Schmitt Trigger
Behavioral model of Schmitt trigger
Description
The Schmitt Trigger block implements a behavioral model
of Schmitt trigger.
The block output logic level is HIGH when the input rises above the
High level input voltage
(VIH) value and does not go
LOW until the input falls below the lower-valued Low
level input voltage (VIL)
value. This logic implements a hysteresis characteristic between input and
output.
In the graphic, VOH and
VOL correspond to the and values, respectively.
The next figure shows a sample output of the block with parameters
VIH = 2V,
VIL = -2V,
VOH = 3V, and
VOL = -3V.
The block determines the logic levels of the gate inputs as follows:
If the gate voltage is greater than the threshold voltage, the block
interprets the input as logic 1.
Otherwise, the block interprets the input as logic 0.
The threshold voltage is the voltage value at midpoint between
the High level input voltage parameter value and the Low
level input voltage parameter value.
Note
To improve simulation speed, the block does not model all the internal individual
MOSFET devices that make up the gate. See Assumptions and Limitations for details.
The block models the gate as follows:
The gate inputs have infinite resistance and finite or zero
capacitance.
The gate output offers a selection of two models:
Linear and Quadratic.
For more information, see Selecting the Output Model for Logic Blocks. Use the
Output current-voltage relationship parameter to
specify the output model.
You can specify propagation delay for both output models. For
Linear output, the block sets the value of the
gate output capacitor such that the resistor-capacitor time constant equals the
Propagation delay parameter value. For
Quadratic output, the gate input demand is lagged
to approximate the Propagation delay parameter
value.
The block output voltage depends on the output model selected:
For Linear model, output high is the High
level output voltage parameter value, and output low is the
Low level output voltage parameter value.
For Quadratic model, the output voltage for High
and Low states is a function of the output current, as explained in Quadratic Model Output and Parameters. For zero load current,
output high is Vcc (the Supply voltage parameter value),
and output low is zero volts.
Assumptions and Limitations
The block does not model the internal individual MOSFET devices that make up the gate
(except for the final MOSFET pair if you select the Quadratic
option for the Output current-voltage relationship parameter). This
limitation has the following implications:
For circuits that involve a feedback path around a set of logic gates, you might need
to set a nonzero propagation delay on one or more gates.
This block is implemented using event equations. This means that you must provide an
initial output state that is consistent with the block input at time zero. For example,
if you set initial output state HIGH, but the initial input voltage is below the
Low level input voltage, then the initial output stays HIGH,
the state only correcting itself when the input voltage rises above the High
level input voltage value.
Ports
Conserving
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A — Input port
electrical
Electrical conserving port associated with the Schmitt Trigger
input.
J — Output port
electrical
Electrical conserving port associated with the Schmitt Trigger
output.
Parameters
expand all
Inputs
Low level input voltage — Low level input voltage
2
V (default)
Voltage value below which the block interprets the input voltage as
logic LOW.
High level input voltage — High level input voltage
3
V (default)
Voltage value above which the block interprets the input voltage as
logic HIGH.
Average input capacitance — Average input capacitance
5
pF (default)
Fixed capacitance that approximates the input capacitance for a MOSFET
gate. The MOSFET capacitance depends on the applied voltage. When you
drive this block with another gate, the Average input
capacitance produces a rise time similar to that of the
MOSFET. You can usually find this capacitance value on a manufacturer
datasheet. Setting this value to zero may result in faster simulation
times.
Outputs
Output current-voltage relationship — Output current-voltage relationship
Linear (default) | Quadratic
Low level output voltage — Low level output voltage
0
V (default)
Voltage value at the output when the output logic level is
LOW.
Dependencies
This parameter is visible only when you select
Linear for the Output
current-voltage relationship parameter.
High level output voltage — High level output voltage
5
V (default)
Voltage value at the output when the output logic level is
HIGH.
Dependencies
This parameter is visible only when you select
Linear for the Output
current-voltage relationship parameter.
Output resistance — Output resistance
25
Ohm (default)
Value of the series output resistor that is used to model the drop in
output voltage resulting from the output current. You can derive this
value from a datasheet by dividing the high-level output voltage by the
maximum low-level output current.
Dependencies
This parameter is visible only when you select
Linear for the Output
current-voltage relationship parameter.
Supply voltage — Supply voltage
5
V (default)
Supply voltage value applied to the gate in your circuit.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Measurement voltage — Measurement voltage
5
V (default)
The gate supply voltage for which mask data output resistances and
currents are defined.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Logic HIGH output resistance at zero current and at I_OH — Logic HIGH output resistance at zero current and at I_OH
[25, 250]
Ohm
A row vector [ R_OH1R_OH2 ] of
two resistance values. The first value R_OH1 is the
gradient of the output voltage-current relationship when the gate is
logic HIGH and there is no output current. The second value
R_OH2 is the gradient of the output
voltage-current relationship when the gate is logic HIGH and the output
current is I_OH.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Logic HIGH output current I_OH when shorted to ground — Logic HIGH output current I_OH when shorted to ground
63
mA (default)
The resulting current when the gate is in the logic HIGH state, but
the load forces the output voltage to zero.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Logic LOW output resistance at zero current and at I_OL — Logic LOW output resistance at zero current and at I_OL
[30, 800]
Ohm (default)
A row vector [ R_OL1R_OL2 ] of
two resistance values. The first value R_OL1 is the
gradient of the output voltage-current relationship when the gate is
logic LOW and there is no output current. The second value
R_OL2 is the gradient of the output
voltage-current relationship when the gate is logic LOW and the output
current is I_OL.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Logic LOW output current I_OL when shorted to Vcc — Logic LOW output current I_OL when shorted to Vcc
-45
mA (default)
The resulting current when the gate is in the logic LOW state, but the
load forces the output voltage to the supply voltage Vcc.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Propagation delay — Propagation delay
25
ns (default)
Time it takes for the output to swing from LOW to
HIGH or HIGH to
LOW after the input logic levels change.
Protection diode on resistance — Protection diode on resistance
5
Ohm (default)
The gradient of the voltage-current relationship for the protection
diodes when forward biased.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Protection diode forward voltage — Protection diode forward voltage
0.6
V (default)
The voltage above which the protection diode is turned on.
Dependencies
This parameter is visible only when you select
Quadratic for the Output
current-voltage relationship parameter.
Initial Conditions
Output initial state — Output initial state
Low (default) | High
Specify whether the initial output state of the block is
High or Low.
This parameter is used for both linear and quadratic output states,
provided that the Propagation delay parameter is
greater than zero and the Solver
Configuration block does not have the
Start simulation from steady state option
selected.
Extended Capabilities
C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.
Introduced in R2015a
