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SMASH Hi-Z Nets Detection
Hi-Z, also known as high impedance, tri-stated, or floating, is the state of a net or output terminal which is not currently driven by the circuit. High impedance nets are useful when used appropriately, e.g. in amplifiers for large voltage gains without large current consumption, in tri-state outputs for data busses, in standard cell library cells… However, unintentional high impedance nets are a frequent cause of non-working circuits, e.g. due to major leakage resulting from short-circuits to ground… while practically undetectable with functional simulations.
SMASH provides the analysis capabilities to detect these high-impedance nets.
Key features
- Interconnect-based high impedance net detection at circuit elaboration
- Dynamic high impedance net detection at operating-point and during transient analysis
- Capacitive and resistive net impedance measurement for all types of analyses
- Analog net coverage analysis to detect test bench shortcomings
- Graphic tracing of net impedance in the waveform viewer
- Measuring of min, max and average impedance
- Characterization of input & output cell impedance at operating-point
- Characterization of net sensitivity to parasitic capacitances before layout
- Avoid expensive non-functional first-pass test-chip or silicon qualifier
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Description of the solution
Designers must be aware that functional simulation will generally fail to detect high impedance nets, as well as the propagation of the high impedance states, because the simulation time of a test bench is generally much shorter than the time needed to measure drift effects from leakage current. Furthermore, functional test benches are also generally insufficient, both due to application of incomplete test vector sets and to omission of effects such as noise coupling on supplies or on the substrate…
To detect potentially crippling high impedance nets, SMASH provides an automated impedance analysis that identifies them during simulation based on a designer specified impedance threshold. SMASH extends functional simulation to include an analog net “coverage like” analysis allowing to analyze impedance variation, measure capacitive and resistive net impedances and detect high impedance nets.
Thanks to SMASH you can increase your productivity and benefit from dynamic detection of high impedance nets, thereby avoiding mal-functioning circuits, yield losses, excessive leakage power… and costly silicon runs!
Logic Hi-Z
In logic circuits, output ports can have a value of 0, 1, X, or Z. Z output stands for the output port being disconnected from the rest of the circuit, putting the output in a high impedance state where the connected signal is not actively driven. The intent of this state is to allow multiple circuits to share the same output line or bus without affecting each other. Such a signal is like an open circuit or floating wire: when connecting it to a component, the signal will instead be pulled to the same voltage as the actively driven output.
Analog Hi-Z
In analog circuits, a high impedance net is one that does not have any low impedance paths to any other nets. High impedance nets have high thermal noise and are prone to Noise Pick-Up (NPU). They are also often difficult to probe as the impedance of a measurement instrument can load down the node.
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Hi-Z is the name of a high impedance input jack on preamps. This is the jack where you plug in your guitar, bass, or other musical instruments with pickups. Preamps with Hi-Z jacks include Direct Boxes and guitar and bass amplifiers. |
Characterization of input/output impedances
The resistive and capacitive impedances of nets and sources are automatically computed at the issue of the operating-point search. Therefore, the characterization of input/output cells’ impedance no longer needs specific simulations as the corresponding impedances are directly available with the operating-point result.
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