A half adder is an arithmetic combinational circuit that takes in two binary digits and adds them. The half adder gives out two outputs, the SUM of the operation and the CARRY generated in the operation. Write a VHDL code using structural, behavioural and data flow modelling.
5 To 32 Decoder Vhdl Code For Serial Adder
Another type of demultiplexer is the 24-pin, 74LS154 which is a 4-bit to 16-line demultiplexer/decoder. Here the individual output positions are selected using a 4-bit binary coded input. Like multiplexers, they can also be cascaded together to form higher order devices.
In this section, various testbenches for combinational circuits are shown, whereas testbenches for sequential circuits are discussed in next section. For simplicity of the codes and better understanding, a simple half adder circuit is tested using various simulation methods.
In this case, the VHDL code implements a cascade of balanced adder equalizing the delay over the adder structure. A possible VHDL code description using FOR-LOOP approach of the adder cascade is given below
Altera Quartus II implements the different VHDL description of the adder in the same way. As clear from Figure 5, the RTL viewer reports different implementation for the VHDL code of cascaded adder or balanced tree adder.
Xilinx ISE implements the different VHDL description of the adder in a slightly different way and the implementation depends on the VHDL RTL code. The FOR-LOOP implementation triggers different hardware architecture. A timing and area report for the two design is reported in
As clear, on a Xilinx Spartan 3 FPGA, equivalent to the Cyclone II Altera FPGA, the area and timing report for the two-different implementation differs both on area and timing. The maximum timing achievable is in the balanced VHDL code version and it is about 80 MHz in the cascaded adder implementation and about 107 MHz in the balanced tree adder implementation.
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