Write down the VHDL behavioural description of the same D type flip flop entity and compare and contrast the two descriptions stating when one would use either model with particular reference to the simulation of the D type flip flop.

$[5]$

QUESTION THREE

a$)$ Figure Q$\mathrm{.}3$a is the circuit diagram of a logic circuit based on a four to one multiplexer. Derive from first principles the logic function which is implemented by it$.$ Using the same multiplexer circuit as a universal logic module $($ULM$),$ use it to implement the following Boolean functions indicated in figure Q$\mathrm{.}3.$

$[8]$

b$)$

c$)$ Explain the use of the same multiplexer as a look up table LUT$-2$ and draw the circuit diagram of a Basic Logic Element $($BLE$)$ with storage used as a basic building block in Field Programmable Gate Arrays.

d$)$ Figure Q$\mathrm{.}3$b is the internal circuit configuration of a Field Programmable Logic Array $($FPLA$)$ with fusible links. Explain how multiplexers and "programmed" bits stored in static CMOS latches can be used to create a reprogrammable FPLA to replace each dot in both the AND plane and the OR plane. Draw the circuit diagram of such a multiplexer in each of the planes clearly explaining how it simulates a blown fuse or one which is still intact.

$[4]$

QUESTION FOUR

a$)$ An FPGA is made up of programmable cells interconnected by a "sea" of routing trunks. Draw a diagram which shows how programmable logic blocks are interconnected together through a network of horizontal and vertical routing channels using the island routing architecture.

$[4]$

b$)$ Draw the functional block diagrams of both bidirectional and unidirectional routing schemes and provide the details of the multiplexer based switching which allows programmable interconnections.

$[10]$

c$)$ Explain the use of pass transistors or ideal transmission gates for bidirectional connections. Draw the circuit diagram of an ideal transmission gate made up of paralleled n channel and p channel MOSFETs and explain how. logic $\text{'}0\text{'}$ and logic $\text{'}1\text{'}$ are transmitted bi$-$directionally through it$.$

$[6]$

$3$

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