Sequential circuits William Sandqvist [email protected]If the same input may produce different output signal, we have a sequential logic circuit. It must then have an internal memory that allows the output to be affected by both the current and previous inputs! Logic circuit Same input can produce different output
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If the same input may produce different output signal, we have a sequential logic circuit. It must then have an internal memory that allows the output to be affected by both the current and previous inputs! Logic circuit
Sequence Detector. If w has been 1 in two (or more) consecutive clock then z = 1.
w w z
C Clk
Specification Sequence circuit has an input w and an output z If input w has been 1 in during the current and previous clock cycle then z will be set to 1 Use a Moore machine with D-flip-flops to realizing the design.
The Next state decoder consists of two logic networks available as input network to the two flip-flops. In order to minimize logic networks, we enter the truth tables in the form of Karnaugh maps.
t 0 t 1 t 2 t 3 t 4 t 5 t 6 t 7 t 8 t 9 t 10 1 0 1 0
1 0
1 0
Clock
y
w
z
• The output may change during the clock period, since it is a function of the input signal • Compared to Moore machine the Mealy machine is moore 'responsive' (bit sequence is detected in t4 compared to t5 for the Moore-machine)
• The disadvantage of the Mealy machine is that the output can be changed during the entire clock period • You can add a register (flip-flop) at the end so to synchronize the output with the clock edge
• Moore-machine output values depend only on the current state • Mealy-machine output values depend on the current state and the values of the input signals • Mealy-machine often uses fewer states • Mealy-machine output signals are not inte synchronized with the clock, why you often has to add an output register
• One-hot-encoding uses one flop-flop per state • For each state one bit is ‘hot’ (1), all other bits are 0, eg. 0001, 0010, 0100, 1000 • One-hot encoding minimizes the combinatorial logic, but increases the number of flip-flops!
• There is not a code that is the best in every situation, it all depends on the state diagram • You can also have your 'own codes' that fits into the design, eg. 00, 11, 10, 01