D Latch

Behavioral model of D latch

Since R2024a

• 

Libraries:
Simscape / Electrical / Integrated Circuits / Logic

Description

The D Latch block implements a behavioral model of a D-type latch. The block stores a one-bit value, either 0 (low) or 1 (high).

The block has two input ports that control the latch: the data pin D and the enable pin E. When the logic level at E is 1, the block transfers the data to the output pin Q. When the logic level at E is 0, the block holds the output in its previous state. This truth table summarizes the logic levels at the inputs and output. The subscript n represents the value at the current time step and n-1 represents the value at the previous time step. The logic level at the inverted output pin !Q is the inverse of the logic level at Q.

En	Dn	Qn
0	0	Qn-1
0	1	Qn-1
1	0	0
1	1	1

To specify the logic level at Q at the start of the simulation, in the Initial Targets section, set the value of the Output logic parameter to 0 or 1. The block uses this value as the initial logic level at Q only if the initial logic level at E is 0. In this situation, the block normally holds the output in its previous state. For more information about initial targets, see Set Priority and Initial Target for Block Variables.

The gate inputs have an infinite resistance and a finite or zero capacitance.

If the gate voltage is greater than the threshold voltage, then the block takes the logic level at the input as 1. Otherwise, the block takes the logic level at the input as 0. The block calculates the threshold voltage as the average of the Low level input voltage and the High level input voltage parameter values.

The gate output depends on the Output current-voltage relationship parameter. Set this parameter to Linear for faster simulation and Quadratic for more accurate results. For most models, use the Linear option. You can use the Quadratic option to validate the results that you obtain using the simpler linear model. For more information, see Selecting the Output Model for Logic Blocks.

If you set the Output current-voltage relationship parameter to Linear:

• If the logic level at the Q port is 0, the output voltage is equal to the Low level output voltage parameter value.

• If the logic level at the Q port is 1, the output voltage is equal to the High level output voltage parameter value.

• The resistor-capacitor time constant of the gate output capacitor is equal to the Propagation delay parameter value.

If you set the Output current-voltage relationship parameter to Quadratic:

• The output voltage is a function of the output current. For zero-load current:

  • If the logic level at the Q port is 0, the output voltage is zero.

  • If the logic level at the Q port is 1, the output voltage is equal to the Supply voltage parameter value.

• The gate input demand lags to approximate the Propagation delay parameter value.

For more information, see Quadratic Model Output and Parameters.

Plot Input and Output Waveforms

You can plot the input and output waveforms of the D Latch block without building a complete model. Use the plots to explore the impact of your parameter choices on device characteristics. If you parameterize the block from a datasheet, you can compare your plots to the datasheet to check that you parameterized the block correctly. If you have a complete working model but do not know which manufactured part to use, you can compare your plots to datasheets to help you decide.

To plot the basic characteristics, right-click the block and select Electrical > Basic characteristics from the context menu.

Note

The plots you create using the Basic characteristics option do not show the effect of the Propagation delay parameter on the output voltage of logic blocks.

Assumptions and Limitations

• To improve simulation speed, the block does not model all the internal metal-oxide-semiconductor field-effect transistor (MOSFET) devices in the gate individually. The block therefore does not accurately model the response of the gate to input noise and input voltages that are close to the threshold voltage.

• If you are modeling a circuit that has a feedback path around a set of logic gates, you must set the Propagation delay parameter to a nonzero value on one or more of the gates.

Ports

Conserving

D — Data pin

electrical

Electrical conserving port associated with the data pin.

E — Enable pin

electrical

Electrical conserving port associated with the enable pin.

Q — Output pin

electrical

Electrical conserving port associated with the output pin.

!Q — Inverted output pin

electrical

Electrical conserving port associated with the inverted output pin.

Parameters

To edit block parameters interactively, use the Property Inspector. From the Simulink^® Toolstrip, on the Simulation tab, in the Prepare gallery, select Property Inspector.

