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S-R Latch

Behavioral model of an S-R Latch

• Library:
• Simscape / Electrical / Integrated Circuits / Logic

Description

The S-R Latch block is an abstracted behavioral model of a set-reset latch. It does not model the internal individual MOSFET devices (see Assumptions and Limitations for details). Therefore, the block runs quickly during simulation but retains the correct I/O behavior.

If the gate voltage is greater than the threshold voltage ${V}_{TH}$, then the input taken is `1` (`HIGH`). Otherwise, the input is zero (`LOW`). The gate threshold voltage ${V}_{TH}$ is halfway between the Low level input voltage (${V}_{IL}$) and High level input voltage (${V}_{IH}$) parameters.

The block output logic level is either HIGH or LOW, according to the logic levels of the gate inputs and the S-R latch truth table.

SR Q n
00 Q n-1
010
101
111

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 initial conditions depend on the output model selected:

• For `Linear` model, the high initial condition corresponds to the High level output voltage parameter value, `V_OH`, and the low initial condition corresponds to the Low level output voltage parameter value, `V_OL`.

• For `Quadratic` model, the high initial condition is a value close to the Supply voltage parameter value, `Vcc`, and the low initial condition is close to `0`.

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:

• The behavior of this block is abstracted. In particular, response to input noise and inputs that are around the logic threshold voltage can be inaccurate. Also, dynamic response is approximate.

• The linear drop in output voltage as a function of output current is an approximation to the MOSFET or bipolar output behavior.

• Modeling of the output as a controlled voltage source is representative of a totem-pole or push-pull output stage. To model a device with an open-collector:

1. Connect the output pin to the base of an NPN Bipolar Transistor or PNP Bipolar Transistor block.

2. Set the Output resistance parameter to a suitable value.

Ports

Conserving

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Electrical conserving port associated with the S-R latch set pin.

Electrical conserving port associated with the S-R latch reset pin.

Electrical conserving port associated with the S-R latch output pin.

Parameters

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Inputs

Voltage value less than which the block interprets the input voltage as `LOW`.

Voltage value greater than which the block interprets the input voltage as `HIGH`.

Fixed capacitance that approximates the input capacitance for a MOSFET gate. You can usually find this capacitance value on a manufacturer datasheet. Setting this value to zero can result in faster simulation times.

Outputs

Select the output model:

• `Linear` - This is the default value.

• `Quadratic`

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.

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.

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 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.

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.

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.

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.

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.

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.

Time it takes for the output to swing from `LOW` to `HIGH` or `HIGH` to `LOW` after the input logic levels change.

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.

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

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.

See Also

Introduced in R2009b

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