01-12-2012, 04:59 PM
BCD-to-excess-3 code converter
1BCD-to-excess.ppt (Size: 561.5 KB / Downloads: 286)
A code converter is a circuit that makes the two systems compatible even though each uses a different binary code.
To convert from binary code A to binary code B, the input lines must supply the bit combination of elements as specified by code A and the output lines must generate the corresponding bit combination of code B.
BCD-to-excess-3 code converter
Each code uses four bits to represent a decimal digit
Four input and four output variables are present
A, B, C, D => Input variables
W,X,Y,Z => Output variables
4 binary variables have 16 bit combinations, only 10 are listed in truth table, others are don’t care conditions.
Key things:
Like combinational logic circuits, a sequential logic circuit has inputs (labelled with x with subscripts) and outputs (labelled with z with subscripts).
Unike combinational logic circuits, a sequential logic circuit uses a clock.
Also, there is a box inside the circuit called State.
This box contains flip flops. Assume it has k flip flops. The flip flops basically store a k-bit number representing the current state.
The output z is computed based on the inputs (x with subscripts) and the state coming out of the state box (q with subscripts).
The state may be updated at each positive clock edge. When there's not a positive clock edge, the state remains unchanged.
The information needed to update to the state (called the next state) comes from the current state (the current value of q) and the input, which is fed through combinational logic, and fed back into the state box, telling the state box how to update itself.
State Assignment
The cost of the combinational circuit part of a sequential circuit can be reduced by using the known simplification methods for combinational circuits. However, there is another factor, known as the state assignment
State assignment procedures are concerned with methods for assigning binary values to states in such a way as to reduce the cost of the combinational circuit that drives the flip flops.