인하대 VLSI 설계 4주차 XOR

문서 내 토픽

1. XOR Gate

XOR Gate는 두 입력 값이 서로 다른 경우 1을, 서로 같은 경우 0을 출력하는 gate로 배타적 논리합이라고도 한다. 이를 나타내는 진리표를 보면 입력 신호가 서로 같을 경우 0, 서로 다를 경우(배타적인 경우) 1이 출력됨을 알 수 있다. 이 진리표를 토대로 카르노맵을 그려서 입력식을 구하면 X = AB' + A'B가 나온다.
2. Transistor level layout

transistor level layout을 그리는 과정을 살펴보면 NMOS network에 A와 B를 직렬 연결해 AB, A'과 B'을 직렬 연결해 A'B'을 만들고 만든 AB과 A'B'을 병렬 연결하여 NMOS network를 완성한다. 그리고 그 위의 PMOS network는 NMOS Network와 dual로 구성하면 전체 회로가 완성된다.
3. PMOS, NMOS의 Size 설정

Hole의 mobility()가 Electron의 mobility()의 0.5배 정도이기 때문에 PMOS는 NMOS보다 같은 size 대비 저항이 2배 큰 것을 고려하여 PMOS network의 총 저항이 NMOS network의 총 저항과 일치하도록 NMOS 트랜지스터 1개의 size를 0.5x로 설정한다. 따라서 Size 비율을 정수비로 나타내면 PMOS : NMOS = x : 0.5x = 2 : 1이 된다.
4. Size ratio에 따른 그래프 차이

Inverter회로에서 MOSFET의 Size raio가 커지면 성능은 좋아지지만 면적의 손해를 보게 된다. Size ratio가 4배인 경우 가장 전압이 빨리 증가했고 x2가 중간, x1이 가장 느리게 증가함을 확인할 수 있다.
5. Size ratio_Finger

Size ratio를 키우기 위해 Finger라는 구조를 만드는데, Two fingers이고 트랜지스터들이 병렬 연결이면 Size ratio는 One finger에 비해 2배가 되고, Two fingers이고 트랜지스터들이 직렬 연결이면 는 2배가 된다. Finger를 이용하여 Size ratio를 키우는 것이 MOSFET size만 키우는 것보다 더 효율적이다.
6. Layout 설계 시 유의사항

Common-centroid: contact 범위를 넓게 설계하고 공정에서 여러 변수를 고려해 소자들을 동일한 환경에 놓아야 한다. Dummy device: 동일 환경 조성을 위해 dummy를 추가해서 공정 변화에 둔감하도록 한다.
7. XOR Gate Layout 및 시뮬레이션

XOR Layout은 Y = AB' + A'B를 기준으로 작성하였다. Magic Tool을 이용해 추출한 netlist와 직접 손으로 작성한 netlist 바탕의 시뮬레이션 결과, 입력 신호 A, B가 각각 0, 0 또는 1, 1로 일치하면 0을 출력, 두 신호가 서로 다르면 1을 출력하는 것을 확인할 수 있었다. 하지만 값이 튀는 구간이 발생하여 입력 A, B의 주기를 각각 2u, 1.8u로 늘려서 시뮬레이션한 결과 출력 값이 튀는 부분이 거의 제거되고 Time delay없이 잘 나오는 것을 확인할 수 있었다.

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1. XOR Gate

The XOR gate is a fundamental logic gate in digital electronics that performs the exclusive OR operation. It is a widely used component in various digital circuits and systems, including arithmetic logic units, error detection and correction circuits, and cryptographic applications. The XOR gate has the unique property of producing a high output when the two inputs are different, and a low output when the two inputs are the same. This behavior makes the XOR gate useful for a variety of applications, such as parity checking, data encryption, and digital communication protocols. Understanding the design and implementation of the XOR gate is crucial for anyone working in the field of digital electronics, as it serves as a building block for more complex digital circuits and systems.
2. Transistor level layout

Transistor-level layout is a critical aspect of integrated circuit (IC) design, as it involves the physical placement and interconnection of individual transistors on the silicon substrate. This process requires a deep understanding of semiconductor physics, device characteristics, and layout design rules to ensure the proper functioning and performance of the circuit. The transistor-level layout determines the size, shape, and relative positioning of the transistors, as well as the routing of the interconnections between them. Careful consideration must be given to factors such as parasitic capacitances, resistance, and layout-dependent effects to optimize the circuit's speed, power consumption, and reliability. Mastering transistor-level layout is a highly specialized skill that is essential for designing high-performance, low-power, and area-efficient integrated circuits across a wide range of applications, from digital logic to analog and mixed-signal circuits.
3. Size ratio's Graph Difference

The size ratio between PMOS and NMOS transistors in a CMOS circuit can have a significant impact on the behavior and performance of the circuit, which can be observed in the resulting graphs. Varying the size ratio can lead to changes in the switching characteristics, propagation delays, noise margins, and other key parameters of the circuit. For example, increasing the PMOS-to-NMOS size ratio can improve the pull-up strength and noise margins of a logic gate, but it may also increase the power consumption and propagation delay. Conversely, decreasing the size ratio can improve the speed and reduce the power consumption, but it may compromise the noise margins and signal integrity. Understanding the relationship between the size ratio and the resulting circuit characteristics, as reflected in the graphs, is crucial for IC designers to optimize the trade-offs between performance, power, and reliability. Analyzing and interpreting these graphs is an essential skill for designers to make informed decisions during the circuit design and optimization process.
4. Size ratio_Finger

The concept of size ratio and finger layout in transistor design is an important consideration in integrated circuit (IC) design. The size ratio refers to the relative sizing of PMOS and NMOS transistors, which can significantly impact the performance, power consumption, and other characteristics of the circuit. The finger layout, on the other hand, is a technique used to arrange the transistor's source, drain, and gate regions in a parallel configuration to increase the effective channel width without increasing the overall area. By adjusting the size ratio and the number of fingers, designers can optimize the trade-offs between factors such as speed, power, and area. For example, increasing the PMOS-to-NMOS size ratio can improve the noise margins and pull-up strength, but it may also increase the power consumption. Similarly, adding more fingers can increase the current-driving capability of a transistor, but it may also introduce additional parasitic capacitances and layout complexities. Understanding the interplay between size ratio, finger layout, and their impact on circuit performance is a crucial skill for IC designers to create efficient and high-performing integrated circuits.
5. XOR Gate Layout and Simulation

The design and implementation of an XOR gate at the layout level is a crucial aspect of integrated circuit (IC) design. The XOR gate is a fundamental logic gate that performs the exclusive OR operation, and it is widely used in various digital circuits and systems, such as arithmetic logic units, error detection and correction circuits, and cryptographic applications. The layout of an XOR gate involves the physical placement and interconnection of the transistors that make up the gate. This process requires a deep understanding of semiconductor physics, device characteristics, and layout design rules to ensure the proper functioning and performance of the circuit. Factors such as transistor sizing, finger layout, and parasitic effects must be carefully considered to optimize the speed, power consumption, and reliability of the XOR gate. Simulation of the XOR gate layout is an essential step in the design process, as it allows designers to verify the functionality, timing, and other performance characteristics of the circuit before fabrication. By running simulations, designers can identify and address any potential issues, such as signal integrity problems, timing violations, or power consumption concerns, and make necessary adjustments to the layout. Mastering the design and simulation of XOR gate layouts is a valuable skill for IC designers, as it not only enables the creation of efficient and high-performing digital circuits but also serves as a foundation for understanding more complex digital logic and system-level design.