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Industrial Strength Simulation




Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

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Digital Clock Macro


The digital clock macro provides a simple method for implementing a digital clock stimulus in a schematic. It eliminates the complexity some people run into when trying to define a clock stimulus in the traditional way.

The traditional way of defining a digital stimulus with a clock output is to use the Stim1 component. This component is placed in the schematic and has its COMMAND attribute defined with a symbolic name. Any valid text string may be used for the name. In this case, we will use the name Clock. The symbolic name then needs to be specified using a define statement such as the following:

.define Clock
+ Label = Start
+ 0ns 0
+ +50ns 1
+ +50ns Goto Start -1 Times

The define statement can be specified in the text area or directly within the Attribute dialog box of the Stim1 component. In this case, the digital stimulus has been defined as a clock with a 100ns period and a 50% duty cycle. At 0ns, the output of the clock is set to a zero state. 50ns later, the clock output is set to a one state. 50ns after that, the Start label is returned to and the sequence begins again. The 'Goto Start -1 Times' specifies that this loop will repeat forever.

The shortcut for this technique is to use the digital clock macro whose circuit is displayed below.

Digital Clock Macro Circuit

The digital macro circuit consists of just one Stim1 digital stimulus source. There are two parameters defined for this macro: Zerowidth and Onewidth. The Zerowidth parameter defines the time in seconds that the clock will be at the zero state for each period of the clock cycle. The Onewidth parameter defines the time in seconds that the clock will be at the one state for each period of the clock cycle. The period of the clock cycle is the sum of the Zerowidth and Onewidth parameters. The COMMAND attribute for the Stim1 source has been defined as CLOCK. In the text area of the macro circuit, there is a corresponding define statement as follows:

.define CLOCK
+ Label = Start
+ 0 0
+ +Zerowidth 1
+ +Onewidth Goto Start -1 times

The two macro parameters are used here to define the transition times of the stimulus output. In this case, the stimulus output starts at the zero state at time=0. After Zerowidth seconds, the output transitions to the one state. In another Onewidth seconds, the Goto statement is triggered and sets the stimulus output back to the zero state. The Goto loop is set to repeat indefinitely.

That is the entire macro. While simple, it clearly demonstrates one of the advantages of using macros in being able to simplify operations. Now, only two parameters need to be specified to create a digital clock of any period with any duty cycle in a schematic.

The digital clock macro is used in the circuit displayed below. The macro, X1, is used as a clock input to the first JK flip-flop of a 4-bit binary counter. Standard Stim1 sources are used to define the digital waveform inputs for the Clear and Preset pins of all four JK flip-flops. The digital clock macro has its parameters defined as:

Zerowidth = 40n
Onewidth = 60n

These parameters define a digital clock with a period of 100ns whose one state has a 60% duty cycle.

Digital Clock Macro Example Circuit

The resulting analysis is displayed below. The d(CLK) expression plots the output of the digital clock macro. Two horizontal tags have been placed to show the period of the clock and the width of the one state during that period. The width of the one state is shown as 60ns, and the period is shown as 100ns which both match the specifications defined for the macro.

Digital Clock Macro Analysis

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