Cross coupled inverter sense amplifier working

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Cross coupled inverter sense amplifier working

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Content:

• Memory design
• Voltage Sense Amplifiers
• Static random-access memory
• Case Study - SRAM & Caches
• Cross Trail Class C Motorhomes
• 5.Design of the RAM Arrays Used in Aries
• US4536859A - Cross-coupled inverters static random access memory - Google Patents
• Gallium nitride-based complementary logic integrated circuits
• SRAM Cell Operation
• Performance analysis of a 8t SRAM cell in 180 Nm CMOS technology

WATCH RELATED VIDEO: Cross Coupled Pair Part 1

Memory design

Practical Semiconductor Storage Methods 5. Pre-charge Circuit 5. Sense-Amplifier 5. Write-circuit 5. Row Decoders 5. Control Circuitry 5. Layouts of RAM Blocks. This is only possible if:. A number of factors need to be evaluated to see if the above mentioned requirements can be met. It is clear that the efficiency heavily depends on the number m, the data storage method and the width of coefficients as well as width of data involved in the convolution.

Generally speaking, for a convolution over m input cells with a data width of b bits with a set of kernel coefficients to get a result k bits wide, the total amount of required storage can be expressed as. These two expressions clearly show that regardless of the storage method a memory based solution is not very efficient for large values of m.

Yet for a simple building block, large values of m are not needed. Even values of m are also not very practical as most convolutions used in DSP algorithms need the data point plus an equal number of neighbours, commonly resulting in a matrix of odd dimensions.

Table 5. At first sight, the amount of computational complexity that the storage replaces may not seem significant. Multiplication is one of the most demanding and area consuming operations in the digital domain.

The same performance can only be achieved by the utilization of at least two or more multiplier blocks. Moreover the delay of the storage elements is independent of the coefficient dynamics, that is to say a result of any bit length will be delivered at the same rate as the bit result.

This allows for optimization in the bit-length, by which the delays of all pipeline stages can be carefully balanced to match the speed of storage elements, as increasing word lengths accounts for increasing processing in the subsequent combinational adder stages. It is a known fact that the flagship of integrated circuits development is the realization Random Access Memory devices. Current technology is often referred in terms of storage elements per chip such as 1 G-bit technology.

Yet much of these advanced technologies are not available for common digital designs and large on-chip memories are not desirable, mainly because of their area requirements. For small and medium scale storage within the constraints of conventional digital VLSI technology two practical solutions exist:. The most important advantage of the register array, which basically consists of an array of D-type flip-flops, is that the designer can safely use standart cell elements to generate the register array, completely avoiding the time-consuming full custom design effort.

Most of the modern synthesis tools support these arrays and many contemporary designs include relatively large amounts of such structures. But the register array design also has several disadvantages:. Figure 5. Although the advantages of using a RAM block is clear, it requires much more design effort than the standart cell based solution. Some semiconductor foundries have tools to automatically generate RAM blocks, while some others have services for commercial products in which practically the foundry employs a RAM design engineer.

The SRAM array consists of a dense two-dimensional arrangement of the actual storage elements. For small memories it is possible to store one word of data in a row for larger memories one row holds several words of data.

All cells in one column share the same input signals called the bitlines. Prior to read or write operations, the bitlines are charged to a known value by the pre-charge circuits. The row decoders are used to select one row in the array. The storage elements in the row are connected to the common bitlines and either the value within the cell is sensed read by sense-amplifiers or is overwritten by write circuits depending on the mode of operation.

For large memories, an additional column decoder is used to select the desired word within a row. The two switches in 5. A so called wordline controls these pass-transistors. As long as the pass transistors are turned off, the cell retainsone of its two possible steady state. A read operation from this cell is performed by pre-charging the bitlines to a known value e. VDD and enabling the wordline. As during any read operation only one row can be active the row decoder guarantees this , each column bitline can be modelled by a capacitor representing all the parasitic capacitance of the bitline and the input capacitance of all the access transistors.

Depending on the content i. The parasitic bitline capacitance is modeled to be 1pF. The first Read Cycle starts at ns with the activation of the wordline. The stored information is a logic "1", bitline rises to VDD while bitline' is pulled down by the second inverter. Notice that the bitline is not pulled down completely in fact it only reaches 4 Volts but that is enough for the sense amplifier to operate correctly.

