Biphasic waveform

Dear all im new here try to learning verilog and vhdl. i need to
generate biphasic waveform width of the pulse 12.5 ns.
for +ve 12.5 ns and -ve 12.5 ns

for the second channel vary the width of pulse i should change and delay
b/w first and second channel o/p is 10ns.

im using spartan 3e kit and ise design suite 14.7

I want program to generate biphasic waveform. I’m unable to generate
negative pulse. Biphasis is +1,-1 correct