Hardware Design Guidelines for Loongson Processors
Designing hardware with Loongson processors requires attention to power delivery, high-speed interfaces, and thermal management. This guide provides practical guidelines for successful hardware design.
Power Delivery Design
Proper power delivery is critical for reliable operation.
3A5000 Power Requirements
| Rail | Voltage | Current (Max) | Tolerance |
| VDD_CPU | 0.9V | 40A | +/- 50mV |
| VDD_DDR | 1.2V | 10A | +/- 60mV |
| VDD_SOC | 1.0V | 5A | +/- 50mV |
| VDD_IO | 1.8V/3.3V | 3A | +/- 100mV |
Power Design Best Practices
Power Sequencing
Follow proper power-up sequence:
Power-down is reverse order. Use dedicated power sequencer IC or implement in CPLD.
DDR Memory Interface
DDR4 interface requires careful design for signal integrity.
DDR4 Routing Guidelines
| Parameter | Specification |
| Trace impedance | 40 Ohm single-ended, 80 Ohm differential |
| Trace length match | +/- 2mm within byte lane |
| Clock to DQ skew | < 100ps |
| Via count | Minimize, max 2 per net |
| Layer | Route on internal stripline layers |
DDR4 Layout Recommendations
DDR4 Topology
Loongson 3A5000 supports dual-channel DDR4. Each channel is point-to-point:
``
CPU DDR Controller
|
|-- Channel 0 --> DIMM0 (64-bit data + ECC)
|
|-- Channel 1 --> DIMM1 (64-bit data + ECC)
``
For embedded designs with soldered memory, use x16 or x8 devices to achieve 64-bit width.
PCIe Interface Design
PCIe 3.0 requires careful high-speed design.
PCIe Routing Guidelines
| Parameter | PCIe 3.0 Specification |
| Trace impedance | 85 Ohm differential |
| Trace length | < 8 inches for x16 |
| Lane-to-lane skew | < 3ps |
| AC coupling | 100nF caps near receiver |
| Via style | Use back-drill or blind vias |
PCIe Layout Best Practices
PCIe Clocking
- Use 100MHz differential reference clock
- Route clock with data lanes (separate pair)
- Match clock length to data length
- Use spread spectrum clocking if supported
Ethernet Interface Design
Gigabit Ethernet requires proper magnetics and termination.
Ethernet Magnetics Selection
Key parameters:
- Turns ratio: 1:1 for voltage mode PHYs
- Insertion loss: < 1.0dB at 100MHz
- Return loss: > 16dB at 100MHz
- Crosstalk: > 40dB
- Isolation: 1500V minimum
Ethernet Layout Guidelines
Thermal Design
Proper thermal management ensures reliable operation.
Thermal Requirements
| Processor | TDP | Max Junction | Heatsink Required |
| 3A5000 | 35W | 105°C | Yes, active recommended |
| 3C5000 | 120W | 105°C | Yes, active required |
| 2K1000 | 3W | 105°C | Optional, passive OK |
| 2K2000 | 5W | 105°C | Optional, passive OK |
Heatsink Design
For active cooling:
For passive cooling:
Thermal Simulation
Perform thermal simulation early in design:
- Model processor as heat source (TDP value)
- Include heatsink thermal resistance
- Account for TIM thermal resistance
- Model airflow (forced or natural)
- Verify junction temperature < 105°C at max ambient
Signal Integrity
High-speed signals require careful design.
General SI Guidelines
Stackup Recommendations
For 6-layer board:
| Layer | Type | Purpose |
| 1 | Signal | Top layer, high-speed signals |
| 2 | Ground | Solid ground plane |
| 3 | Signal | Internal routing |
| 4 | Power | Power distribution |
| 5 | Ground | Solid ground plane |
| 6 | Signal | Bottom layer, components |
For high-speed designs, consider 8+ layers for better isolation.
