Download CMOS Schmitt Trigger Test Circuit

CMOS Schmitt Trigger
Test Circuit
Mitchell Belser, P.E.
Visiting Instructor
Department of Computer Engineering
Jackson State University
[email protected]
What is a Schmitt Trigger
• A type of comparator.
• A comparator is an electronic circuit whose
output state changes when its input
reaches a certain value.
• A comparator’s output state will change at
the same input value whether the input
voltage is increasing or decreasing.
• A noisy signal can cause the output to
change state randomly.
Schmitt Trigger Operation
• The Schmitt trigger is a comparator circuit that
has two threshold voltages.
• When the input is higher than the upper
threshold, the output is high.
• When the input is lower than the lower threshold,
the output is low.
Schmitt Trigger Operation
• The Schmitt trigger employs positive
feedback.
• When the input is between the two
thresholds the output state does not
change.
• The difference between the two thresholds
is the hysteresis.
Schmitt Trigger Transfer Curve
Schmitt Trigger History
• The Schmitt trigger was first invented by Otto H.
Schmitt in the 1934.
• Vacuum Tube based design.
• A Thermionic Trigger, Journal of Scientific
Instruments, 15 January 1938 pg 24-26
Schmitt Trigger
v-minus
v-plus
R10
R7
R9
output
2
R8
3
V1
1
2
input
3
V2
1
R6
Thermionic Trigger
Schmitt Trigger Implementations
• Comparator with positive feedback
• Bipolar transistor Schmitt trigger
• CMOS transistor Schmitt trigger
Comparator with Positive Feedback
R11
U1
R12
output
Vin
Comparator
with Positive Feedback
Bipolar Schmitt Trigger
vdd
Upper switching point
voltage:
R2
R3
output
Vth = R4/(R4+R3)*Vdd
Q2
R1
Lower switching point
voltage:
Vtl = R4/(R4+R2)*Vdd
Q1
input
R4
R5
BJT Schmitt Trigger
CMOS Schmitt Trigger
VDD
Upper switching point
voltage:
M5
β1/ β3 = W1L3 / W3L1
M6
M4
Lower switching point
voltage:
β5/ β6 = W5L6 / W6L5
GND
U1A
input
M2
VDD
M3
M1
GND
U3A
U4A
output
Applications
• Input buffers for digital signals.
• Crystal oscillator circuits.
• Clean up noisy signals (switch debounce).
• Speed up signals with slow edges.
Problem Description and Motivation
for Improvement
The conventional method for characterizing a CMOS Schmitt trigger is to ramp up the
input voltage until the output state changes. The input voltage that results in the
output changing state is the high threshold voltage VTH. In a similar manner, the low
threshold voltage VTL is determined by reducing the input voltage until a state
change occurs at the output. This method is widely used. The major draw back is
the time that is required to search for the input voltage that results in a state change
at the output.
The proposed method would eliminate the need to perform a ramp search and thus
greatly reduce the time needed to determine the switching threshold voltage levels.
Specific Improvements
Innovation : A CMOS Schmitt trigger circuit containing additional test circuitry that
allows trigger voltage levels to be detected at the input without needing to ramp the
input voltage.
Important components of innovation: The test circuitry consists of combinational
logic and a CMOS switch. Digital input signals determine whether the high or low
threshold voltage is measurable.
Traditional Schmitt Trigger Circuit
VDD
M5
M6
M4
GND
U1A
input
M2
VDD
M3
M1
GND
U3A
U4A
output
Schmitt Trigger Showing Effect of
Test Circuitry
VDD
►Test
circuitry is shown in blue.
►Asserting test mode and vih input
results in sout being shorted to the
input. The gates of the feedback
transistors are driven to vdd. The
input is regulated to the high
input
threshold voltage VTH.
►Asserting test mode and vil input
results in sout being shorted to the
input. The gates of the feedback
transistors are driven to gnd. The
input is regulated to the low
threshold voltage VTL.
►In normal operating mode the
test circuitry has no effect on
Schmitt trigger operation.
M5
M6
M4
GND
sout
sout
U6A
7402N
U7A
U4A
7402N
M2
U8A
VDD
7432N
M3
M1
U2A
7408J
VTL
Test_en
U5A
input
VTH
7408J
M7
GND
sout
M8
U1A
output
Advantages of This Method
Typical method requires input to be driven and output voltage to be monitored in order to
determine the switching threshold voltage levels.
Test circuitry allow thresholds to be measured at the input without driving input or
monitoring the output.
Simply setting a digital input causes the threshold voltage to be present on the input pin
of the Schmitt trigger. .
Simplifies test program and reduces test time resulting in increased profit margin.
Measurement Data
Part
VDD
C VCL-A4-5
VTH
5V
3.3 V
A VCL-A4-3
STATIC
5V
3.3 V
4.21
2.63
4.22
2.64
RAMP
VTL
2.92
1.69
2.98
1.73
VTH
Frequency
VTL
4.2
3.08
1
4.24
3.04
1k
2.64
1.84
1
2.64
1.76
1k
4.24
3.12
1
4.28
3.06
1k
2.66
1.80
1
2.64
1.72
1k
Measurement Waveforms
VDD= 3.3V
Ramp frequency =
1kHz
Summary
• Schmitt trigger threshold test circuit.
• Measure rising and falling threshold levels.
• Ramping input and monitoring output state is no
longer necessary.
• Reduction in test time.
• Increase in profit due to reduction in test time.