CLP: How to verify UPF for back to back ISO

Verifying UPF Functional Equivalency for Back-to-Back Isolation Cells Using CLP (Conformal Low Power)

When implementing back-to-back isolation cells (e.g., for bidirectional signals or redundant isolation), ensuring functional equivalency between the original design and the UPF-annotated netlist is critical. Here’s how to verify this using Synopsys Conformal Low Power (CLP).

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Step 1: Understand Back-to-Back Isolation Cells

• Purpose:
• Used when a signal crosses two power domains (e.g., PD_A → PD_B → PD_A).
• Prevents floating values when either domain is powered off.
• Structure:

  PD_A (ON) → Isolation Cell 1 → PD_B (OFF) → Isolation Cell 2 → PD_A (ON)

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Step 2: UPF for Back-to-Back Isolation

Example UPF Commands

# Define isolation for Domain A (output to Domain B)
set_isolation iso_AB \
  -domain PD_A \
  -isolation_power_net VDD \
  -isolation_ground_net VSS \
  -clamp_value 0 \
  -applies_to outputs

# Define isolation for Domain B (output back to Domain A)
set_isolation iso_BA \
  -domain PD_B \
  -isolation_power_net VDD \
  -isolation_ground_net VSS \
  -clamp_value 0 \
  -applies_to outputs

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Step 3: CLP Verification Flow

Key Checks

1. Isolation Enable Conditions

• Verify isolation cells activate only when their domain is OFF.

1. Clamping Behavior

• Ensure outputs are clamped to 0/1/Z as specified.

1. Bidirectional Signal Integrity

• No combinational loops or undefined states.

CLP Script Example

# Load reference (RTL) and implementation (UPF-netlist)
read_design -golden ../rtl/top.v -verilog
read_design -revised ../netlist/top_upf.v -verilog
read_upf -golden ../upf/power.upf

# Set power-aware comparison mode
set_low_power_compare -golden -revised

# Map power domains and isolation cells
add_ignore_output iso_AB*  # Exclude isolation cells from logic comparison
add_ignore_output iso_BA*

# Verify functional equivalency
verify

# Report mismatches
report_failing_points -verbose

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Step 4: Debugging Common Failures

Failure 1: Isolation Cell Not Triggering

• Cause: Incorrect -isolation_signal mapping.
• Fix:

  set_isolation_control iso_AB -domain PD_A -isolation_signal power_enable_AB

Failure 2: Clamping Conflict

• Cause: Back-to-back cells drive conflicting values (e.g., 0 vs. 1).
• Fix:

  set_isolation iso_BA -clamp_value Z  # High-Z when conflicting

Failure 3: Combinational Loop

• Cause: Bidirectional signal forms a loop when both domains are ON.
• Fix:

  set_disable_domain_checking PD_A:PD_B  # Allow direct connection when both ON

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Step 5: Advanced Checks

Power State-Dependent Verification

# Check behavior when PD_A=ON, PD_B=OFF
set_power_state PD_A -state {ON}
set_power_state PD_B -state {OFF}
verify

# Check behavior when PD_A=OFF, PD_B=ON
set_power_state PD_A -state {OFF}
set_power_state PD_B -state {ON}
verify

Waveform Debugging

# Dump waveforms for failing scenarios
write_waveforms -golden -revised -format VCD -output isolation_debug.vcd

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Key Takeaways

Check	CLP Command	UPF Requirement
Isolation Enable	set_power_state	Correct -isolation_signal.
Clamping Value	report_failing_points	Match -clamp_value in UPF.
Bidirectional Safety	set_disable_domain_checking	Avoid combinational loops.
Power-State Coverage	verify -power_states	Test all ON/OFF combinations.

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Conclusion

1. UPF Must Define:

• Isolation cells for both directions (iso_AB, iso_BA).
• Clamping behavior (0/1/Z).

1. CLP Must Verify:

• Functional equivalency per power state.
• No undefined states or loops.

Pro Tip: Use -debug mode in CLP to trace signal propagation through isolation cells.