The second read operation starts at ns with the activation of another wordline not shown, which in fact is just the inverted first wordline. This time the stored information is a logic "0" and the switching of the bitline s can be seen clearly. A write operation is pretty similar in nature to the read operation. Again the cell is accessed with enabling the wordline , but this time the bitlines are driven to a known state by the write circuitry. This write circuitry is designed to have a stronger current driving capability than the pre-charge and storage cell circuitry, and as a result the bitlines are driven beyond the inversion thresholds of the inverters within the SRAM cell.

The plot in Figure 5. The first three strips are the control signals. Write enable signal activates the write mode, Wordline selects the rows to be accessed and the Data is the value to be written to the selected cell. The voltages of the bitplanes is plotted next followed by the voltages within the SRAM cell. The simulation snapshot displays consecutive write and read cycles. The first write cycle starts at ns.

A logic "0" is written to a cell within the column which is not the cell that we are concentrating. At ns. The Wordline goes high indicating a write to the cell we are observing. Notice the bitline swing for the write operation. Also the last two strips clearly show the switching inverters. A new value has been written. There is a read cycle starting at ns. At ns our cell is accessed for a read operation. Notice the relatively low bitline swing and the perturbation on the inverter.

It is clear that a perturbation as high as the threshold voltage of the nMOS transistor may cause the inverter switch state and destroy the content of the cell. Kang and Leblebici [19] give a conservative analytical expression for the sizing of access and pass transistors to prevent overwrite during read. The simulation shows a second write operation starting at ns.

At ns logic "1" is written to the cell activated by the Wordline. As each port must be able to access the cell independent of each other the basic access lines Bitline , Bitline' and Wordline need to be duplicated. Two more pass transistors are added to control access of the second port See Figure 5.

An efficient layout for the basic cell is the key point of any dense RAM circuit layout. There are a few important factors to consider when designing a custom cell like this:. The basic cell measures A very simple pre-charge circuit is used for the realization of the RAM.

A clocked pre-charge circuit would have needed a very accurate and complicated clock timing. The schematic of the static pre-charge circuit structure can be seen in Figure 5. The pMOS transistors driving the bitlines have width of 6um , all the other transistors have dimensions of 2um.

While providing a very simple column pre-charge mechanism, this structure also has some drawbacks:. The most important is that as there is a constant pre-charge voltage in the bitline , any effort to pull the bitline down must fight against this pre-charge circuit.

One possible remedy would be to use minimum-size pMOS transistors in the pre-charge, but this is not feasible as a fast recovery is required to pull the bitlines from their low states to their high states.

A weak pull-up would result in a slower response time in read access. On the other hand, the write operation needs to pull down the bitline even further, in order to be able to switch the internal inverters. A relatively strong write circuit easily accomplishes this task. This in turn has another negative effect. After any write operation one of the bitlines is pulled down to a relatively low voltage level. Thus, a subsequent read operation may under certain circumstances overwrite the content of the cell.

Fortunately, this is not a major issue, since write operations are only required to update the weights; therefore, the write timing is not critical. In Aries, a write operation must be followed by an idle state to help recover the bitlines prior to a read operation. This restriction does hardly effect the operation of the circuit.

The main issue in the RAM design for Aries was the speed of read operation. The sense amplifier is the most important block that dictates the speed of read operations. The sense amplifier was developed by B.

Aksoy [20]. A number of different architectures were evaluated. Finally, a two stage amplifier with a cross-coupled pMOS amplifier as the first stage and a conventional differential amplifier as the second stage was found to give the best performance.

Voltage Sense Amplifiers

Static RAM is a type of random-access memory that uses latching circuitry flip-flop to store each bit. To get insight of mem4kBytesOr32kbits design Specifications, check this. During write operation i. To write logic 0, Bitlline should be logic 0 and BLbar is complemented to 1.

Static random-access memory

This is to certify that Ms. The processor communicates with the memory system memory interface. The processor sends the address over the address bus. For read, MemWrite is 0 and the memory returns the data on the ReadData bus. For write, MemWrite is 1 and the memory returns the data on the WriteData bus. The process is shown in the block diagram below. The address lines i. Since to obtain such a large decoder logic with and gates makes it slower, cascading of multiple smaller decoders is done. Here, there are 2 approaches that were considered:.

