shinei-nouzen-arch/fpga-agent/vivado-synth

Name: vivado-synth
Author: shinei-nouzen-arch

Claude Code and agent skill by shinei-nouzen-arch/fpga-agent

Trust tier 3

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Use this skill when the user needs help with Vivado synthesis strategy selection, synthesis attribute configuration, synth_design option tuning, resource inference control (RAM/DSP/SRL/BRAM), hierarchy optimization (flatten_hierarchy, KEEP_HIERARCHY), OOC (out-of-context) synthesis, incremental synthesis, RTL linting, Block-Level synthesis (BLOCK_SYNTH), or any synthesis optimization decision. Trigger when the user mentions synthesis strategies, synthesis directives, synthesis attributes, resource mapping control, FSM encoding, retiming, or asks how to optimize synthesis results for area/timing/power. This skill provides decision-making knowledge — for TCL command execution, use vivado-tcl skill.

Install

npx vskill@latest install shinei-nouzen-arch/fpga-agent/vivado-synth

SKILL.md

Vivado Synthesis Decision Guide

Based on UG901 (v2025.2). This skill helps choose the right synthesis strategies, options, and attributes. For attribute syntax see REFERENCE.md; for HDL coding templates (RAM/DSP/ROM/SRL/FSM/etc.) see examples/ directory — consult the index table in REFERENCE.md to locate the right file.

synth_design Complete Options

Hierarchy Control

Option	Values	Default	Effect
`-flatten_hierarchy`	none/full/rebuilt	rebuilt	none: preserve RTL hierarchy. full: flatten to top only. rebuilt: flatten then rebuild similar hierarchy (best QoR + readability)

Timing Optimization

Option	Values	Default	Effect
`-directive`	See strategy table below	Default	Selects preconfigured optimization strategy
`-global_retiming`	auto/on/off	auto	Register balancing across combinational logic. Auto = on for Versal, off for others
`-no_timing_driven`	flag	—	Disables timing-driven synthesis (faster, worse timing)

Resource Control

Option	Values	Default	Effect
`-max_bram`	integer	-1 (max)	Limit Block RAM usage
`-max_uram`	integer	-1 (max)	Limit UltraRAM usage (UltraScale+)
`-max_dsp`	integer	-1 (max)	Limit DSP block usage
`-shreg_min_size`	integer	3	Min chain length for SRL inference
`-srl_style`	register/rl/srl_reg/reg_srl/reg_srl_reg	—	Global SRL implementation style
`-cascade_dsp`	auto/tree/force	auto	DSP adder chain implementation
`-max_bram_cascade_height`	integer	-1	Max BRAM cascade depth
`-max_uram_cascade_height`	integer	-1	Max UltraRAM cascade depth
`-no_srlextract`	flag	—	Disable all SRL extraction
`-no_lc`	flag	—	Disable LUT combining

FSM & Logic

Option	Values	Default	Effect
`-fsm_extraction`	auto/one_hot/sequential/gray/johnson/user_encoding/off	auto	FSM encoding strategy
`-resource_sharing`	auto/on/off	auto	Arithmetic operator sharing
`-keep_equivalent_registers`	flag	off	Prevent merging of equivalent registers
`-control_set_opt_threshold`	auto/integer/0	auto	Fanout threshold for control set optimization. 0 = disable

Clock & IO

Option	Values	Default	Effect
`-bufg`	integer	12	Max global clock buffers to infer
`-gated_clock_conversion`	off/on/auto	off	Convert gated clocks to enables

Design Entry

Option	Values	Default	Effect
`-top`	module name	—	Top module
`-part`	part number	—	Target FPGA part
`-constrset`	fileset name	—	Constraint fileset to use
`-include_dirs`	paths	—	Verilog include search dirs
`-generic`	name=value	—	Override Verilog parameters / VHDL generics
`-verilog_define`	macro[=text]	—	Define Verilog macros
`-mode`	default/out_of_context	default	OOC mode: no IO buffers

Special Modes

Option	Values	Default	Effect
`-rtl`	flag	—	Elaborate only, don't synthesize
`-lint`	flag	—	Run RTL Linter only (see section below)
`-incremental_mode`	default/quick/off	default	Incremental synthesis mode
`-sfcu`	flag	—	Single-file compilation unit mode

Debug & Performance

Option	Values	Default	Effect
`-debug_log`	flag	—	Extra debug info in log
`-assert`	flag	—	Enable VHDL assertions

Multi-Threading

tcl

set_param general.maxThreads 8  ;# 1-8 threads for synthesis

Strategy Decision Table

Use `-directive <strategy>` to select:

