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.

UNDETERMINED strength means that the strength of the signal is unknown.

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
  • 0S = LOW-STRONG
  • 1S = HIGH-STRONG
  • XS = UNKNOWN-STRONG
  • 0R = LOW-RESISTIVE
  • 1R = HIGH-RESISTIVE
  • XR = UNKNOWN-RESISTIVE
  • 0Z = LOW-HI-Z
  • 1Z = HIGH-HI-Z
  • XZ = UNKNOWN-HI-Z
  • 0U = LOW-UNDETERMINED
  • 1U = HIGH-UNDETERMINED
  • XU=UNKNOWN-UNDETERMINED

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