PCB Layout Checklist
Before manufacturing, verify:
Power
- [ ] All power rails present with correct voltage
- [ ] Adequate copper width for current capacity
- [ ] Sufficient decoupling capacitors placed
- [ ] Power sequencing implemented correctly
- [ ] Test points on all power rails
DDR Memory
- [ ] Length matching within specifications
- [ ] Impedance control verified
- [ ] Reference planes continuous
- [ ] Termination resistors correct
- [ ] Decoupling adequate
High-Speed Interfaces
- [ ] PCIe traces length matched
- [ ] Ethernet magnetics correct
- [ ] Impedance controlled for all high-speed signals
- [ ] Via stubs minimized
- [ ] Crosstalk analyzed
Thermal
- [ ] Heatsink mounting provisions
- [ ] TIM selection appropriate
- [ ] Airflow path clear
- [ ] Thermal simulation completed
Manufacturing
- [ ] DFM review completed
- [ ] Test points accessible
- [ ] Component footprints verified
- [ ] Silkscreen clear and complete
Design Validation
After PCB fabrication:
Use oscilloscope to verify:
- Power rail ripple and noise
- Clock signal quality
- High-speed signal eye diagrams
Reference Designs
Loongson provides reference designs for:
- 3A5000 desktop motherboard
- 2K1000 embedded board
- 3C5000 server board
Use these as starting points for custom designs. Reference designs include:
- Complete schematics
- PCB layout files
- BOM with validated components
- Design documentation
Getting Help
For hardware design support:
- Review reference designs thoroughly
- Use provided layout guidelines
- Engage FAE team for design review
- Consider design services for complex projects
Proper planning and following guidelines will result in successful hardware design.
💡 FAE Insights
Technical Logic
Hardware design is about managing risk through proven practices. The hierarchy of importance is: 1) Power delivery - without clean power, nothing works reliably, 2) Memory interface - DDR is the most timing-critical interface, 3) High-speed I/O - PCIe and Ethernet need proper impedance control, 4) Thermal - prevents long-term reliability issues, 5) General layout - good practices prevent EMI and signal integrity issues. Each layer builds on the previous. Get the power and memory right, and the rest is manageable. Start with reference designs and modify incrementally, validating at each step.
📋 Customer Cases
Industrial Computer Manufacturer
Hardware
Challenge
Customer designed their first 3A5000 motherboard but encountered stability issues under heavy load. System would randomly crash during memory-intensive operations.
Solution
Redesigned with 4-phase VRM and re-routed DDR signals with proper length matching. Added additional bulk capacitance near VRM. Implemented proper power sequencing.
Customer Feedback
"System stable under all test conditions. Passed 72-hour burn-in testing. Design entered production and has shipped 1000+ units with <1% RMA rate."
Frequently Asked Questions
1. What is the minimum PCB layer count for Loongson designs?
Minimum PCB layer count depends on design complexity. For simple 2K1000 designs with minimal I/O, 4 layers may suffice. For 3A5000 desktop motherboards with DDR4 and PCIe, 6-8 layers are typical. For complex server designs, 10+ layers may be needed. The key factors are: number of high-speed interfaces, DDR memory channels, power distribution requirements, and EMI constraints. More layers provide better signal integrity and power distribution but increase cost. For production designs, 6 layers is a good balance for most 3A5000 applications.
2. How do I ensure DDR signal integrity on my design?
Ensuring DDR signal integrity requires attention to multiple factors. First, use proper stackup with controlled impedance (40 Ohm single-ended, 80 Ohm differential). Second, route DDR on internal stripline layers with solid ground reference. Third, match trace lengths carefully - within 2mm for byte lanes, within 5mm across channels. Fourth, minimize vias - use via-in-pad or blind vias if possible. Fifth, provide adequate decoupling - 100nF per power pin. Sixth, simulate the design using signal integrity tools. Finally, validate with memory stress testing on actual hardware. Following reference design layouts is the safest approach.
3. What thermal solution do I need for the 3A5000?
The 3A5000 (35W TDP) requires active cooling for most applications. A typical solution includes: aluminum heatsink with 40x40mm base and 20-30mm fin height, 40mm or 60mm fan with 10-20 CFM airflow, and thermal interface material (1-2 W/mK). For compact designs, consider heat pipes to spread heat. For industrial applications, ensure fan has temperature control or use high-reliability fan. Thermal simulation should verify junction temperature stays below 105°C at maximum ambient temperature. In thermally challenging environments, consider derating or enhanced cooling.
4. Can I use standard PC components with Loongson motherboards?
Many standard PC components work with Loongson motherboards. DDR4 memory modules from major brands (Samsung, Micron, Hynix) are compatible. SATA SSDs and hard drives work without issues. USB keyboards, mice, and storage devices are supported. Ethernet cables and switches are standard. Graphics cards require AMD Radeon with Loongson driver support - NVIDIA is not supported. Power supplies use standard ATX connectors. Cases follow standard ATX/mATX form factors. For embedded designs, verify component operating temperature ranges match your requirements.
5. What are common mistakes when designing hardware?
Common mistakes include insufficient planning and requirements analysis, not following reference designs closely enough, inadequate testing at each development phase, overlooking thermal and power design requirements, and not engaging technical support early. Other issues include insufficient validation of software compatibility, inadequate documentation of design decisions, and not planning for long-term software maintenance. We recommend following our development guides, using reference designs as starting points, and engaging our FAE team for design review.