Unit – 5

Semiconductor memories and Programmable logic devices

Q1) Define memory request ad word size.

A1)

Memory Request:
Memory request contains the address along with the control signals. For Example, When inserting data into the stack, each block consumes memory (RAM) and the number of memory cells can be determined by the capacity of a memory chip.

Word Size:
It is the maximum number of bits that a CPU can process at a time and it depends upon the processor. Word size is a fixed size piece of data handled as a unit by the instruction set or the hardware of a processor.

Q2) Explain ROM and its types.

A2)

• ROM stands for Read Only Memory.
• It can only read but cannot write on it.
• This memory is non-volatile.
• The information is stored permanently during manufacturing.
• It stores instructions that are required to start a computer and is referred to as bootstrap.
• They are not only used in the computer but also in other electronic devices like washing machine, microwave oven etc.

Advantages :

• Non-volatile in nature
• Cheaper than RAMs
• Easy to test
• More reliable

Types:

• Programmable Read-Only Memory (PROM)
• Electrically Programmable Read-Only Memory (EPROM)
• Electrically Erasable Programmable Read-Only Memory (EEPROM; also called Flash ROM)
• Electrically Alterable Read-Only Memory (EAROM)

They are often referred to as programmable ROM. It was designed for specific data which contains the start-up code.

Q3) What is RAM? Explain.

A3)

RAM (Random Access Memory) is the internal memory of the CPU which is used for storing data, program, and result. It is a read-write memory which stores data only if the machine is working and as the machine is switched off, data gets erased.

Access time is independent of the address hence each storage location inside the memory is easy to reach and takes the same amount of time. Here, Data can be accessed randomly and is very expensive.

RAM is volatile in nature therefore data stored in it is lost when we switch off the computer or in case of power failure.

Hence, a Uninterruptible Power System (UPS) is used as a backup with computers. RAM is small in size and in the amount of data it can hold.

Q4) What is CAM? Explain.

A4)

Content-addressable memory (CAM) is silicon chip architecture that is purpose-built for extremely fast but very specific type of memory lookups. Lookups using a CAM is conceptually similar to associative array logic in data structures but the output are highly simplified.  When key is passed to a CAM sub-system it returns the associated value to that key. As a result, a “key -> value” pair is created that can be referenced further as an object. The most important feature is that a lookup of an entry in a CAM can be performed in a single clock cycle in the silicon. Compare this with a RAM module that requires multiple clock cycles to make a single memory fetch.

Q5) Write a short note on CAM and CCD.

A5)

CCD

A charge coupled device (CCD) is an integrated circuit etched onto a silicon surface forming light sensitive elements called pixels. Photons incident on this surface generate charge that can be read by electronics and turned into a digital copy of the light patterns falling on the device. CCDs come in a wide variety of sizes and types and are used in many applications from cell phone cameras to high-end scientific applications.

CAM

Content-addressable memory (CAM) is silicon chip architecture that is purpose-built for extremely fast but very specific type of memory lookups. Lookups using a CAM is conceptually similar to associative array logic in data structures but the output are highly simplified.  When key is passed to a CAM sub-system it returns the associated value to that key. As a result, a “key -> value” pair is created that can be referenced further as an object. The most important feature is that a lookup of an entry in a CAM can be performed in a single clock cycle in the silicon. Compare this with a RAM module that requires multiple clock cycles to make a single memory fetch.

Q6) Give example and explain PLA in detail.

A6)

• Programmable Logic Array (PLA) is a fixed architecture logic device with programmable AND gates followed by programmable OR gates.
• It is basically a type of programmable logic device used to build reconfigurable digital circuit.
• PLDs have undefined function at the time of manufacturing but they are programmed before made into use.
• PLA is a combination of memory and logic.
• PLA is similar to a ROM in concept; however it does not provide full decoding of variables and does not generate all min terms as in the ROM.
• It does not require any type of programming like in C and C++.

F1 = AB’C’ + AB’C + ABC’ + ABC
on simplifying we get : F1 = AB’ + AC

F2 = A’BC + AB’C + ABC
on simplifying we get: F2 = BC + AC

Q7) Implement the following Boolean expression with the help of programmable array logic (PAL)

X =AB + AC’
Y= AB’ + BC’

A7)

Q8) Comparison between PLA and PAL

A8)

Q9) Write short note on CPLD.

A9)

A complex programmable logic device comprises of a group of programmable FBs (functional blocks). The inputs and outputs of these functional blocks are connected together by a GIM (global interconnection matrix). This interconnection matrix is reconfigurable, so that we can modify the contacts between the functional blocks. There will be some input and output blocks that let us to unite CPLD to external world. The architecture of CPLD is shown below.

Generally, the programmable FB looks like the array of logic gates, where an array of AND gates can be programmed and OR gates are stable. But, each manufacturer has their way of thinking to design the functional block. A listed o/p can be found by operating the feedback signals attained from the OR gate outputs.

                                                         Fig.: CPLD architecture

In CPLD programming, the design is first coded in Verilog or VHDL language once the code is (simulated and synthesized. During synthesis, the CPLD model (target device) is handpicked and a technology-based mapping net list is produced. This list can be close-fitting to the genuine CPLD architecture using a place and route process, typically achieved by the place-and-route software of CPLD Company’s proprietary. Then the operator will do some confirmation processes. If everything is good, he will utilize the CPLD, else he will rearrange it.

Q10) Compare CPLD and FPGA

A10)

       CPLD                                                                               FPGA