Enhancements over XSPICE

• Gate delays in XSPICE are stored i.e. like a transmission line not like a real gate. SIMetrix gate delays are inertial so if a pulse shorter than the propagation delay is received, it is swallowed not transmitted.
• Automatic interface creation. In XSPICE you have to explicitly join digital and analog nodes via interface bridges. In SIMetrix this is done automatically.
• Fan out implemented. The underlying mechanism for load dependent delay was there but none of the models supported it. Static loading effects (as in bipolar logic) was not supported at all. In SIMetrix it is.
• Input load reflected in analog to digital interfaces. The AD interfaces in XSPICE have infinite input impedance regardless of what the digital output is driving. SIMetrix AD interfaces reflect the digital capacitative and static load at their inputs.
• Output strength reflected in digital to analog interfaces. The DA interfaces in XSPICE have zero output impedance regardless of what is driving them. SIMetrix DA interfaces reflect the strength of the digital output driving the input. A hi-z logic state will look like a hi-z logic state when transferred to the analog domain. This is not the case with XSPICE.
• AD interface threshold detection. All AD interfaces switch at a particular input threshold. In the XSPICE system the output switched at the first analog timepoint that exceeded the threshold. This could be a long way passed the threshold if the analog time steps are large. In SIMetrix a mechanism has been implemented that cuts back the time step so that the threshold is hit within a specified time tolerance.
• Arbitrary logic block device. This allows the definition of any logic device using a simple descriptive language. The language accommodates combinational logic, synchronous and asynchronous registers as well as look up tables (i.e. ROMS) and arrays (i.e. RAMs).
• Arbitrary analog to digital converter. Up to 32 bits with specified input range and offset, conversion time and maximum conversion rate. Output may be in two's complement or offset binary.
• Arbitrary digital to analogue converter. Up to 32 bit with specified input range and offset and output slew time. Input may be in two's complement or offset binary.
• Voltage controlled oscillator (analog in digital out). There was one of these in the original XSPICE code but it suffered a number of problems and was scrapped. The SIMetrix version is all new.