Many HBM controllers that pass verification still face issues when they reach
silicon.
Not because engineers are incompetent. Not because tools are broken.
But because most verification environments focus on confirming correctness rather than
challenging the design under real stress.
If your controller has only been tested against ideal memory behavior, it may give you confidence
rather than true verification.
Before We Start: The Open-Source Reality Check
Here's something that should help most teams working on HBM.
The open-source community has already produced substantial work on HBM and DRAM simulation. Cycle-accurate timing. Bank state machines. Refresh modeling. Power estimation. Configuration files for HBM2, HBM2E, HBM3. All freely available. All well-documented.
| Project | Type | HBM Support | License |
|---|---|---|---|
| DRAMSim3 | Cycle-accurate C++ simulator | HBM, HBM2, DDR3/4/5, GDDR5/6 | BSD |
| Ramulator 2.0 | Cycle-accurate C++ simulator | HBM2, DDR4/5, LPDDR5 | MIT |
| DRAMPower 5.0 | Power model | HBM3, DDR5, LPDDR5 | BSD |
| CACTI | Cache/memory modeling | Various DRAM configs | BSD |
These projects have been used in hundreds of academic papers. They contain years of validated timing models, configuration parameters, and behavioral knowledge.
The uncomfortable question: If you're building an HBM controller and you haven't studied DRAMSim3's HBM configuration files, haven't looked at Ramulator's bank state machine implementation, haven't examined how these simulators model refresh — you should start with them first to save time.
This isn't proprietary. This isn't hidden. This is sitting on GitHub, waiting to be read.
We say this not to promote these specific projects, but to make a point: the knowledge exists. The timing parameters are documented. The behavioral models are written. What's missing is not information — it's the integration and the stress modeling that turns academic simulators into silicon-ready verification environments.
If you haven't explored what's already available, start there. Then come back.
What We Assume Is Already Done
This article is not an HBM tutorial. If you're still debugging basic functionality, stop here.
| Item | Status |
|---|---|
| JEDEC command legality (ACT, RD, WR, PRE, REF) | Implemented |
| Timing closure (tRCD, tRP, tRAS, tRC, tRFC, tREFI) | Implemented |
| AXI/TileLink protocol compliance | Implemented |
| Basic read/write data integrity | Implemented |
| Bank state machine correctness | Implemented |
We start from here.
The Actual Problem
HBM failures don't come from violating the spec. They come from multiple legal behaviors overlapping.
| Behavior | Individually Legal? | Collectively Dangerous? |
|---|---|---|
| Refresh stealing cycles | Yes | Yes |
| Bank conflicts under random access | Yes | Yes |
| Out-of-order response completion | Yes | Yes |
| Backpressure from downstream | Yes | Yes |
| Thermal throttling reducing bandwidth | Yes | Yes |
JEDEC defines legality. Silicon exposes interaction.
Your controller can be spec-compliant and still deadlock when refresh storms hit during bank conflicts while thermal throttling reduces bandwidth. Every individual behavior is legal. The combination kills you.
Why Open-Source Timing Models Alone Are Not Enough
DRAMSim3 and Ramulator are excellent. They model DRAM timing accurately. They track bank states correctly. They handle refresh properly.
But they model nominal behavior.
| Property | Open-Source Simulator | Real HBM Silicon |
|---|---|---|
| Read latency | Deterministic (tCL = 24) | Variable (thermal, congestion) |
| Bank arbitration | Stable round-robin | Skewed under load |
| Bandwidth | Smooth, predictable | Bursty, collapses during refresh |
| Response ordering | In-order or simple OOO | Complex reordering under stress |
| Refresh impact | Predictable pause | Cascading stalls |
These simulators give you the foundation. They don't give you the stress. Accurate timing models without realistic stress can lead to over-confidence.
The gap: open-source timing models + what we add = silicon-ready verification.
The Verification Architecture
Our approach: don't model perfect memory, model stressed memory.
The Stress Injection Layer sits between controller and memory. It never violates protocol. It violates expectations.
The memory timing model builds on knowledge from open-source simulators — timing parameters, bank state logic, refresh behavior. We don't reinvent what already works. We add what's missing.
