Blog

Introduction

When working with digital designs in QuestaSim (ModelSim), understanding how to analyze signals programmatically using TCL commands is essential. This post will guide you through the basics of signal analysis and demonstrate how to create a useful clock frequency measurement utility.

Basic Signal Access in QuestaSim

Understanding Signal Paths

In QuestaSim, signals can be accessed in several equivalent ways:

sim:/design/instance/my_clock    # Full path with dataset
/design/instance/my_clock        # Full path without dataset
design/instance/my_clock         # Relative path

The “sim:” prefix represents the current dataset name, which you can verify using:

dataset current    # Returns "sim" for active simulation

Examining Signal Values

To check a signal’s value, use the examine command:

examine design/instance/my_clock           # Current value
examine -time {1000 ns} design/instance/my_clock  # Value at specific time

Signal Logging and Wave Window

An important note: When a signal is added to the Wave window using add wave, it is automatically logged to the WLF (Wave Log Format) file. You don’t need to use the log command separately for signals already in the Wave window.

Creating a Clock Frequency Analysis Tool

Let’s create a powerful utility to measure clock frequencies. Save this as clock_utils.tcl:

###############################################################################
# Clock Frequency Calculation Utility
#
# Description:
#   This procedure calculates the clock frequency by finding two consecutive 
#   rising edges of a given signal starting from a specified time.
#
# Usage:
#   calculate_clock_frequency <start_time> <signal_path>
#
# Parameters:
#   - start_time: The time from which to start searching for rising edges (e.g., "1000ns")
#   - signal_path: Full path to the clock signal in the design
#
# Returns:
#   - Frequency in MHz
#
# Examples:
#   # Basic usage
#   set freq [calculate_clock_frequency 1000ns /design/instance/my_clock]
#
#   # If you want to just print the frequency
#   calculate_clock_frequency 1000ns /design/instance/my_clock
###############################################################################

# Time unit conversion helper
proc convert_to_ns {time_str} {
    set time_str [string trim $time_str]
    array set unit_factors {
        "fs" 0.001
        "ps" 1.0
        "ns" 1000.0
        "us" 1000000.0
        "ms" 1000000000.0
        "sec" 1000000000.0
    }

    foreach {unit factor} [array get unit_factors] {
        if {[string match "*$unit" $time_str]} {
            set numeric_value [scan [string range $time_str 0 end-[string length $unit]]
            return [format 
        }
    }
    return "${time_str}ns"
}

# Main frequency calculation procedure
proc calculate_clock_frequency {start_time signal} {
    # Find first rising edge
    set first_time [lindex [lindex [searchlog -count 1 -rising $start_time $signal] 0] 0]

    # Find second rising edge
    set second_time [lindex [lindex [searchlog -count 2 -rising $start_time $signal] 0] 0]

    # Convert times to ns
    set first_time_ns [convert_to_ns $first_time]
    set second_time_ns [convert_to_ns $second_time]

    # Strip 'ns' and convert to numeric
    set first_numeric [scan [string range $first_time_ns 0 end-2]
    set second_numeric [scan [string range $second_time_ns 0 end-2]

    # Calculate period and frequency
    set period [expr {$second_numeric - $first_numeric}]
    set frequency [expr {1000.0 / $period}]

    puts "First Edge:  $first_time_ns"
    puts "Second Edge: $second_time_ns"
    puts "Period:      $period ns"
    puts "Frequency:   [format 

    return $frequency
}

Understanding Key Commands

1. searchlog Command:

   searchlog -count 1 -rising $start_time $signal

• -count 1: Find first occurrence
• -rising: Look for rising edge
• Returns a nested list containing time and match count

1. List Processing:

   [lindex [lindex [...] 0] 0]

• Extracts time value from nested list structure
• First lindex gets outer list element
• Second lindex gets inner time value

1. restart Command:

• Resets simulation to time 0
• Reloads design elements that have changed
• Maintains current signal logging setup

Using the Utility

1. Source the utility file in your simulation:

source clock_utils.tcl

2. Calculate clock frequency:

calculate_clock_frequency 1000ns /design/instance/my_clock

Conclusion

This toolset provides a robust way to analyze clock signals in your QuestaSim simulations. By combining TCL’s powerful list processing with QuestaSim’s signal analysis commands, we can create sophisticated measurement utilities.

Remember:

• Signals in Wave window are automatically logged
• Use full paths for reliable signal access
• The current dataset (usually “sim:”) is optional in paths
• TCL’s list processing is key for handling QuestaSim command outputs

This approach to signal analysis can be extended to create more complex analysis tools for your specific needs.