Behavioral model of Schmitt trigger

Logic

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** (*V*_{IH})
value and does not go `LOW`

until the input falls
below the lower-valued **Low level input voltage** (*V*_{IL})
value. This logic implements a hysteresis characteristic between input
and output.

In the graphic, *V*_{OH}
and *V*_{OL} correspond to the **High
level output voltage** and **Low level output
voltage** values, respectively.

The next figure shows a sample output of the block with parameters *V*_{IH} =
2V, *V*_{IL} = -2V, *V*_{OH} =
3V, and *V*_{OL} = -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.

To improve simulation speed, the block does not model all the internal individual MOSFET devices that make up the gate. See Basic 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.

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:

The block does not accurately model the gate's response to input noise and inputs that are around the logic threshold voltage.

The block does not accurately model dynamic response.

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.

**Low level input voltage**Voltage value below which the block interprets the input voltage as logic

`LOW`

. The default value is`2`

V.**High level input voltage**Voltage value above which the block interprets the input voltage as logic

`HIGH`

. The default value is`3`

V.**Average input capacitance**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. The default value is`5`

pF. Setting this value to zero can result in faster simulation times.

**Output current-voltage relationship**Select the output model,

`Linear`

or`Quadratic`

. The default value is`Linear`

.**Low level output voltage**Voltage value at the output when the output logic level is

`LOW`

. The default value is`0`

V. This parameter is available when you select the`Linear`

option for the**Output current-voltage relationship**parameter.**High level output voltage**Voltage value at the output when the output logic level is

`HIGH`

. The default value is`5`

V. This parameter is available when you select the`Linear`

option for the**Output current-voltage relationship**parameter.**Output resistance**Value of the series output resistor that is used to model the drop in output voltage resulting from the output current. The default value is

`25`

Ω. You can derive this value from a datasheet by dividing the high-level output voltage by the maximum low-level output current. This parameter is available when you select the`Linear`

option for the**Output current-voltage relationship**parameter.**Supply voltage**Supply voltage value applied to the gate in your circuit. The default value is

`5`

V. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.**Measurement voltage**The gate supply voltage for which mask data output resistances and currents are defined. The default value is

`5`

V. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.**Logic HIGH output resistance at zero current and at I_OH**A row vector [

*R_OH1**R_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*. The default value is`[ 25 250 ]`

Ω. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.**Logic HIGH output current I_OH when shorted to ground**The resulting current when the gate is in the logic

`HIGH`

state, but the load forces the output voltage to zero. The default value is`63`

mA. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.**Logic LOW output resistance at zero current and at I_OL**A row vector [

*R_OL1**R_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*. The default value is`[ 30 800 ]`

Ω. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.**Logic LOW output current I_OL when shorted to Vcc**The resulting current when the gate is in the logic

`LOW`

state, but the load forces the output voltage to the supply voltage Vcc. The default value is`-45`

mA. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.**Propagation delay**Time it takes for the output to swing from

`LOW`

to`HIGH`

or`HIGH`

to`LOW`

after the input logic levels change. The default value is`25`

ns.**Protection diode on resistance**The gradient of the voltage-current relationship for the protection diodes when forward biased. The default value is

`5`

Ω. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.**Protection diode forward voltage**The voltage above which the protection diode is turned on. The default value is

`0.6`

V. This parameter is available when you select the`Quadratic`

option for the**Output current-voltage relationship**parameter.

**Output initial state**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. The default value is`Low`

.

The block has the following ports:

`A`

Electrical input port

`J`

Electrical output port

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