# How to build a lsfr based on sequence $s_i = s_{i-1} + s_{i-4}$? [closed]

How do I know where my XOR gates go? What does the F2 stand for here? Also the next task is to generate the sequence (with initialisation vector $$s_0 = 1, s_1 = s_2 = s_3 = 0$$) until it becomes periodic which I'm fairly certain I can do however even a few rows would be helpful as I won't have any other answers to these (not official homework assignment).

edit: my guess is now the xor port goes on top $$s_3$$, because of unknown reason. How will this affect the sequence? When generating it I shift and do I xor the last $$s_3$$ and first $$s_1$$ in order to receive the new bit?

edit2: my solution with sequence

• I have tried looking at this among others, but I am still unsure. My guess is that si-1 = s0 and si-4 = s3, but I am still unsure how to draw it and why it becomes as such? Commented Oct 18, 2021 at 13:51
• Also checked that before. This is in different form. I believe I can draw from polynomial form but not from this. Also what F2 stands for is still unknown to me and it isn't mentioned anywhere in your replies. Commented Oct 18, 2021 at 13:56
• The sequence definition already gives you the hint. the next feedback is the sum( xor?) of previous and 4th previous? Commented Oct 18, 2021 at 13:59
• So si-4 = s3 and the xor goes on top of s3? Commented Oct 18, 2021 at 14:00
• Run you solution? Commented Oct 18, 2021 at 14:06