What is a Fabric?
A collection of physical logic (gates) and interconnect (wires)
These can be fixed or flexible
Describe an ASIC
An ASIC is an application specific integrated circuit which is created, designed and optimised for one specific task.
It is the most expensive option but has the highest performance.
Describe an FPGA
An FPGA is a Field Programmable Gate Array which is a pre-built collection of physical logic blocks and interconnects that can be adapted to perform many different logic tasks.
It is cheaper than an ASIC but offers worse performance
Describe what the LUT does in an FPGA
The Look Up Table can be thought of as a truth table in a box. It can perform many different combinatorial logic functions despite being pre-constructed. We use the program bits to set the particular function of the LUT.
eg. AND, NOR, etc.
Draw the simplified diagram for an FPGA cell. (The set and reset to the D flip-flop can be omitted)
Describe how the interconnect in an FPGA works.
Mention the Clock signal and data signals
What happens with many signals in one place?
The interconnect is programmable with program bits again. This allows us to route the output of each logic cell to the inputs of other ones as necessary.
The clock signal has its own dedicated interconnect however.
When too many signals pass through a single interconnect block there is routing congestion. We cannot simply add more wires as in an ASIC but we can use an entire logic cell as a wire by setting the multiplexers in a certain way so F=A.
What is fan-in ? Describe why it is important in an FPGA
Fan-in is the number of inputs connected to a gate/cell.
In an FPGA, the LUT has a physical limit on the fan-in, which must be designed around.
Describe fan-out. What is the effect of increased fan-out on circuit operation?
Fan-out is the number of outputs from a gate/cell.
Increased fan-out results in increased propagation delay.