Scenario	Strategy	Key Effect
Default / first try	`default`	Balanced optimization
Fast iteration / debug	`runtimeoptimized`	Fewer optimizations, faster runtime
Resource-constrained	`AreaOptimized_high`	Force ternary adders, includes AreaMapLargeShiftRegToBRAM + AreaMultThresholdDSP
Resource-constrained (moderate)	`AreaOptimized_medium`	Area-optimized MUX, ternary adders, lower multiplier→DSP threshold
Timing-critical	`PerformanceOptimized`	Logic level reduction at expense of area
Routing congestion	`AlternateRoutability`	Less MUXF/CARRY usage for better routability
Minimum area	`LogicCompaction`	Compact CARRY+LUT into fewer SLICEs (negative timing impact)
DSP-heavy	`AreaMultThresholdDSP`	Lower threshold for multiplier→DSP inference
Carry-chain issues	`FewerCarryChains`	Higher threshold to use LUTs instead of carry chains
Large shift registers	`AreaMapLargeShiftRegToBRAM`	Implement large shift regs in Block RAM

Note: Directive values are case-sensitive in v2025.2. Use exactly as shown above.

Synthesis Attributes Quick Reference (by scenario)

Preventing Optimization

Attribute	Values	Where	When to Use
`DONT_TOUCH`	TRUE/FALSE	RTL+XDC	Prevent optimization AND preserve through P&R. Strongest protection
`KEEP`	TRUE/FALSE	RTL only	Prevent signal absorption into LUTs. Synthesis-only, NOT forwarded to P&R
`KEEP_HIERARCHY`	TRUE/SOFT/FALSE	RTL+XDC	SOFT (recommended): allow constant propagation. TRUE: block all cross-boundary optimization

Resource Inference Control

Attribute	Values	Where	When to Use
`RAM_STYLE`	block/distributed/registers/ultra/mixed/auto	RTL+XDC	Force specific RAM implementation
`ROM_STYLE`	block/distributed/ultra	RTL+XDC	Force specific ROM implementation
`USE_DSP`	yes/no/logic/simd	RTL+XDC	Force/prevent DSP block usage. `logic`=XOR→DSP, `simd`=SIMD mode
`SHREG_EXTRACT`	yes/no	RTL+XDC	Enable/disable SRL inference
`SRL_STYLE`	register/srl/srl_reg/reg_srl/reg_srl_reg/block	RTL+XDC	Specific SRL implementation
`RAM_DECOMP`	power/area	RTL+XDC	RAM split: `power` = address decode (saves power), `area` = smallest
`CASCADE_HEIGHT`	integer (0=disable)	RTL+XDC	BRAM/URAM cascade chain length (UltraScale+ only)
`RW_ADDR_COLLISION`	auto/yes/no	RTL only	Read-write collision handling

Timing Optimization

Attribute	Values	Where	When to Use
`RETIMING_FORWARD`	integer (0=off)	RTL+XDC	Move register forward through logic
`RETIMING_BACKWARD`	integer (0=off)	RTL+XDC	Move register backward through logic
`CRITICAL_SIG_OPT`	true/false	RTL+XDC	Shannon decomposition on critical feedback loops. Trades area for timing
`MAX_FANOUT`	integer (-1=no limit)	RTL+XDC	Fanout limit → triggers register replication. Best on local signals only

Debug

Attribute	Values	Where	When to Use
`MARK_DEBUG`	TRUE/FALSE	RTL+XDC	Mark nets for ILA debug probing
`ASYNC_REG`	TRUE/FALSE	RTL+XDC	Mark CDC synchronizer registers

FSM

Attribute	Values	Where	When to Use
`FSM_ENCODING`	one_hot/sequential/gray/johnson/user_encoding/none	RTL+XDC	Override FSM encoding
`FSM_SAFE_STATE`	auto_safe_state/reset_state/power_on_state/default_state	RTL+XDC	Add invalid state recovery logic

IO & Clock

Attribute	Values	Where	When to Use
`IOB`	TRUE/FALSE	RTL only	Pack register into IOB
`IO_BUFFER_TYPE`	NONE	RTL only	Disable automatic IO buffer insertion
`CLOCK_BUFFER_TYPE`	BUFG/BUFH/BUFIO/BUFMR/BUFR/none	RTL+XDC	Specify clock buffer type
`GATED_CLOCK`	yes	RTL+XDC	Mark signal as clock for gated clock conversion

