Adaptive Design Approaches and Performance Uncertainties in IC Design
1) What are the different adaptive design approaches to address performance uncertainties in IC design?
Consider three sources of performance uncertainties: Process variation, temperature variations, IR drop. For each, consider both global and local effects. From this set of 6 sources, list which variations can be addressed with the following adaptive design approaches: • a canary circuit that is fixed at design time and has no further post silicon tuning. • a canary circuit which is tuned post silicon for each chip. • always correct in situ delay detection (i.e, triple latch monitor) • an in situ "let fail and correct" method such as Razor
2) Why do we need metal fill?
3) List two local random variation sources.
4) How do the variance of random and systematic variations scale with the population?
5) What are the reasons that may prevent you from skewing gates too aggressively in pulse static logic?
Answers:
1) Adaptive design approaches can address different performance uncertainties in different ways:
- Process variation: Both global and local process variations can be addressed with adaptive design approaches. For a canary circuit that is fixed at design time and has no further post-silicon tuning, it can help mitigate process variations by providing a reference point for comparison. A canary circuit that is tuned post-silicon for each chip can also help compensate for process variations by adjusting the circuit parameters based on the specific chip's characteristics.
- Temperature variations: Global temperature variations can be addressed with adaptive design approaches. A canary circuit that is fixed at design time may not be able to address temperature variations directly, as it is not designed to be adjustable. However, a canary circuit that is tuned post-silicon for each chip can take temperature variations into account and make appropriate adjustments.
- IR drop: Both global and local effects of IR drop can be addressed with adaptive design approaches. A canary circuit that is fixed at design time may not have the ability to directly compensate for IR drop variations. However, a canary circuit that is tuned post-silicon for each chip can dynamically adjust the circuit parameters to minimize the impact of IR drop.
2) Metal fill is needed in integrated circuit (IC) design to improve the performance and reliability of the circuit. Metal fill is used to fill in empty spaces on the metal layers of the IC. It helps to ensure uniformity of the metal layers, reduce the effects of process variations, and improve the electromigration resistance of the metal interconnects. Metal fill also helps to reduce the capacitance and inductance variations in the circuit, which can affect the overall performance of the IC.
3) Two local random variation sources in IC design are:
- Local process variations: Within a single chip, there can be variations in the fabrication process that lead to differences in the electrical properties of the transistors and interconnects. These local process variations can result in variations in device characteristics, such as threshold voltage or resistance, which can affect the performance and reliability of the circuit.
- Local voltage and current variations: The voltage and current supplied to different parts of the circuit may vary due to factors such as resistive and capacitive effects in the interconnects or power supply noise. These local variations can lead to differences in timing, power consumption, and signal integrity within the circuit.
4) The variance of random variations and systematic variations scale with the population as follows:
- Random variations: The variance of random variations scales with the square root of the population size. This means that as the population size increases, the random variations become relatively smaller. For example, if the random variations have a standard deviation of 1, then the variance will be proportional to the square root of the population size.
- Systematic variations: The variance of systematic variations does not scale with the population size. Systematic variations are inherent to the design or manufacturing process and are not affected by the number of instances or chips. These variations can include factors such as systematic errors in design, variations in process parameters, or equipment limitations.
5) In pulse static logic, skewing the gates along the path can improve the transition delay. However, there are two reasons that may prevent aggressive skewing:
- Setup and hold time violations: Aggressive skewing of the gates can potentially lead to setup and hold time violations. Setup time violation occurs when the input to a flip-flop changes too close to the clock edge, resulting in incorrect data capture. Hold time violation occurs when the input to a flip-flop changes too close to the clock edge, resulting in unstable data.
- Power and area overhead: Skewing gates aggressively can lead to increased power consumption and area overhead. Skewed gates may require additional circuitry, such as delay elements, which can increase power consumption. Additionally, the increased complexity of the skewed gates can result in larger area requirements, impacting the overall chip size and cost.