VHDL Nonretriggerable One-Shot
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Simulation of the Nonretriggerable One-Shots
Comparison of this VHDL OS to an IC OS
– VHDL OS output does not change state until a rising edge clock signal
occurs whereas an IC OS output changes “immediately” when triggered.
– The trigger signal for the VHDL OS must be active during a rising clock
edge so as to be recognized (the IC OS trigger is an asynchronous signal,
i.e., it can occur whenever).
– The VHDL OS is level-triggered whereas the IC OS is edge-triggered.
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Detecting Edges
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VHDL Retriggerable One-Shot with
Edge Trigger
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Simulation of the Edge-Triggered Retriggerable
One-Shot
Comparison to Previous VHDL OS
– Just as the case before, the output of this OS does not change state until a
rising edge clock signal occurs.
– Unlike the previous OS, this OS is edge-triggered.
– Just as the case before, the trigger signal for this OS must be active during
a rising clock edge so as to be recognized.
ECE 3450
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Final Notes on VHDL OS
• To minimize the effects of a delayed output pulse
with respect to a rising edge trigger, the clock
frequency and the initial value of the count can be
increased so that an output pulse of the same
width is produced.
• An asynchronously triggered OS can be created in
VHDL but the output pulse generated will
fluctuate in width by up to one clock period.
ECE 3450
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Example of a Dual One Shot Circuit
trigger (10 KHz)
clock (100 KHz)
Q1
Q2
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-- retriggerable edge-triggered one-shot
-- time delay = delay * clock period
OS1 VHDL
LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.all;
USE IEEE.STD_LOGIC_ARITH.all;
USE IEEE.STD_LOGIC_UNSIGNED.all;
ENTITY os1 IS
PORT (
clock, trigger, reset
delay
q
END os1;
: IN BIT;
: IN INTEGER RANGE 0 TO 15;
: OUT BIT);
ARCHITECTURE a OF os1 IS
BEGIN
PROCESS (clock, reset)
VARIABLE count
: INTEGER RANGE 0 TO 15;
VARIABLE trig_was
: BIT;
BEGIN
IF reset = '0' THEN count := 0;
ELSIF (clock'EVENT AND clock = '0' ) THEN
IF trigger = '0' AND trig_was = '1' THEN
count := delay;
trig_was := '0';
ELSIF count = 0 THEN count := 0;
ELSE count := count - 1;
END IF;
IF trigger = '1' THEN trig_was := '1';
END IF;
END IF;
IF count /= 0 THEN q <= '1';
ELSE q <= '0';
END IF;
END PROCESS;
END a;
ECE 3450
-- load counter
-- "remember" edge detected
-- hold @ 0
-- decrement
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