Enable & Reset

Attribute	Values	Where	When to Use
`DIRECT_ENABLE`	yes	RTL+XDC	Force signal to CE pin of register
`DIRECT_RESET`	yes	RTL+XDC	Force signal to reset pin of register
`EXTRACT_ENABLE`	yes/no	RTL+XDC	Control enable extraction to CE pin
`EXTRACT_RESET`	yes/no	RTL+XDC	Control reset extraction (sync reset only)

DSP Folding

Attribute	Values	Where	When to Use
`DSP_FOLDING`	yes/no	RTL only	Fold two MAC structures into one DSP
`DSP_FOLDING_FASTCLOCK`	yes/no	RTL only	Designate fast clock port for DSP folding

Verilog Case Control

Attribute	Values	Where	When to Use
`FULL_CASE`	(presence)	RTL only	All case values covered — suppress latch inference
`PARALLEL_CASE`	(presence)	RTL only	Build as parallel if-elsif, no priority

Other

Attribute	Values	Where	When to Use
`BLACK_BOX`	(presence)	RTL only	Force module to black box
`TRANSLATE_OFF/ON`	comment-based	RTL only	Exclude code from synthesis

Block-Level Synthesis (BLOCK_SYNTH)

Apply per-instance synthesis settings via XDC:

tcl

set_property BLOCK_SYNTH.<option> <value> [get_cells <instance>]

Option	Type	Values	Description
STRATEGY	STRING	DEFAULT/AREA_OPTIMIZED/ALTERNATE_ROUTABILITY/PERFORMANCE_OPTIMIZED	Per-instance strategy
RETIMING	INT	0/1	Enable/disable retiming
ADDER_THRESHOLD	INT	4-128	Adder size → CARRY chain threshold
COMPARATOR_THRESHOLD	INT	4-128	Comparator size → CARRY chain threshold
SHREG_MIN_SIZE	INT	3-32	SRL inference threshold
FSM_EXTRACTION	STRING	OFF/ONE_HOT/SEQUENTIAL/GRAY/JOHNSON/AUTO	FSM encoding
LUT_COMBINING	INT	0/1	Enable/disable LUT combining
CONTROL_SET_THRESHOLD	INT	0-128	Control set optimization threshold
MAX_LUT_INPUT	INT	4-6	4=no LUT5/6, 5=no LUT6, 6=all
MUXF_MAPPING	INT	0/1	Enable/disable MUXF7/F8/F9
KEEP_EQUIVALENT_REGISTER	INT	0/1	Merge or retain equivalent registers
PRESERVE_BOUNDARY	INT	any	Mark hierarchy as changing (for incremental)
LOGIC_COMPACTION	INT	1	Compact CARRY+LUT into fewer SLICEs
SRL_STYLE	STRING	REGISTER/SRL/SRL_REG/REG_SRL/REG_SRL_REG	SRL implementation

Note: BLOCK_SYNTH hardens the instance hierarchy. Use only where needed.

OOC (Out-of-Context) Synthesis

When to use: Large IP modules, 3rd-party netlists, modules that rarely change.

tcl

# Non-Project Mode
synth_design -top <module> -part <part> -mode out_of_context
 
# Project Mode: right-click module → Set As Out-of-Context for Synthesis

Key rules:

OOC modules are synthesized independently, treated as black boxes in top-level synthesis
No IO buffers are created
Incremental synthesis is NOT supported for OOC runs
Do NOT use OOC if module has AMD IP in lower levels
Do NOT use OOC if module ports use user-defined types

Incremental Synthesis

Detects RTL changes and only re-synthesizes modified sections. Reduces runtime and QoR fluctuation.

tcl

# Project Mode: Settings → Synthesis → Incremental synthesis
# Non-Project Mode:
synth_design -top <top> -part <part> -incremental_mode default

Mode	Effect
`default`	Full incremental (detect + selective re-synthesis)
`quick`	Faster, less thorough incremental
`off`	Full re-synthesis

Note: Not supported for OOC runs.

RTL Linter

Pre-synthesis code quality check:

tcl

synth_design -lint -top <top> -part <part>
# For OOC runs:
synth_design -lint -srcset [get_property SRCSET [get_runs my_IP_core_synth_1]]

This is the start of the file. Read the full SKILL.md on GitHub.

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Source

SKILL.md in the repositoryCheckingvivado-synth/SKILL.md

Popularity

Repository stars: 132
Repository forks: 16
7-day trend score: 24

Stars and forks are for the whole GitHub repository, not this skill alone. Refreshed 17 May 2026, 5 months ago.

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