492 test_friday2
492 : test_friday2
- Author: Niles Peter
- Description: class
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8-bit KoggeStone Adder
Author: Niles Villaverde, Joshua Cho Language: Verilog
How it works
The KoggeStone Adder computes in parallel, first the sum from the two different inputs and then computes the carry-out for each bit. Then uses the calculated carry-out and sum of each bit to compute the final result of the adder. Note: No carry-out so values greater than 255 can not be outputted
In the project.v file, there are 5 different modules: BigCircle, SmallCircle, Square, Triangle, and tt_um_koggestone_adder8.
Shown in figure 1 below is the block diagram for the flow for the KoggeStone Adder
Figure 1: KoggeStone Adder Block Diagram
BigCirle Module
The BigCircle module represents the carry generator for the KoggeStone Adder. It calculates the generated and propagated signal in each bit stage in the Adder. In comparison to carry-ripple adders, the KoggeStone adder allows for the carry information to propagate efficiently to multiple bit positions. This allows for the number of sequential steps in calculating the final carry-out to be reduced.
The BigCircle takes in the generate and propagate signals from the current position in the adder and the previous position in the adder. Using these signals, BigCircle updates the generate signal for the bit position to reflect if the carry is generated from this bit position or propagated from the previous. Then calculates the propagation signal to decide whether if a carry can be passed through this position.
SmallCircle Module
The SmallCircle module passes the carry in signal and generated carry signal to the next position
Square Module
The Square module calculates the current generate and propagate signal by ANDing the inputs A and B as well as XORing the inputs A and B respectively.
Triangle Module
The Triangle module calcualtes the sum bit by XORing the propagate bit with the previous carry-in bit.
tt_um_koggestone_adder8 Module
The tt_um_koggestone_adder8 module takes in two 8-bit inputs, ui_in and uio_in. The module also outputs an 8-bit output, uo_out. Input Signals: Two 8-bit, a and b which are mapped to ui_in and uio_in, respectively. Cin, carry-in for the addition which is set to zero. g and p, generate and propagation signal for each bit. c, carries for each bit position.
The first sequence is to use the Square Module to create the initial generate and progagate calculations. Then uses the BigCircle Module to calculate the intermediate generate and propagation signals of each bit. In the second stage of the BigCircle Module, by combining the signals over groups of 4 bits, it further propagates the carry. In the third stage of the BigCircle Module, it continues the carry propagation over an even wider spans of bits. Then using the SmallCircle Module, the final Carry-Out signals for each position are calculated. Then the final sum is calculated using the Triangle Modules.
How to test
The two different inputs, ui_in[7:0] and uio_in[7:0] are iterated through each possible combination of 8-bit numbers to test all corner cases. The outputs are set to the calculated values calculated by the KoggeStone Adder. If the sum between the two values are greater than 255, the test is skipped as limitations on the hardware prevent us from having a carry-out value.
External hardware
no external hardware
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | a[0] | sum[0] | b[0] |
| 1 | a[1] | sum[1] | b[1] |
| 2 | a[2] | sum[2] | b[2] |
| 3 | a[3] | sum[3] | b[3] |
| 4 | a[4] | sum[4] | b[4] |
| 5 | a[5] | sum[5] | b[5] |
| 6 | a[6] | sum[6] | b[6] |
| 7 | a[7] | sum[7] | b[7] |