Model an S-R Latch behaviorally

Logic

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 Basic 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.

S | R | Q |
---|---|---|

0 | 0 | 1 |

0 | 1 | 0 |

1 | 0 | 1 |

1 | 1 | 1 |

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 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:

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

Set the

**Output resistance**parameter to a suitable value.

**Low level input voltage**Voltage value less than which the block interprets the input voltage as

`LOW`

. The default value is`2`

V.**High level input voltage**Voltage value greater than which the block interprets the input voltage as

`HIGH`

. The default value is`3`

V.**Average input capacitance**Fixed capacitance that approximates the input capacitance for a MOSFET gate. 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`

.

This block has the following ports:

`S`

Electrical input port corresponding to the set pin

`R`

Electrical input port corresponding to the reset pin

`Q`

Electrical output port corresponding to the output pin

Was this topic helpful?