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Nexxim Simulator >
Nexsys Discrete Time Domain Analysis >
   Nexsys MATLAB User Defined Models >
       Example MATLAB Model Files >
           Digital Clock Model

Digital Clock Model

Here is a model for a digital clock source. This SOURCE type model has no inputs and one output. Here is a listing of the model file, dclock.m, with explanations.

The initial comment line (identified by %NEXSYS) is the model definition line. Nexsys reads the model definition into a structure that contains various information the simulator needs.

% NEXSYS FUNCTIONAL_MODEL ("dclock", "0","", "", "1","r", "1","NO_DATA_REQ", "NOT_APPLICABLE", "PERIOD,DELAY=0,DUTY=0,NSAMP,SAMPLE_RATE")

Here are explanations for the entries in the dclock model definition

“dclock”	Model name. The model name, dclock, is also the Matlab function name.
“0”	No inputs
“”	No input data types
“”	No input number of samples
“1”	One output
“r”	The single output is real
“1”	One (1) sample is to be generated per invocation
“NO_DATA_REQ”	No extra data reference is required
“NOT_APPLICABLE”	No probe domain applies
"PERIOD,DELAY=0,DUTY=0,NSAMP,SAMPLE_RATE"	Parameters for the model

Here is the Matlab function declaration for the digital clock. The function name, dclock in this example, is the same as model name.

function [ y, SV ] = dclock (p, SV, status_in)

The input and output variables for the function are:

p	Parameter values of the model
SV	State variable (in)
status_in	Input status
y	Output signal
SV	State variable (out)

The first step is to read the values from the parameters passed to the function.

% Get parameters from function argument p

period = p(1)
delay = p(2)
duty = p(3)
sample_rate = p(5)
time_step = 1.0/p(5)

During the initialization call, the dclock model sets up its state variables structure SV. During later calls, the model retrieves the data.

total_num_out = 0;

% (status_in == 0) => Analysis initialization stage

if status_in == 0

delay_mode = 0;

if (delay > 0)

delay_mode = 1;

end

time_pos = 0.0;

total_out_num = 0;

SV = [delay_mode time_pos total_num_out]

else

delay_mode = SV(1);

time_pos = SV(2);

total_num_out = SV(3);

end

Here is how dclock calculates the output (digital clock) values:

out_value = 0.0;

if delay_mode == 1

time_elapsed = (total_num_out + 1) / sample_rate;

if (delay > time_elapsed)

out_value = 0.0;

else

delay_mode = 0;

end

else

if ((time_pos>=0) && (time_pos < (duty * period)))

out_value = 1.0;

else

out_value = 0.0;

end

%re-calculate the time position

time_pos = time_pos + 1.0 / sample_rate;

if (time_pos >= period)

time_pos = time_pos - period;

elseif (time_pos == period)

time_pos = 0.0;

end

y(1) = out_value;

total_num_out = total_num_out + 1;

Finally, dclock saves its state variables SV and returns.

SV = [delay_mode time_pos total_num_out];

return

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