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Cells are the basic gates in your design. A cell can also refer to the basic digital structures that are present and used in the Synthesis tool.
When a cell is used in a design, it is called an instance. For example, you have the final top-level module in your design where you call the lower-level module a number of times and connect them together. These are called instances of the leaf/lower-level module.
Instances are also the basic cells that are used when synthesizing your design. For example, if you design a counter and after synthesis the tool calls or uses the DFF cell in the tech library, the D-FF that is used is called an instance and the number of times the D-FF is used or called are called "number of instances". If the D-FF is used 5 times..then it has 5 instances in the design.
cells are basic logic gates. Example. Say you have an inverter with drive strength 1X. You may be having thousands, lakhs of 1X inverters in your design. How will you differentiate them?. By naming it individually called instance names.
Cells are none other than the normal cells as nand, nor etc..,
Even instances are same as cells. But in the tool perspective we can have many instances of the same cell.
This is usually because if the same cell name (eg:nand) is used many times in the design, and later u want to do any optimization of a particular cell then it is impossible for the tool to go and optimize that cell.
so what u can do is instantiate the same cell as many times as required for the design and make the job for the tool easier. This is what exactly uniqueness is.
Cell is a basic module which could be Nand, Nor, Latch & Flops etc.. In a large design, these cells are used many times.. This module is called top module & its made up of instances which are nothing but your basic cells.