What the Stress Layer Does
| Stress Type | Mechanism | What It Exposes |
|---|---|---|
| Latency jitter | Randomized delay within legal timing window | Pipeline stalls, timeout handling |
| Response reordering | Legal out-of-order completion | Completion tracking bugs, data corruption |
| Backpressure | Variable accept windows, credit starvation | Deadlock, livelock, flow control bugs |
| Bandwidth throttling | Artificial reduction simulating thermal limits | Graceful degradation, priority inversion |
| Refresh clustering | Bursts of refresh commands | Scheduler starvation, timing violations |
Every response is still legal. Every timing is still compliant. But the controller sees what real silicon sees — not what simulators show.
HBM-Specific Complications
HBM is not just wide DDR. It's a 3D-stacked structure with unique failure modes.
Physical Structure
- 8 channels, 2 pseudo-channels each → 16 independent access points
- 1024-bit total bus width (HBM2E), 2048-bit in HBM4
- 8-12 DRAM dies stacked vertically via TSV
- Logic base die handling PHY, training, temperature sensing
Why This Matters for Verification
| Aspect | Planar DRAM | HBM |
|---|---|---|
| Latency variation | PCB trace length | TSV congestion + thermal |
| Thermal coupling | Weak (spread out) | Strong (stacked) |
| Failure propagation | Localized | Vertical cascade |
| Arbitration scope | Single controller | Multiple pseudo-channels competing |
We model TSV effects as configurable latency skew. We model thermal effects as bandwidth shaping. We don't simulate physics — we simulate observable effects.
Integration: HBM in Multi-Die Systems
HBM typically sits on an interposer next to compute dies. In chiplet architectures, the controller may be on a separate die from the CPU/NPU.
This means your HBM controller now has additional latency and potential backpressure from the D2D link. Verification must include this path.
Our verification environment supports plugging in die-to-die interfaces (UCIe, AIB) between traffic generator and HBM controller. End-to-end stress testing across die boundaries.
Verification Completion Criteria
Passing individual tests is meaningless. The controller is verified when:
| Condition | Required |
|---|---|
| Sustained traffic during refresh storms | No deadlock, no data loss |
| Out-of-order completion under load | Correct data, correct ordering to requestor |
| Bandwidth collapse (70% throttle) | Graceful degradation, no livelock |
| Backpressure from all channels simultaneously | No deadlock, fair progress |
| Random stress combination (1000+ transactions) | 100% data integrity |
Surviving combinations matters. Passing tests does not.
What We Built
HBM Controller (Reference)
- 8-channel, 16-pseudo-channel architecture
- 1024-bit data path
- FR-FCFS scheduler with bank-aware arbitration
- Per-bank refresh with temperature compensation hooks
- AXI4 and TileLink interfaces
- PHY layer with training state machine
Stress Injection Layer
- Configurable latency jitter (0-50% variance)
- Response reordering (configurable depth)
- Credit-based backpressure injection
- Bandwidth throttling (0-80% reduction)
- Refresh storm generation
Memory Model
- Bank state machines (IDLE/ACTIVE/PRECHARGE)
- JEDEC timing enforcement
- Actual data storage and retrieval
- Integration hooks for DRAMSim3/Ramulator via DPI-C
What This Is For
Engineers who:
- Design HBM controllers and need to prove silicon-readiness
- Debug silicon failures that "passed verification"
- Distrust ideal simulation environments
- Want to see failure modes before tape-out
- Believe open-source foundations should be leveraged, not ignored
This is a reference implementation and methodology. Not production IP. Not foundry-qualified PHY. A working example of how to verify memory controllers against reality, not against optimism.
Final Point
HBM verification is not about proving correctness.
It's about proving that nothing collapses when everything degrades.
If your controller only works reliably when memory behaves ideally, it may not yet be ready for real silicon conditions.
The timing knowledge is freely available. The behavioral models exist. What’s often missing is the extra step of integrating them with a stress layer that builds true silicon resilience.
Passing simulation tests is not always the same as surviving real silicon.
For technical questions or collaboration: contact us through wiowiz.com