Inputs

Low level input voltage — Input voltage below which logic is 0

2 V (default) | nonnegative scalar

Input voltage below which the block interprets the input logic as 0.

High level input voltage — Input voltage above which logic is 1

3 V (default) | positive scalar

Input voltage above which the block interprets the input logic as 1.

Average input capacitance — Input capacitance for MOSFET gate

5 pF (default) | nonnegative scalar

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 parameter produces a rise time similar to that of the MOSFET. You can find this capacitance value on a manufacturer datasheet. For fast simulation, set this parameter to zero.

Outputs

Output current-voltage relationship — Model for output current-voltage relationship

Linear (default) | Quadratic

Model that determines the relationship between the output current and output voltage. Set this parameter to Linear for faster simulation and Quadratic for more accurate results. For more information, see Selecting the Output Model for Logic Blocks.

Low level output voltage — Output voltage when logic is 0

0 V (default) | nonnegative scalar

Output voltage when the logic level at the output pin is 0.

Dependencies

To enable this parameter, set Output current-voltage relationship to Linear.

High level output voltage — Output voltage when logic is 1

5 V (default) | positive scalar

Output voltage when the logic level at the output pin is 1.

Dependencies

To enable this parameter, set Output current-voltage relationship to Linear.

Output resistance — Output resistance

25 Ohm (default) | nonnegative scalar

Output resistance that the block uses to model the drop in output voltage resulting from the output current. You can calculate this value from a datasheet by dividing the high-level output voltage by the maximum low-level output current.

If you set the Propagation delay parameter to zero, you must set the Output resistance parameter to a positive value.

Dependencies

To enable this parameter, set Output current-voltage relationship to Linear.

Supply voltage — Supply voltage

5 V (default) | positive scalar

Voltage you supply to the gate in the circuit.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Measurement voltage — Measurement supply voltage

5 V (default) | positive scalar

Gate supply voltage at which you quote the output resistances and currents.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Logic HIGH output resistance at zero current and at I_OH — Resistance when output logic is 1

[25, 250] Ohm | 2-D vector of positive scalars

Resistance when the logic level at the output is 1. Specify this parameter as a row vector of two resistance values [R_OH1, R_OH2]. The first value, R_OH1, is the gradient of the output voltage-current relationship when there is no output current. The second value, R_OH2, is the gradient of the output voltage-current relationship when the output is shorted to ground.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Logic HIGH output current I_OH when shorted to ground — Output current when logic is 1

63 mA (default) | positive scalar

Output current when the logic level at the output is 1, but the load forces the output voltage to zero.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Logic LOW output resistance at zero current and at I_OL — Resistance when output logic is 0

[30, 800] Ohm (default) | 2-D vector of positive scalars

Resistance when the logic level at the output is 0. Specify this parameter as a row vector of two resistance values [R_OL1, R_OL2]. The first value, R_OL1, is the gradient of the output voltage-current relationship when there is no output current. The second value, R_OL2, is the gradient of the output voltage-current relationship when the load forces the output voltage to the Supply voltage parameter value.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Logic LOW output current I_OL when shorted to Vcc — Output current when logic is 0

-45 mA (default) | negative scalar

Output current when the logic level at the output is 0, but the load forces the output voltage to the Supply voltage parameter value.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Propagation delay — Time for output voltage to change states

25 ns (default) | nonnegative scalar

Time it takes for the output voltage to change from the low to high or high to low states after the input logic levels change.

If you set the Propagation delay parameter to zero, you must set the Output resistance parameter to a positive value.

Protection diode on resistance — Protection diode on resistance

5 Ohm (default) | positive scalar

Gradient of the voltage-current relationship for the protection diodes when they are forward biased.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Protection diode forward voltage — Protection diode forward voltage

0.6 V (default) | positive scalar

Voltage above which the protection diode turns on.

Dependencies

To enable this parameter, set Output current-voltage relationship to Quadratic.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2024a