lut5 - Arbitrary 5-input logic function based on a look-up table
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loadrt lut5 [count=N|names=name1[,name2...]] |
lut5 constructs a logic function with up to 5 inputs using a look-up table. The value for function can be determined by writing the truth table, and computing the sum of all the weights for which the output value would be TRUE. The weights are hexadecimal not decimal so hexadecimal math must be used to sum the weights. A wiki page has a calculator to assist in computing the proper value for function.
https://wiki.linuxcnc.org/cgi-bin/wiki.pl?Lut5
Note that LUT5 will generate any of the 4,294,967,296 logical functions of 5 inputs so AND, OR, NAND, NOR, XOR and every other combinatorial function is possible.
A 5-input and function is TRUE only when all the inputs are true, so the correct value for function is 0x80000000.
A 2-input or function would be the sum of 0x2 + 0x4 + 0x8, so the correct value for function is 0xe.
A 5-input or function is TRUE whenever any of the inputs are true, so the correct value for function is 0xfffffffe. Because every weight except 0x1 is true the function is the sum of every line except the first one.
A 2-input xor function is TRUE whenever exactly one of the inputs is true, so the correct value for function is 0x6. Only in-0 and in-1 should be connected to signals, because if any other bit is TRUE then the output will be FALSE.
| Weights for each line of truth table | |||||
|---|---|---|---|---|---|
| Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 | Weight |
| 0 | 0 | 0 | 0 | 0 | 0x1 |
| 0 | 0 | 0 | 0 | 1 | 0x2 |
| 0 | 0 | 0 | 1 | 0 | 0x4 |
| 0 | 0 | 0 | 1 | 1 | 0x8 |
| 0 | 0 | 1 | 0 | 0 | 0x10 |
| 0 | 0 | 1 | 0 | 1 | 0x20 |
| 0 | 0 | 1 | 1 | 0 | 0x40 |
| 0 | 0 | 1 | 1 | 1 | 0x80 |
| 0 | 1 | 0 | 0 | 0 | 0x100 |
| 0 | 1 | 0 | 0 | 1 | 0x200 |
| 0 | 1 | 0 | 1 | 0 | 0x400 |
| 0 | 1 | 0 | 1 | 1 | 0x800 |
| 0 | 1 | 1 | 0 | 0 | 0x1000 |
| 0 | 1 | 1 | 0 | 1 | 0x2000 |
| 0 | 1 | 1 | 1 | 0 | 0x4000 |
| 0 | 1 | 1 | 1 | 1 | 0x8000 |
| 1 | 0 | 0 | 0 | 0 | 0x10000 |
| 1 | 0 | 0 | 0 | 1 | 0x20000 |
| 1 | 0 | 0 | 1 | 0 | 0x40000 |
| 1 | 0 | 0 | 1 | 1 | 0x80000 |
| 1 | 0 | 1 | 0 | 0 | 0x100000 |
| 1 | 0 | 1 | 0 | 1 | 0x200000 |
| 1 | 0 | 1 | 1 | 0 | 0x400000 |
| 1 | 0 | 1 | 1 | 1 | 0x800000 |
| 1 | 1 | 0 | 0 | 0 | 0x1000000 |
| 1 | 1 | 0 | 0 | 1 | 0x2000000 |
| 1 | 1 | 0 | 1 | 0 | 0x4000000 |
| 1 | 1 | 0 | 1 | 1 | 0x8000000 |
| 1 | 1 | 1 | 0 | 0 | 0x10000000 |
| 1 | 1 | 1 | 0 | 1 | 0x20000000 |
| 1 | 1 | 1 | 1 | 0 | 0x40000000 |
| 1 | 1 | 1 | 1 | 1 | 0x80000000 |
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lut5.N |
lut5.N.in-0
bit in
lut5.N.in-1 bit in
lut5.N.in-2 bit in
lut5.N.in-3 bit in
lut5.N.in-4 bit in
lut5.N.out bit out
lut5.N.function u32 rw
and(9), logic(9), not(9), or2(9), xor2(9).
Jeff Epler
GPL