SIMPLIS Parts

Digital Lookup Table with Don't Care Input Definitions

The Digital Lookup Table with Don't Care Input Definitions models combinatorial logic defined with a truth table. The truth table maps logical combinations of input states to output states. Unlike the Digital Lookup Table, this device allows the input space to include don't care inputs. A default output state determines the output if the input state is not defined in the table.

The digital lookup table with don't care input definitions is even more powerful than the digital lookup table, especially for defining state machines or complex logic. Using the lookup table with don't care input definitions, you can easily implement, test, and revise extremely complex logic, all without changing the schematic view of the design.

If you do not require input definitions which include don't care states, see Digital Lookup Table. The points definition of the digital lookup table is more flexible and it is slightly easier to configure.

In this Topic ShowHide

Editing the Digital Lookup Table with Don't Care Input Definitions
Table Definition
Examples
Waveforms
Subcircuit Parameters
Common File Formatting Issues

Model Name:	Digital Lookup Table with Don't Care Input Definitions
Simulator:		This device is compatible with the SIMPLIS simulator.
Parts Selector Menu Location:	Digital Functions \| Functions
Symbol Library:	None - the symbol is automatically generated when placed or edited.
Model File:	None - the device model is generated before simulation.
Subcircuit Name:	SIMPLIS_DIGI1_D_LOOKUP_TABLE_DONTCARE_N: Without Ground Reference SIMPLIS_DIGI1_D_LOOKUP_TABLE_DONTCARE_Y: With Ground Reference
Symbol:
Multiple Selections:	Only one device at a time can be edited.

Editing the Digital Lookup Table with Don't Care Input Definitions

To configure the digital lookup table with don't care input definitions, follow these steps:

Double click the symbol on the schematic to open the editing dialog to the Parameters tab.
Make the appropriate changes to the fields described in the table below the image.

Label	Parameter Description
Output Delay	Delay from when the input state changes until the output changes
Number of Input Bits	Number of input bits for the digital lookup table
Number of Output Bits	Number of output bits for the digital lookup table
Redefine Table?	Determines whether the lookup table is to be redefined after the dialog is closed. Checked: The lookup table will be redefined after the dialog is closed. Unchecked: The lookup table definition remains unchanged after the dialog is closed.
Default Value	Default value of lookup table output if the input value is not found in the lookup table.
Initial Condition	Initial condition of the lookup table output in decimal
Ground Ref	Determines whether or not a device has a ground reference pin. Any digital component that has an input or output pin connected to an analog circuit node must have its Ground Ref pin connected to an analog node. This is usually ground on the schematic. For more information on the use of Ground Ref for digital components, see When is Ground Ref Required?

To define the parameters for the interface between this digital component and each analog component connected directly to an input or output pin, follow these steps from the Edit Digital Lookup Table with Don't Care Input Definitions dialog box:

Click on the Interface tab.
Make the appropriate changes to the fields described in the table below the image.

Label

Parameter Description

Input Resistance

Input resistance of each input pin

Threshold

Hysteresis

The Threshold (T) and Hysteresis (H) of the Schmitt trigger input buffer on each Lookup Table input. To determine the low-to-high threshold (TH) and the high-to-low threshold (TL), substitute Threshold (T) and Hysteresis (H) in each of the following formulas :

Input Logic Transition	Actual Threshold
0 ➞ 1	TH = Threshold + 0.5 * Hysteresis
1 ➞ 0	TL = Threshold - 0.5 * Hysteresis

Output Resistance

Output resistance of the lookup table output pin(s)

Output High Voltage

Output high voltage for the lookup table output pin(s)

Output Low Voltage

Output low voltage for the lookup table output pin(s)

Table Definition

You define the digital lookup table with don't care input definitions with an ASCII text file. Unlike the Digital Lookup Table, this device requires a single column to define each input and output bit. The points definition file is otherwise similar to the definition of a regular digital lookup table.

The input file is whitespace delimited, with spaces or tabs, or a combination of the two. Each line in the definition file defines the input states which correspond to a set of output states. The delimited line has three components - the input definitions, the delimiter (space or tab) which separates the input definitions from the output definitions, and the output definitions.

A points definition file example can be downloaded here: state_machine_lookup_table. This file is also included in the following zip archive: simplis_042_lut_dont_care_example.zip.

Input Definitions

Each line starts with the input definitions, starting with the most significant bit (MSB). Each bit in the input space is defined with one of three characters:

0 The input bit is required to be logic low for this input combination to test true.
1 The input bit is required to be logic high for this input combination to test true.

- The input bit can be either logic low or logic high for this input combination to test true.

