A word in advance: SPICE is not a digital simulator. There are much better specialized tools out there for this purpose.
Having said that, LTSPICE does have limited digital simulation capabilities. They are provided for those cases where a mixed mode simulation (analog + digital) cannot be avoided. Since the use of digital logic in LTSPICE is not self-explanatory, here are some tips to get you started:
Figure 1 basic AND gate
Figure 1 shows a basic AND gate( other types of gate are available, see LTSPICE help). The gate has 5 inputs (1...5) to the left, one common return pin (8) at the bottom plus 2 outputs, one inverting, the other non-inverting (note that the pin numbers are not shown on the LTSPICE schematic - I have put them manually into figure 1 for explanation only). Thus the gate may be used either as an AND gate or as a NAND gate. Use the gate as follows (cf. figure 2):
Figure 2 complete NAND circuit
Note that I added a fixed voltage to each trace in figure 2. Thus the traces are stacked on top of each other to better distinguish between the different traces. A good offset is n*(Vcc+0.5 V) for traces 1...max (assuming the bottom trace is trace 0). In figure 2 Vcc=1 V (see below), thus the offsets are 0 V (trace 0), 1.5 V (trace 1) and 3 V (trace 2).
In addition it has to be noted that the LTSPICE logic gates by default operate with 0V...1V logic levels and a threshold of 0.5 V:
Figure 3 shows the attribute dialog (in LTSPICE: right click on the symbol to open the dialog) for the NAND gate of figure 2 where the output voltage for logic High is set to 5 V and the threshold is set to 2.5 V (if you do not set the threshold manually, it is calculated as 1/2*(Vhigh+Vlow).
Figure 3 Attributes of the NAND gate
Other parameters may be set as well (from the TLSPICE help):
The propagation delay of an LTSPICE logic gate defaults to 0 ns and has to be set to a meaningful value using the parameter td in the attribute dialog (figure 3).
SPICE help topics to look at: Special functions
Harald Kapp, 2014-05-13
Having said that, LTSPICE does have limited digital simulation capabilities. They are provided for those cases where a mixed mode simulation (analog + digital) cannot be avoided. Since the use of digital logic in LTSPICE is not self-explanatory, here are some tips to get you started:
Figure 1 basic AND gate
Figure 1 shows a basic AND gate( other types of gate are available, see LTSPICE help). The gate has 5 inputs (1...5) to the left, one common return pin (8) at the bottom plus 2 outputs, one inverting, the other non-inverting (note that the pin numbers are not shown on the LTSPICE schematic - I have put them manually into figure 1 for explanation only). Thus the gate may be used either as an AND gate or as a NAND gate. Use the gate as follows (cf. figure 2):
- Any unused input and/or output has to be connected to pin 8 (LTSPICE will recognize that these I/Os are unused and remove them from the simulation).
- Pin 8 (plus the unused I/Os) has to be connected to the common ground of the circuit.
- The used output needs a path to ground, so you have to connect at least a resistor to ground. Otherwise LTSPICE will throw an error.
- There is no power supply for the logic gates (other than the common return pin 8).
Note that I added a fixed voltage to each trace in figure 2. Thus the traces are stacked on top of each other to better distinguish between the different traces. A good offset is n*(Vcc+0.5 V) for traces 1...max (assuming the bottom trace is trace 0). In figure 2 Vcc=1 V (see below), thus the offsets are 0 V (trace 0), 1.5 V (trace 1) and 3 V (trace 2).
In addition it has to be noted that the LTSPICE logic gates by default operate with 0V...1V logic levels and a threshold of 0.5 V:
- U <0.5 V -> logic low
- U >0.5 V -> logic high
Figure 3 shows the attribute dialog (in LTSPICE: right click on the symbol to open the dialog) for the NAND gate of figure 2 where the output voltage for logic High is set to 5 V and the threshold is set to 2.5 V (if you do not set the threshold manually, it is calculated as 1/2*(Vhigh+Vlow).
Figure 3 Attributes of the NAND gate
Other parameters may be set as well (from the TLSPICE help):
The propagation delay of an LTSPICE logic gate defaults to 0 ns and has to be set to a meaningful value using the parameter td in the attribute dialog (figure 3).
SPICE help topics to look at: Special functions
Harald Kapp, 2014-05-13
