Using a 74LS670 chip (Linkto chip diagram/info) I need to program it to act like

Question

Using a 74LS670 chip (Linkto chip diagram/info) I need to program it to act like a halfadder.

The 74LS670 is a memory register with 4x4 bits, capable of somehowcreating up to 4 2-bit boolean functions.

Here is the given info, which I am having issues understanding howto relate to what it is asking me to do, so any assistance orthoughts would be appreciated.

To program D3D2D1D0 to act as an AND, OR, NAND, and NOR gatesrespectively, program it as follows:

WaWb D3
D2 D1 D0
00 0
0 1 1 01 0 1 1 0 10 0 1 1 0 11 1 1 0 0
From this, I now need to program the 74LS670 to act like ahalf-adder. The hint is Ra and Rb are the "inputs" to the "gates",and Er must be set to low to turn on Q3Q2Q1Q0 as the outputs.

Any help would be greatly appreciated, and of course rated in atimely manner.

WaWb D3
D2 D1 D0

Explanation / Answer

For others who might ever have my problem, it is actually a lotsimpler than it may seem. Each combination of Wa and Wb forms a "word", of D3D2D1D0. You setthe word by setting Wa and Wb to the desired setting, and settingD3, D2, D1, and D0 to the settings you desire (it can be anythingwithin the 4 bit limit). A register can "act" like gates dependingon what you set these things to be. Once you have set the 4 wordsto whatever you choose, you turn off Ew (high) and turn on Er,which will read back the settings you put in D3D2D1D0 to Q3Q2Q1Q0,the outputs. The outputs and inputs only function when theirrespective E is set to low. For my question, a half adder adds the two bits together, so thetruth table would look like: WaWb D3 D2 D1 D0 00 0 0 0 0 01 0 0 0 1 10 0 0 0 1 11 0 0 1 0 As you can see, D1 acts like the "carry", and D3 and D2 aren't usedat all. D1D0 acts like a two bit counter for a half adder, thus"acting" like a half adder. You don't actually have to mess withthe gates, since you are simply programming an output. To get the readings, set Er to low, and toggle Ra and Rb to thecorresponding values of Wa and Wb to get the outputs you previouslyprogrammed.

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