I need help fixing my Bisection method code.

I'm trying to find the root of t , using the bisection method, where P=Ce^kt, with a percent error of .05%,or .0005. Here's the code:

function [t]=HW7_JAS_1(P,k,C)
P1=P;
k1=k;
C1=C;
t_lower=1;
t_upper=15;
t_error=.0005;
t_step=[1:t_error:t_upper];
P2=NaN(1,length(t_step));
for j=0:t_upper+1
    for f=0:t_error:t_upper+1
        P2(j+1)=C1*exp(f*k1);
    end
end
while abs(1-(P2(t_upper)-P2(t_lower)))>t_error
    t_mid=(t_lower+t_upper)/2;
    if P2(t_mid)<P1 && P2(t_upper)>P1
        t_upper=t_mid;
        t_mid=((t_upper+t_lower)/2);
    elseif P2(t_mid)>P1 && P2(t_lower)<P1
        t_lower=t_mid;
        t_mid=((t_upper+t_lower)/2);
    else
        t_upper=t_upper*2;
        t_lower=t_lower-t_upper;
    end
end
t=t_mid;
t
end

Sorry for how scrappy it is.