Brenkung Adder Design Essay

1048 WordsFeb 11, 20125 Pages
Executive Summary – Adder Design The objective of the project is to design a 16-bit adder. The goal of the project is to minimize the delay while keeping power within constraints, as well as minimizing the area. The constraints are as follows: Energy/operation has to be less , t trise han t0.5nJ and fall < 200ps, and use the standard cell layout, which must obey all the design rules. Throughout the course there were several adder topologies with different advantages been introduced, and we finally came to the decision to choose Brent-Kung adder architecture. The reason we chose this topology was because its implementation was simpler than Radix-4 Kogge-Stone tree. As a result the layout of the circuit would be less complicated. Even though it is a bit slower than the Kogg-Stone, it would save us time doing the layout in the end. After choosing the topology, we had to come up with the logic style to implement the bl We had implemented the logic blocks using transmission gates to gain more speed. However, for simplicity in sizing the Motivations: Since we wanted the adder to operate as fast as possible, we came up with some ideas to minimize the propagation delay such as: we modified the Brent-Kung tree, added 3 more dot products and cut the number of stages for the critical path from 7 to 5. For the Brent-Kung tree in order to get the carryout C14, the adder has to wait for the product of C7&C11, then C11&13 and finally C13&P14. On the other hand for the modified Brent- Kung tree C7 is directly producted with C13 at the fourth stage. This helps the C14 to be available at the fifth stage. The modified Brent-Kung tree is shown below as well as the critical path, which is highlighted in red. Loading estimate: Rwire=Rsq*L/W=0.75*1.8mm / 4λ=281.2Ohm Cin = 24.7fF(from the sum of all the gate input capacitance of the very first stage.) Since

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