What is SPICE Model

What is SPICE Model

SPICE (Simulation Program with Integrated Circuit Emphasis). As I/O switching frequencies increase and voltage levels decrease, accurate analog simulation of I/O becomes a very important part of modern high-speed digital system design. By accurately simulating I/O buffers, terminals, and board traces, you can significantly reduce the time to market for new designs. By identifying signal integrity related issues at the beginning of the design, the number of board fixing points can be reduced.

Traditionally, SPICE analysis has been used in areas such as IC design where high accuracy is required. However, in the context of PCBs and systems, the SPICE method has several disadvantages for both users and device vendors.

Because SPICE simulations model circuits at the transistor level, they contain detailed information about circuit and process parameters. Most IC vendors consider this information proprietary and refuse to make their models public.

Although SPICE simulations are accurate, simulation speeds are particularly slow for transient simulation analysis (commonly used when evaluating signal integrity performance). Also, not all SPICE simulators are fully compatible. The default simulator options may vary from one SPICE simulator to another. Because some powerful options control accuracy, convergence, and algorithm type, any inconsistent options may result in poor correlation of simulation results from different simulators. Finally, because there are SPICE variants, models are not always compatible between simulators in general; they must be screened for a specific simulator.

SPICE models are text-based descriptions of circuit devices used by SPICE simulators, which can mathematically predict the electrical behavior of components under different circumstances. SPICE models range from the simplest one-line descriptions of passive components such as resistors to extremely complex subcircuits that use hundreds of lines of description.

SPICE models should not be confused with pSPICE models. pSPICE is a dedicated circuit simulator provided by OrCAD. Although some pSPICE models are compatible with SPICE, full compatibility is not guaranteed. SPICE is the most widely used circuit simulator and is an open standard.

What is Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC)

Electromagnetic interference (Electromagnetic Interference) includes two types: conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of a signal on an electrical network to another electrical network through a conductive medium. Radiated interference refers to the coupling (interference) of an interference source to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may become radiated interference sources with antenna characteristics, which can emit electromagnetic waves and affect the normal operation of other systems or other subsystems within the system.

Since the emergence of electronic system noise reduction technology in the mid-1970s, mainly due to the US Federal Communications Commission in 1990 and the European Union in 1992, regulations on commercial digital products were proposed. These regulations require companies to ensure that their products meet strict susceptibility and emission standards. Products that meet these regulations are called EMC (Electromagnetic Compatibility).

What is signal integrity?

Signal integrity refers to the quality of the signal on the signal line. A signal with good signal integrity means that it has the necessary voltage level value when needed. Poor signal integrity is not caused by a single factor, but by multiple factors in board-level design. The main signal integrity problems include reflection, oscillation, ground bounce, crosstalk, etc.

What is reflection?

Reflections are echoes on a transmission line. Part of the signal power (voltage and current) is transmitted to the line and reaches the load, but part of it is reflected. If the source and load have the same impedance, reflections will not occur. Impedance mismatch between the source and load causes reflections on the line, and the load reflects part of the voltage back to the source. If the load impedance is less than the source impedance, the reflected voltage is negative, and vice versa, if the load impedance is greater than the source impedance, the reflected voltage is positive. Changes in the geometry of the wiring, improper line termination, transmission through connectors, and discontinuities in the power plane can all cause such reflections.

What is crosstalk?

Crosstalk is the coupling between two signal lines. The mutual inductance and mutual capacitance between the signal lines cause noise on the line. Capacitive coupling causes coupling current, while inductive coupling causes coupling voltage. The parameters of the PCB board layer, the spacing between the signal lines, the electrical characteristics of the driver and receiver, and the line termination method all have a certain impact on crosstalk.

What is overshoot and undershoot?

Overshoot is the first peak or valley that exceeds the set voltage - the highest voltage for a rising edge and the lowest voltage for a falling edge. Undershoot is the next valley or peak. Excessive overshoot can cause protection diodes to operate, leading to premature failure. Excessive undershoot can cause false clock or data errors.

What is ringing and rounding?

The phenomenon of oscillation is repeated overshoot and undershoot. Oscillation and ringing of the signal are caused by excessive inductance and capacitance on the line. Oscillation is underdamped and ringing is overdamped. Signal integrity problems usually occur in periodic signals, such as clocks. Oscillation and ringing are caused by many factors, just like reflections. Oscillation can be reduced by proper termination, but it is impossible to completely eliminate it.

What is ground plane bounce noise and return noise

When there is a large current surge in the circuit, it will cause ground plane bounce noise (referred to as ground bounce). For example, when the outputs of a large number of chips are turned on at the same time, a large transient current will flow through the power plane of the chip and the board. The inductance and resistance of the chip package and the power plane will cause power noise, which will cause voltage fluctuations and changes on the real ground plane (0V). This noise will affect the operation of other components. The increase in load capacitance, the decrease in load resistance, the increase in ground inductance, and the increase in the number of switching devices will all lead to an increase in ground bounce. Due to the division of the ground plane (including power and ground), for example, the ground layer is divided into digital ground, analog ground, shielding ground, etc., when the digital signal reaches the analog ground area, ground plane reflux noise will be generated. Similarly, the power layer may also be divided into 2.5V, 3.3V, 5V, etc. Therefore, in multi-voltage PCB design, the bounce noise and reflux noise of the ground plane need special attention.

What is the difference between the time domain and the frequency domain?

The time domain is the process of voltage or current changes based on time, which can be observed with an oscilloscope. It is usually used to find pin-to-pin delays, skew, overshoot, undershoot, and settling times.

The frequency domain is the process of voltage or current variation based on frequency, which can be observed with a spectrum analyzer. It is often used to compare waveforms with FCC and other EMI control limits.

What is impedance?

Impedance is the ratio of input voltage to input current on a transmission line (Z0=V/I). When a source sends a signal to the line, it will hinder it from driving until 2*TD, the source does not see its change, where TD is the delay of the line.

What is settling time?

Settling time is the time required for an oscillating signal to settle to a specified final value.

What is pin-to-pin delay?

Pin-to-pin delay is the time between a change in state at the driver and a change in state at the receiver. These changes typically occur at 50% of a given voltage, with the minimum delay occurring when the output first crosses a given threshold and the maximum delay occurring when the output last crosses the voltage threshold, all measured in these cases.

What is skew?

The skew of a signal is the time difference between the arrival of the same network at different receivers. Skew is also used to measure the time difference between the arrival of clock and data at logic gates.

What is Slew Rate?

Slew rate is the edge slope (the ratio of a signal's voltage to time change). The I/O specification (such as PCI) states that the state is between two voltages, which is the slew rate, and it can be measured.

What is a quiescent line?

It does not toggle during the current clock cycle. Also called a "stuck-at" line or static line. Crosstalk can cause a static line to toggle during a clock cycle.

What is false clocking?

Spurious clocking occurs when a clock crosses a threshold and changes state unintentionally (sometimes between VIL or VIH). It is usually caused by excessive undershoot or crosstalk.