Let’s walk through this step by step—this is a common timing constraint task when interfacing an ADC to an FPGA, so I’ve got you covered. First, let’s recap the key parameters we’re working with:

• AD9238 CLK-to-DATA propagation delay: t_pd_ad_min = 2ns, t_pd_ad_max = 6ns
• Board-level delay (ADC data output to FPGA data input): t_pd_board_min = 0.5ns, t_pd_board_max = 1ns
• FPGA main clock: 80MHz (period T = 12.5ns), which generates the ADC’s sampling clock (we’ll assume this is a synchronous source—no asynchronous division, so the ADC clock is locked to the FPGA’s main clock)

1. Understand Input Delay (The Foundation of Setup/Hold Constraints) #

For FPGA timing analysis, we don’t directly "specify" setup/hold times for the data ports themselves. Instead, we define input delays relative to the FPGA’s capture clock. The FPGA tool (like Lattice Diamond) will then use these input delays plus its own internal register timing specs to verify if setup and hold margins are met.

Input delay measures how long data takes to reach the FPGA’s input pins after the ADC’s sampling clock edge. We need two critical values:

• Maximum input delay: Worst-case scenario for setup time (data arrives as late as possible)
• Minimum input delay: Worst-case scenario for hold time (data arrives as early as possible)

Calculate Input Delays #

• Max Input Delay: Sum the longest possible delays in the data path
  t_input_max = t_pd_ad_max + t_pd_board_max = 6ns + 1ns = 7ns
• Min Input Delay: Sum the shortest possible delays in the data path
  t_input_min = t_pd_ad_min + t_pd_board_min = 2ns + 0.5ns = 2.5ns

2. Apply Constraints in Your FPGA Tool #

For Lattice Diamond, we use Tcl commands to formalize these constraints—this is the standard approach for timing closure:

Step 1: Define the Main Clock #

First, constrain your FPGA’s 80MHz main clock (adjust the port name to match your design):

# Create constraint for the 80MHz main clock on port "clk_main"
create_clock -period 12.5 -name clk_main [get_ports clk_main]

Step 2: Define the ADC Capture Clock #

If the ADC’s clock (clk_adc) is generated by dividing the main clock (e.g., a 1:1 divider for the same 80MHz frequency), define it as a generated clock to link it to the main clock source:

# Replace "clk_divider/Q" with the actual output pin of your clock divider module
create_generated_clock -name clk_adc -source [get_ports clk_main] [get_pins {clk_divider/Q}]

Step 3: Apply Input Delay Constraints to Data Ports #

Now apply the input delays to your ADC data ports (e.g., adc_data[11:0] for the 12-bit AD9238):

# Max input delay for setup time analysis (relative to clk_adc's rising edge)
set_input_delay -max 7.0 -clock clk_adc [get_ports adc_data*]

# Min input delay for hold time analysis (relative to clk_adc's rising edge)
set_input_delay -min 2.5 -clock clk_adc [get_ports adc_data*]

3. Adjustments for Divided ADC Clocks #

If you’re using a slower ADC clock (e.g., dividing the 80MHz main clock to get 40MHz, period 25ns), you only need to update the create_generated_clock command to reflect the new period. The input delay values stay the same—since the ADC’s CLK-to-DATA delay doesn’t change with clock frequency. The tool will automatically use the new period for setup time calculations.

4. Verify Timing Closure #

After applying these constraints, run the timing analysis in your FPGA tool. You’ll want to confirm:

• Setup margin: Positive value means data arrives early enough before the capture clock edge
• Hold margin: Positive value means data stays stable long enough after the capture clock edge

If margins are negative, you can optimize by adding clock buffers to reduce skew, using faster I/O registers in the FPGA, or shortening board-level data traces to cut down on propagation delay.

内容的提问来源于stack exchange，提问作者ercegovac