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How to use EasyEDA Schmitt Trigger input logic gates as an oscillator

4 years ago 1513
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Description

Using the EasyEDA SCHMITTINVEE Schmitt Trigger input Inverter gate and SCHMITTNAND2EE symbol and spice model from the EasyEDA Libs panel to form relaxation oscillators.

EasyEDA logic device models are different from the basic XSPICE logic gates in the ngspice because it does not need ADC and DAC Bridges to connect analogue signals, loads and probes into it.

It is also different from logic gates in many other simulation tools because it does require a power supply. This is so that if a simulation is run with the supply ramped up from zero - to assist ngspice in finding the right initial conditions for the simulation - or if the sim is run at different supply voltages, then the input threshold levels and the output voltage swing track the supply voltage. Behavioural logic devices in many other simulators do not track the supply voltage and this can cause confusion where logic devices with a wide supply range such as 74C and 4000 series or some of the low voltage CMOS devices are used with variable or poorly regulated supplies.

The supply pins for the device are named VDD for the positive and GND for the negative rails respectively. To reduce the wires needed to be shown in the schematic, the VDD and GND supply pins for the gate are hidden. To connect to them all that is needed is for a positive rail with a VDD netlabel and a negative rail with a GND symbol to exist in the schematic.

If the rails in the schematic have different names, such as RAIL and VSS then all that is required is to edit the supply pin names. to do this, select U1, press the 'I' hotkey, edit the supply pin names and then click 'OK'.

Alternatively, the rails can be wired directly to the device supply pins in the normal way. To do this the device pins must be made visible. Then the supply rail netlabels have no effect on the supply connectivity to the device.

To do this, select U1, press the 'I' hotkey and untick 'Hidden Pin' to show the supply pins.

Other features of this EasyEDA logic device are that input and output over-rail voltage excursions are clamped to the appropriate rail and so will cause excessive current flow as seen in a real device. A supply voltage above an absolute maximum of approximately 18V or a reverse polarity will also cause excessive current flow.

These simulation shows a basic relaxation oscillator and a simple gated version.

Note that either a .ic initial condition or a power supply ramp-up from zero may be required to kick-start the circuit into oscillation.

For more about this see:

https://easyeda.com/Doc/Simulation-eBook/Initial-conditions-and-starting-up-circuits.htm#Initial-conditions-and-starting-up-circuits

To run this simulation: CTRL+R

For more information about the SCHMITTINVEE model, do:

Super Menu > Miscellaneous > Netlist for Document > Spice...

and inspect the netlist.

Documents

SCHMITTNINVEE oscillator

SCHMITTNAND2EE gated oscillator

BOM

ID Name Designator Quantity
1 5 VRAIL 1
2 470p COSC 1
3 4.7k ROSC 1
4 SCHMITTINVEE U1 1
5 SCHMITTNAND2EE U1 1
6 PULSE(0 5 2u 100n 100n 15u) V1 1

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