Timer

Behavioral model of a timer integrated circuit

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Libraries:
Simscape / Electrical / Integrated Circuits

Description

The Timer block is a behavioral model of a timer integrated circuit such as the NE555.

The following figure shows the implementation structure.

The Potential divider component resistance parameter sets the values of the three resistors creating the potential divider. The two comparator inputs have infinite input resistance and zero input capacitance. The S-R Latch block provides the functionality of the set-reset latch. It includes an output capacitor and a resistor with values set to match the Propagation delay parameter value. The block models the output stage inverter using a CMOS NOT block. You define the output resistance, low-level output voltage, and high-level output voltage for the CMOS gate in the Timer block dialog box. The discharge switch approximates the NPN bipolar transistor on a real timer as a switch with defined switch on-resistance and off-resistance values.

Examples

PWM Circuit Using 555 Timer

A pulse-width-modulated (PWM) output implemented using a 555 Timer in astable mode. The duty cycle is set by a potentiometer, P1. The potentiometer is controlled during run-time via Duty Cycle Control Knob. The scope shows the resultant output from the 555 Timer. To end the simulation, click on the Stop button.

• Open Model

Assumptions and Limitations

• The behavior is abstracted. Results are not as accurate as a transistor-level model.

• Delay in response to changing inputs depends solely upon the RC time constant of the resistor-capacitor network at the output of the latch. In practice, the delay has a more complex dependency on the device structure. Set this value based on the output-pulse rise and fall times.

• The drop in output voltage is a linear function of output current. In practice, the relationship is that of a bipolar transistor push-pull pair.

• The controlled switch arrangement used by the block is an approximation of an open-collector arrangement.

• The power supply connects internally within the component, and the block assumes that the GND pin is grounded.

Ports

Conserving

THRES — Threshold pin

electrical

Electrical conserving port associated to the timer threshold pin

TRIG — Trigger pin

electrical

Electrical conserving port associated to the timer trigger pin

CONT — Control pin

electrical

Electrical conserving port associated to the timer control pin

RESET — Reset pin

electrical

Electrical conserving port associated to the timer reset pin

OUT — Output pin

electrical

Electrical conserving port associated to the timer output pin

DISCH — Discharge pin

electrical

Electrical conserving port associated to the timer discharge pin

Parameters

Supply

Power supply voltage — Power supply voltage

15 V (default)

The voltage value $$V_{cc}$$ that the block applies internally to the timer component.

Outputs

Low level output voltage — Low level output voltage

0 V (default)

The output voltage when the timer output is low and no output current is drawn.

High level output voltage — High level output voltage

14.1 V (default)

The output voltage $V_{OH}$ when the timer output is high and no current is drawn.

Output resistance — Output resistance

8 Ohm (default)

The ratio of output voltage drop to output current. Set this parameter to $(V_{OH}-V_{OH}{}_1)/I_{OH}{}_1$, where $V_{OH}{}_1$ is the reduced output high voltage when the output current is $I_{OH}{}_1$.

Propagation delay — Propagation delay

100e-9 s (default)

Set this value to the input-pulse or output-pulse rise time.

Discharge

Discharge switch on-resistance — Discharge switch on-resistance

12 Ohm (default)

A representative value is the discharge pin saturation voltage divided by the corresponding current.

Discharge switch off-resistance — Discharge switch off-resistance

500e6 Ohm (default)

A representative value is the discharge pin leakage current divided by the corresponding pin voltage.

Potential Divider

Potential divider component resistance — Potential divider component resistance

5 kOhm (default)

A typical value for a 555-type timer is 5 kΩ. You can measure it directly across the positive supply and control pins when the chip does not connect to a circuit.

Extended Capabilities

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

Version History

Introduced in R2009b