Look up tables

Question 1

How is a look up table made?

Answer 1

From memory cells

Question 2

How many memory cells is required for a look up table?

Answer 2

Num Cells = 2^n

n = Number of inputs

Question 3

How many inputs are there for a look up table?

Answer 3

> Between 3 and 4 because memory is expensive.

> Expanders can be used

Question 4

What is the benefit using a look up table?

Answer 4

> There is no hard limit to the number of terms in the SOP expression
More can be done with fewer resources

Question 5

What is a mask (when referring to a look up table)?

Answer 5

This is the way that a look up table is programmed

Question 6

What does each memory cell represent on a look up table?

Answer 6

It represents a possible input combination for all the input combinations

Question 7

How is a mask produced?

Answer 7

For each SOP expression posibility expressed from ~A~B~C to ABC:
> a 1 is put in that memory cell if its part of the the solution
> The mask is then read from bottom to top (ABC to ~A~B~C)
> This is then turned into a hexadecimal
[Picture4]
In this example the mask is 0b10101001 or 0xA9

Question 8

How can the number of inputs be expanded?

Answer 8

> By feeding the output from one LUT into the input of another LUT
Or there may be an expansion interconnect