I tried to find on web something about counters with R-S flip-flops and I can't find anything.
How should they look?
As a particular counter I need one to obtain this sequence: 0, 1, 3, 2, 6, 4, 5, 7.
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
As Andy points out use a 3 bit (stage) binary counter and logic gates to get the sequence. As I suspect this is a homework question I'll leave you with the logic gates to work out for yourself.
You can build a S-R flip flop from simple two input logic gates. I have shown an S-R type using two NOR gates and a NOT S-R type using NAND gates.
The problem is the simple S-R Flip flop (two cross coupled gates) cannot act as a counter (divider) by itself. Also if both inputs are HIGH (in the case of the S-R NOR flip flop) the output is uncertain.
By adding more logic gates around the S-R flip flop you can prevent this happening (resolve the conflict) and create a circuit capable of dividing an input pulse by two (a one stage binary counter). These circuits sense the current state of the outputs (Q and notQ) and 'steer' the next input pulse (clock) to the appropriate input on the flip flop causing the circuit to change state
A commonly used counter is the D type which uses TWO internally connected S-R flip flops. The 'not Q' output is connected to the D or Data input. By connecting up the D type flip flops as show below you can make a binary counter any length required. For this problem you need three stages (Binary 000 - 111, Decimal 0 - 7). All that you need to do is work out the logic required to change the binary count sequence to the sequence you require.