Logic States

The digital simulator is described as '12-state' which means that a digital signal can be in 1 of 12 states. These 12 states are combined from 3 levels and 4 strengths as follows:

Logic levels Strengths
HIGH STRONG
LOW RESISTIVE
UNKNOWN HI-IMPEDANCE
UNDETERMINED
Logic levels HIGH and LOW are self-explanatory. UNKNOWN means the signal could be either HIGH or LOW but which is not known at this stage. The start up state of a flip-flop is an example of an UNKNOWN state. Strength refers to the driving force behind the signal. STRONG is the highest with HI-IMPEDANCE the lowest. It is used to resolve conflicts when two outputs are connected together. For example consider a LOW-RESISTIVE signal (as possessed by a pull-down resistor) connected to a HIGH-STRONG signal There is a conflict between the two logic levels but as they are different strengths, the stronger wins and therefore the resulting level is HIGH.

State resolution table

The following table defines how a state is decided when two outputs are connected:

0S 1S XS 0R 1R XR 0Z 1Z XZ 0U 1U XU
0S 0S XS XS 0S 0S 0S 0S 0S 0S 0S XS XS
1S XS 1S XS 1S 1S 1S 1S 1S 1S XS 1S XS
XS XS XS XS XS XS XS XS XS XS XS XS XS
0R 0S 1S XS 0R XR XR 0R 0R 0R 0U XU XU
1R 0S 1S XS XR 1R XR 1R 1R 1R XU 1U XU
XR 0S 1S XS XR XR XR XR XR XR 1U XU XU
0Z 0S 1S XS 0R 1R XR 0Z XZ XZ 0U XU XU
1Z 0S 1S XS 0R 1R XR XZ 1Z XZ XU 1U XU
XZ 0S 1S XS 0R 1R XR XZ XZ XZ XU XU XU
0U 0S XS XS 0U XU XU 0U XU XU 0U XU XU
1U XS 1S XS XU 1U XU XU 1U XU XU 1U XU
XU XS XS XS XU XU XU XU XU XU XU XU XU

◄ Digital Simulation: Overview

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