Case Study - SRAM & Caches

Semiconductor memories are an integral part of all systems to store large quantities of digital information. One of the important types of semiconductor memory is Static Random-Access Memory which is mainly used as memory caches and processor registers. Its SRAM architecture consists of bit cell array with peripherals like address decoder, the word line drivers, the column multiplexer, the precharge circuitry, the sense amplifier, the write drivers and the control logic. It requires custom cells like bitcell, sense amplifier, write drivers, tristate buffers and D-flip flop for each technology added to it. It uses the technology files, custom cells designed and configuration script to generate SPICE netlists, layout, timing and power models and other IP deliverables required for fabricating SRAM as shown in the above figure.

Cross Trail Class C Motorhomes

A sense amplifier for use in a memory device and in a memory-resident system. The sense amplifier operates on a lower voltage consistent with the voltage range of the differential input data and the sense amplifier further operates on a higher voltage to level-shift the output signal concurrently with the sensing operation. The sense amplifier includes a pair of differential cross-coupled inverters whose inputs are coupled to receive the data from the memory. Once the input nodes of the cross-coupled inverters are charged, the cross-coupled inverters are Once the input nodes of the cross-coupled inverters are charged, the cross-coupled inverters are further coupled to pull-up and pull-down circuits that span the higher voltage range for performing the level-shifting functionality.

5.Design of the RAM Arrays Used in Aries

The emerging nanoscale technologies inherently offer transistors working with low voltage levels and are optimized for low-power operation. Moreover, the voltage headroom, ESD properties, the maximum current densities, parasitic effects, process fluctuations, aging effects, and many other parameters are superior in verified-by-time CMOS processes using planar transistors. This is the main reason, why low-voltage, low-power high-performance analog and mixed-signal circuits are still being designed in mature process nodes. In the proposed chapter, we bring an overview of main challenges and design techniques effectively applicable for ultra-low-voltage and low-power analog integrated circuits in nanoscale technologies. New design challenges and limitations linked with a low value of the supply voltage, the process fluctuation, device mismatch, and other effects are discussed. In the later part of the chapter, conventional and unconventional design techniques bulk-driven approach, floating-gate, dynamic threshold, etc. Examples of ultra-low-voltage analog ICs blocks an operational amplifier, a voltage comparator, a charge pump, etc. The design of ultra-low-voltage ULV and low-power LP analog and mixed-signal ICs in modern nanotechnologies represents a real challenge for circuit designers and researches, since it introduces several limitations in numerous aspects.

US4536859A - Cross-coupled inverters static random access memory - Google Patents

During the read cycle, the bit-lines are initially precharged by bit-line load transistors. When the selected word-line is activated, one of the two bit-lines is pulled low by driver transistor, while the other stays high. The bit-line pull-down speed is very slow due to the small cell size and large bit-line load capacitance.

Gallium nitride-based complementary logic integrated circuits

RELATED VIDEO: sense amplifier 1

SRAM is volatile memory ; data is lost when power is removed. In , [4] Arnold Farber and Eugene Schlig, working for IBM, created a hard-wired memory cell , using a transistor gate and tunnel diode latch. They replaced the latch with two transistors and two resistors , a configuration that became known as the Farber-Schlig cell. In , Benjamin Agusta and his team at IBM created a bit silicon memory chip based on the Farber-Schlig cell, with 80 transistors, 64 resistors, and 4 diodes.

SRAM Cell Operation

Voltage Type Sense Amplifier. Latch Type Sense Amplifier. Sense amplifiers SA are an important component in memory design. The choice and design of a SA defines the robustness of bit line sensing, impacting the read speed and power. Due to the variety of SAs in semiconductor memories and the impact they have on the final specs of the memory, the sense amplifiers have become a separate of circuits.

Performance analysis of a 8t SRAM cell in 180 Nm CMOS technology

Hochschule Kempten. Publications Fachgebiet Elektronik, Prof. Increasing memory sizes, smaller feature sizes and lower operating voltages make it more important to understand the cell signal sensing operation, the signal of memory cells and the limiting factors. This paper presents a measurement method to evaluate the signal created by the memory cell and the sense amplifier uniformity at product level.