Data Logging at regular interval. Rpm calculation from Encoder data

Program is Calculating rpm through encoder data which is in the form of series of binary data. Encoder resolution is 20 pulses/rev.

N=1000;                            %No.of data samples
encdata=zeros(1,N);                %array to store encoder data
rpm=0;
rpmdata=zeros(1,N);                %array to store rpm data
tic;                               %Start Timer

for i = 2:N;                     
  encdata(1,i)= ->coming from the encoder

    if xor(data(i-1),data(i))==1     %Detects change in state
       sum=sum+1;                    %Counts no. of pulses.
    end

    if mod(round(toc),60)== 0;       %Every 60 seconds
        rpm=sum/20;                  %since 20 pulses per rev
        sum=0;
    end

   rpmdata(1,i)=rpm;

end

This code is not working the way I am expecting it to. Sum calculations are correct so I know the code works for that 'if' loop. Although, the rpm loop does not work right, I presume it has something to do with toc.

It works if I replace

 if mod(round(toc),60)== 0;

with

 if mod(i,100)== 0; %Every 100th sample.

Please suggest if you have a better way to solve this problem :)