ALTERA FPGA timing analysis

Timing analysis is a critical step in FPGA design to ensure that your design meets the required timing constraints and operates reliably at the target clock frequency. For Altera FPGAs (now part of Intel FPGAs), the Quartus Prime software provides powerful tools for timing analysis. Below is a detailed guide to performing timing analysis in Altera FPGAs using Quartus Prime.

1. What is Timing Analysis?

Timing analysis ensures that all signals in your design meet setup and hold time requirements, propagate through the logic within one clock cycle, and do not violate any timing constraints. It involves:

• Setup Time: The time before the clock edge when data must be stable.
• Hold Time: The time after the clock edge when data must remain stable.
• Clock-to-Output Delay: The time it takes for data to propagate from a register's input to its output.
• Clock Skew: The difference in arrival times of the clock signal at different registers.

2. Types of Timing Analysis

1. Static Timing Analysis (STA):

• Analyzes the design without requiring simulation.
• Checks all possible paths for timing violations.

1. Dynamic Timing Analysis:

• Uses simulation to verify timing under specific conditions.
• Less comprehensive than STA but useful for functional verification.

3. Steps for Timing Analysis in Quartus Prime

Step 1: Set Timing Constraints

1. Define Clock Frequencies:

• Specify the clock frequencies for all clocks in your design.
• Example: Create a 100 MHz clock constraint.

tcl

create_clock -name clk -period 10 [get_ports clk]

1. Set Input/Output Delays:

• Define the timing requirements for input and output signals relative to the clock.
• Example: Set input delay for a signal.

tcl

set_input_delay -clock clk 2 [get_ports data_in]

1. Set False Paths and Multicycle Paths:

• Specify paths that do not require timing analysis (false paths) or paths that require multiple clock cycles (multicycle paths).
• Example: Set a false path.

tcl

set_false_path -from [get_clocks clk1] -to [get_clocks clk2]

Step 2: Compile the Design

1. Run the Analysis & Synthesis and Fitter stages in Quartus Prime.

2. The compiler maps your design to the FPGA resources and performs initial timing analysis.

Step 3: Run the TimeQuest Timing Analyzer

1. Open the TimeQuest Timing Analyzer from the Quartus Prime tools menu.

2. Read SDC File:

Load the timing constraints file (.sdc).

1. Generate Timing Netlist:

Create a timing netlist for analysis.

1. Perform Timing Analysis:

• Run Setup Analysis and Hold Analysis to check for violations.
• Use the Report Timing tool to view detailed timing reports.

Step 4: Analyze Timing Reports

1. Setup and Hold Violations:

• Check for paths that fail setup or hold timing requirements.
• Example: A setup violation occurs if data arrives too late at a register.

1. Slack:

• Positive slack indicates that timing requirements are met.
• Negative slack indicates a timing violation.

1. Critical Path:

Identify the longest path in the design (critical path) and optimize it.

Step 5: Optimize the Design

1. Pipeline the Design:

Add pipeline stages to break long combinational paths.

1. Reduce Logic Levels:

Simplify logic to reduce propagation delays.

1. Use Register Retiming:

Move registers to balance delays across paths.

1. Increase Clock Period:

If possible, reduce the clock frequency to meet timing requirements.

Step 6: Recompile and Verify

1. Recompile the design after making optimizations.

2. Rerun the timing analysis to ensure all violations are resolved.

4. Common Timing Issues and Solutions

1. Setup Violations:

Cause: Data arrives too late at the destination register.

Solution:

• Reduce combinational logic delay.
• Use pipelining.
• Increase clock period.

2. Hold Violations:

Cause: Data changes too soon after the clock edge.

Solution:

• Add delay buffers.
• Adjust clock skew.

3. Clock Skew:

Cause: Uneven clock distribution.

Solution:

• Use global clock networks.
• Balance clock tree synthesis.

5. Advanced Timing Analysis Features

1. Clock Domain Crossing (CDC) Analysis:

• Verify synchronization for signals crossing clock domains.
• Use synchronizers (e.g., double-flop synchronizers) to avoid metastability.

2. Timing Exceptions:

Define false paths, multicycle paths, and maximum/minimum delays.

3. On-Chip Variation (OCV) Analysis:

Account for process, voltage, and temperature variations.

4. Post-Fit Simulation:

Simulate the design with actual routing delays to verify timing.

6. Example: Timing Constraints File (.sdc)

tcl

# Clock definition
create_clock -name clk -period 10 [get_ports clk]

# Input delay
set_input_delay -clock clk 2 [get_ports data_in]

# Output delay
set_output_delay -clock clk 3 [get_ports data_out]

# False path
set_false_path -from [get_clocks clk1] -to [get_clocks clk2]

# Multicycle path
set_multicycle_path -setup 2 -from [get_registers reg1] -to [get_registers reg2]

7. Tools in Quartus Prime

• TimeQuest Timing Analyzer: For static timing analysis.
• Signal Tap Logic Analyzer: For debugging timing issues on hardware.
• Chip Planner: For visualizing and optimizing placement and routing.

Summary

Timing analysis in Altera FPGAs involves:

1. Setting timing constraints.
2. Compiling the design.
3. Running the TimeQuest Timing Analyzer.
4. Analyzing and optimizing the design to meet timing requirements.

By following these steps, you can ensure that your FPGA design operates reliably at the target clock frequency.