CMOS Schmitt Trigger Test Circuit

Mitchell Belser, P.E.Visiting Instructor

Department of Computer EngineeringJackson State University

mbelser@ieee.org

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 2

2

3

1

V 1

2

3

1

R6

R7

R8

R9R10

v-m inus

input

v-p lus

Thermionic Trigger

output

Schmitt Trigger Implementations

• Comparator with positive feedback

• Bipolar transistor Schmitt trigger

• CMOS transistor Schmitt trigger

Comparator with Positive Feedback

U1

R11

R12

V inoutput

Comparator w ith Positive Feedback

Bipolar Schmitt Trigger

                                                                                                                      

                                                               

Upper switching point voltage:

Vth = R4/(R4+R3)*Vdd

Lower switching point voltage:Vtl = R4/(R4+R2)*Vdd Q 1

Q 2R1

R2 R3

R4 R5

vdd

input

output

BJT Schmitt Trigger

CMOS Schmitt Trigger

Upper switching point voltage:

β1/ β3 = W1L3 / W3L1

Lower switching point voltage:

β5/ β6 = W5L6 / W6L5

M 1

M 2

M 3

M 4

M 5

M 6

G ND

G ND

V DD

V DD

U1A U3A U4A outputinput

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

M 1

M 2

M 3

M 4

M 5

M 6

G ND

G ND

V DD

V DD

U1A U3A U4A outputinput

Schmitt Trigger Showing Effect of Test Circuitry

►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 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.

M 1

M 2

M 3

M 4

M 5

M 6

G ND

G ND

V DD

V DD

U4A outputinput

U2A

7408J

U5A

7408J

U6A

7402N

U7A

7402N

U8A

7432N

V TL

V TH

Te st_e n

M 7 M 8 U1A

sout

soutsout

input

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 STATIC RAMP FrequencyC VCL-A4-5 VTH VTL VTH VTL

5 V 4.21 2.92 4.2 3.08 1

4.24 3.04 1k

3.3 V 2.63 1.69 2.64 1.84 1

2.64 1.76 1k

A VCL-A4-3 5 V 4.22 2.98 4.24 3.12 1

4.28 3.06 1k

3.3 V 2.64 1.73 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.