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 |
-
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