function tot = tot_sq_err(x)
% Calculate total squared error between diode model and 
%  measured data.
%
% Requires globally defined arrays:
%  v_diode = array of measured voltages for data points
%  i_diode = array of measured currents for data points
% Example:
%  If the data pts as ordered pairs of (voltage, current) were:
%   (0.0, 0.13), (0.1, 0.24), (0.2, 0.37).
%  The corresponding arrays would be:
%   v_diode = [0.0, 0.1, 0.2];
%   i_diode = [0.13, 0.24, 0.37];

global v_diode
global i_diode

% Extract Is and T from array x that is function argument.
Is = x(1);
T = x(2);

% Define physical constants.
k = 1.38e-23;  % J/degK Boltzmann constant
q = 1.602e-19; % C electron charge

% Clear the total squared error variable.
tot = 0;

% Add up the squared errors for all the data points.
for ind = 1 : length(v_diode)

  % Calculate the diode current from the model.
  i_model = Is * (exp(v_diode(ind) / (k*T/q)) - 1);

  % Square the error in the model prediction for this data point.
  i_err_sq = (i_diode(ind) - i_model)^2;

  % Add the squared error to the running sum.
  tot = tot + i_err_sq;
end

return