Memory testing

INTRODUCTION

Testing is a process of checking the fabricated chip against manufacturing defects, to determine the presence of a fault in a given circuit, no amount of test can guarantee that the circuit is fault free we carry out testing to validate the circuit. Two types of approaches are used to verify the circuit first is simulation-based approach and the second are the formal method [3]System board, chip, gate switch interconnection, logic function every part is tested. Adding some extra logic such that the memory can test itself whenever the test is enabled is called Design for Testing (DFT). There are some testing issues first one is testing at higher level cost higher, test application time grows exponentially e.g. For a combinational circuit with 60 input we need 2^60 test patterns are required, the third one is the lack of controllability and observability of flip-flops (test generation for sub sequential Circuit is difficult) basic test principal is input patterns are applied to the CUT and output is compared with the previously decided golden response to check whether it is good or not. [4]Memory is not composed of logic gates or flip-flops rather than these it is made up of a number of cells arranged in a symmetric manner As they have symmetry consumes less area than flip-flop or logic gates etc., Arranged in a compressed manner. Decrease in the memory price per bit due to quadrupled capacity every three years. High density implies decrease in size of capacitor used to store a bit (in experiments, it is found that if high dielectric capacity material is used in place of old ones like barium, tritium etc., Facility high capacitance maintained in less area) This memory cell only works to read and write these Cells as this cell comprises of 1 and 0 only. If we use flip-flop for memory building then we can observe it consumes more area with this it also not profitable in the market. Like general circuit, we do not discard faulty memory chips. Since every chip has defects so the faults are not only detected, but cell number is also diagnosed. [14] All memory cells will have a redundant circuit with them and it could be replaced by a faulty circuit. Capacitors are used to store bits if they charged they represent 1 if they are not they represents 0 values. Instead of the capacitor if flip-flops are used they lead to a very large area. The manufacturer builds some extra memory with the original one. Redundant circuit is switched by multiplexing arrangement or by blowing up with laser .

Memory cell

This is the unit which composed of a number of cells as they are arranged in a symmetric manner and it is a part where all RAM memory is present it holds value 1 and 0 in different cells, it has address bus and data bus in a parallel manner and they also have crossed arrangement of address and data bus, if address bus is active and it send 1 in data bus, the capacitor gets charged and shows 1 bit, and if in data bus we send 0 then its capacitor doesn’t charge and shows 0, in this manner we can write data or read data in the memory.

Decoders

Two types of Decoders are found in the memory as the name implies they work for decoding data they are Row Decoder and column decoder.
The Row or Column Decoder are Implemented using logic gates, as they are digital circuits, they together collect information of row and column and fetch data from where it is to be accessed.

Sense Amplifier

This is an analog circuit, this amplifier used to sense whether to read data or not if it senses the command of read instruction it fetches the data, it has two units different to read and write.

Driver

The Driver is used to writing data on cells.

MEMORY TESTING AND FAULT TYPES

As we know memory is made up of the combination cells, arranged in a sequential manner these cells may be affected by many reasons such as they could have manufacturing faults, operational faults etc. These faults fail the system to perform an accurate operation. The memory unit has four major parts
A Read/write logic,
An Address decoder
A Sense amplifier
The Memory cell,
When any of this part is affected may cause failure, so it becomes essential to test and remove the fault. Among them, a few are explained here.