Input-to-Output Delimiter

The pipe character | must occur on every line after the input definitions and before the output definitions. As with all entries on the line, the pipe character should be separated by a space or tab character.

Output Definitions

After the input bits and the pipe delimiter, the outputs are defined starting with the MSB. Each output bit is defined with one of three characters:

0 The output bit is low for this input combination.
1 The output bit is high for this input combination.
x The output bit is in the unknown state for this input combination. The "x" character is case-insensitive.

Examples

The test circuit used to generate the waveform examples in the next section can be downloaded here: simplis_042_lut_dont_care_example.zip.

Waveforms

This example uses a digital lookup table with don't care input definitions and a digital register to implement a simple state machine. The state machine has four states (S00 through S03), four inputs (A, B, C, and D) and four state indication outputs. The definition of the state machine is described graphically in the following state transition diagram.

State Diagram

Lookup Table Definition

State Transition Diagram

Inputs							Outputs						Action
A	B	C	D	Current State			Next State		S11	S10	S01	S00
A	B	C	D	MSB	LSB		MSB	LSB	S11	S10	S01	S00
0	-	-	-	0	0	\|	0	0	0	0	0	1	Remain in S00
1	-	-	-	0	0	\|	0	1	0	0	0	1	Transition from S00->S01
-	0	-	-	0	1	\|	0	1	0	0	1	0	Remain in S01
-	1	-	-	0	1	\|	1	0	0	0	1	0	Transition from S01->S10
-	-	0	-	1	0	\|	1	0	0	1	0	0	Remain in S10
-	-	1	-	1	0	\|	1	1	0	1	0	0	Transition from S10->S11
-	-	-	0	1	1	\|	1	1	1	0	0	0	Remain in S11
-	-	-	1	1	1	\|	0	0	1	0	0	0	Transition from S11->S00

In this example, the initial state is S00. The state machine transitions to the next state only on a rising clock edge when the A input is high. These conditions are met at time=0.5us. Shortly after the state machine moves to the S01 state, the A, C, and D inputs transition high for a clock cycle to test that the machine will not change state erroneously. The cycle repeats every 1us with the state machine transitioning through all four states.

Subcircuit Parameters

The digital lookup table model is generated by a template script when the simulation is executed. For this reason, there is no fixed model which can be inserted into a netlist. The template script for this device is simplis_make_signal_source_model.sxscr, which you, as a licensed user, can download in a zip archive of all built-in scripts.

To download this zip file, follow these steps:

Note: You will be prompted to log in with the user name and password given to you when you registered.

Click here to go to the product-installation page on the SIMetrix website.
Click Download Links in the first paragraph of the product-installation web page.
Scroll down to the Built-in Scripts section.
Right click on Download and select Save target as..., and then navigate to a location on your computer to save the zip file.

Common File Formatting Issues

An input file without an extra empty line after the last point definition:

If you examine the file example, it contains 6 lines: 4 are comments, one is an input definition, and the last one is an empty line. This empty line MUST be present at the end of each definition files. If the line is not present, the following error occurs:

*** ERRORS REPORTED BY SIMPLIS ***
**************************************** <<<<<<<< Error Message ID: 1179 >>>>>>>> points file AND_NA~1.TXT No data point has been defined in this file. This model requires 1 data points to be defined.*** END SIMPLIS ERROR REPORT ***
An input file that contains non-ASCII characters:

A points file that contains non-ASCII characters cannot be read by SIMetrix/SIMPLIS; as a result, the lookup table creation is aborted and the following errors are generated:Error : The file 'and_nand_points.txt' contained some non-ascii characters File read has been aborted*** ERRORS REPORTED BY SIMPLIS *** **************************************** <<<<<<<< Error Message ID: 1057 >>>>>>>> input file file_path\SIMPLIS_Data/simplis_041_digitallookuptable_file_example.deck, line 76: + VOL=0 VOH=5 DEFAULT=2 OUT_DELAY=1e-011 NPTS=0 NUMBITS_OUT=2 NUMBITS_IN=10 FILE=AND_NA~1.TXT A positive integer to represent the value for `NPTS' is expected at the location where `0' occupies.*** END SIMPLIS ERROR REPORT ***
Comments at the end of the point-definition line:

If you have a comment in a point-definition line, the following error occurs:

*** ERRORS REPORTED BY SIMPLIS ***
**************************************** <<<<<<<< Error Message ID: 1179 >>>>>>>> points file AND_NA~1.TXT, line 5 1023 0 1 *** illegal comment ^ Unexpected entry `***' when end of line is expected while reading point #1.*** END SIMPLIS ERROR REPORT ***