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Altera_Forum
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12 years ago

Question regarding CyclonV ALMs

Hi, I am new to Altera FPGAs, so I did some study on the internal design reading the following document: http://www.altera.com/literature/hb/cyclone-v/cv_52001.pdf What confuses me a bit, ...
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Altera_Forum
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12 years ago

The ALM is a physical structure on-chip. It can be used in several different modes.

In 6-LUT mode, it can compute any function of 6 input bits. So why does it have 8 inputs?

The ALM can also be split into two smaller units. You can get two completely independent 4-input functions from one ALM. (You can also get two 5-input functions, providing that the two 5-input functions share at least two inputs in common.) In this mode, each half-ALM can implement the same functions as a Cyclone IV LE, so (roughly speaking) 1 ALM = 2 LEs.

When it's used in an adder (yet another mode), each ALM will produce 2 bits of the result. For simple functions - add, compare etc you'll need 1 4-LUT per bit.

Depending on how you're handling overflow, an 8-bit add may require a 9-bit output, which would be 5 ALMs. A comparison is similar.