LOAD

Net loading element for backannotation.

BACKANNOTATION

Description

The LOAD primitive is a backannotation element used to add capacitive loading to nets in a circuit. It does not perform logic evaluation—it exists solely to specify incremental net loading that is added to the loading already present at the specified nodes.

Backannotation elements like LOAD and DELAY allow designers to incorporate extracted parasitics and timing information from layout tools back into the simulation model without modifying the original circuit description.

Note: NET is an alias for LOAD—both type names refer to the same primitive and are interchangeable.

Equivalent Type Statement

Type=LOAD o=Q
Type=NET o=Q

Usage Syntax

Nets are assigned loading by instantiating the built-in LOAD element using one of two forms:

Form 1 - With explicit part name:

P=<name> T=load O=<net-name> OLOD=<value>

Form 2 - Using net name as part name:

P=<net-name> T=load OLOD=<value>

Where:

<net-name> is the hierarchical name of the net to which loading is added
<value> is the loading value (single value or typ;min;max triplet)

Loading Value Formats

Loading can be specified in several formats:

Single value: OLOD=10 - typical loading only
Min/typ/max triplet: OLOD=3;2;5 - typical=3, minimum=2, maximum=5
Range notation: OLOD=5;4;6 or OLOD=7.89

Examples

Example 1: Simple Net Loading

Add 10 units of loading to signal 'a':

PART=a TYPE=load OLOD=10

Example 2: Hierarchical Signal with Min/Typ/Max Loading

Assign typical=3, minimum=2, maximum=5 loading to signal 'a.b.c':

PART=a.b.c TYPE=load OLOD=3;2;5

Example 3: Backannotation File Usage

A typical backannotation file (.ann extension) containing LOAD elements:

Remark= This adds loading to the signals
Remark= named 'a' and 'main.q'

!format p= t= olod=

A           load    10
main.q      load    17

This file would be read with the command:

>>: get afile=mybackannotation.ann

Technical Notes

Backannotation Element: The LOAD primitive is one of two backannotation elements, along with DELAY.
Logic Handling: This primitive does not perform logic evaluation; it does not drive any value onto the net. It exists solely to provide loading metadata that affects signal propagation delays.
Alias: NET is an alias for LOAD—both are treated identically by the simulator and refer to the same primitive type.
Incremental Loading: The loading specified by LOAD elements is added to the loading already present at the specified nodes, not replaced.
Backannotation Files: LOAD elements are commonly used in backannotation files (default .ann extension) that are read with the GET AFILE= command.
Restoring Original Loading: Subsequent GET AFILE commands will restore the loading specified in the network description before adding the new loading from the file.
Delay Propagation: Net loading affects the propagation delays of drivers connected to the net. Higher loading typically results in longer delays.
Global Signals: Using LOAD on global signals (like ONE or ZERO) does not trigger "Output connected to Logic ONE or ZERO" warnings, making them suitable for modeling power rail loading.