FCE LAB $3$

Q $3$: What is the characteristic equation of a flip$-$flop?

Q $4$: Write down the characteristic equations for the JK$-$FF$,$ D$-$FF$,$ T$-$FF$.$

Q $5$: What is the difference between level$-$triggered and edge$-$triggered flip$-$flops?

$4$ Sequential Circuits

Combinatorial circuits can be described using a set of equations. For combinatorial circuits the state of the outputs depends only on the state of the inputs $($see lab $2).$ For sequential circuits, the state of the outputs depends not only on the state of the inputs, but also on the inner state of the circuit. Similar to combinatorial circuits, sequential circuits can be described by a set of characteristic equations, like flip$-$flops, which are the simplest sequential circuits. The characteristic equations for the FF outputs are obtained from a truth table which describes which next states $Q^{n+1}$ results from the current states $Q^n.$

$4\mathrm{.}1$ Register

Registers are the simplest circuits built from multiple flip$-$flops. A register is a synchronous circuit of D$-$FFs$.$ A register stores not only $1$ bit but multiple bits. A $N$ bit register needs exactly N flip$-$flops. In Figure $12$ a $3-$bit register with D$-$FFs drawn in Qucs is shown. The reset inputs of the D$-$FFs are connected together to a single $R$ input, so that the whole register can be reset at once.

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