# is it posssible to make a 4-bit up/down asychronous counter using jk flip flops, xnor gates and nothing else?

the image below is supposed to represent the last segment of the circuit, where the 'updown' input would change the direction of the counting: when updown=0 it counts backwards (f,e,d,...1,0,f...) when =1, it counts upwards (0,1,2,...e,f,0...)

yet i am even strugling to design one with two bits. i have tried different variations of the circuit below but none seem to work the way it has to

note that i am only allowed to use j-k flip-flops (74LS112) and XNOR gates (TTL 74266)

could someone give me a hint or something because i am in severe pain

• We won't do your homework for you but we can give you hints. Start by creating a state transition table. Use that to create K-maps or truth tables for each of the J and K inputs. Take it at least that far, and show us your work. Then let us know if you have a specific question. Commented May 16, 2021 at 19:15
• i reworked it and have came to the conclusion that (x= updown input) j3=k3=xq2q1q0+x'q2'q1'q0' , j2=k2=xq1q0+x'q1'q0' , j1=k1=xq0+x'q0', j0=k0=1. turns out i cant simplify these function any further tho. any ideas? ill keep working on it of course Commented May 16, 2021 at 22:54
• The two-bit circuit you show is not asynchronous, it is synchronous. Commented May 17, 2021 at 6:21
• Its possible to make any of the other standard logic gates (AND, NAND, OR, NOR, NOT, XOR) from just XNOR gates. Therefore, if you can make something out of those other gates, then you can certainly make it out of XNOR gates. Commented May 17, 2021 at 14:53