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SiFive launches first RISC-V SBC that runs Linux

Feb 5, 2018 — by Eric Brown 5,634 views

SiFive has opened pre-sales on a “HiFive Unleashed” SBC that runs Linux on its new RISC-V based, quad-core, 1.5GHz U540 SoC, with 8GB DDR4, a GbE port, and an FMC connector.

SiFive followed up on its October announcement of the SiFive Freedom U540 — the world’s first Linux-driven RISC-V processor SoC — with a Crowd Supply campaign to launch a HiFive Unleashed SBC that showcases the U450. The board is available for a steep $999 for a June 30 shipment, or $1,250 if you want the few remaining first-run boards that ship by Mar. 31.

HiFive Unleashed
(click images to enlarge)

The HiFive Unleashed ships with a U540 SoC, 8GB DDR4 RAM with ECC and 32MB quad SPI flash from ISSI. Other features include a microSD slot, a Gigabit Ethernet port, and an FMC connector “for future expansion.” There also appears to be a power jack and a reset button.

No more specs were detailed, and there is likely not much more to be said about this minimalist board. The Crowd Supply page suggests the SBC will be offered as open hardware to match the open source nature of the SoC itself.


There were no details on software support beyond the Linux compatibility. Earlier, however, SiFive said the HiFive Unleashed would ship with “a rich SDK with demo software and an easy-to-install binary toolchain.” Standard development and debug tools such as OpenOCD, GDB, and an Eclipse IDE, are also available.

U540 SoC

Originally referred to as the “U54-MC Coreplex,” the real-time capable U540 is the first multi-core SoC featuring the open source RISC-V ISA, and the first to offer cache coherence. The processor is intended to run Linux for applications including AI, machine learning, networking, gateways and smart IoT devices. Fabricated with TSMC’s 28nm HPC process, the SoC integrates 4x U54 RV64GC cores at up to 1.5GHz, as well as an E51 RV64IMAC management core with Sv39 virtual memory support.

U540 block diagram (left) and tray of U540 SoCs
(click images to enlarge)

Each of the 0.234mm square cores features a “highly efficient” five-stage in-order pipeline, along with 32KB L1 instruction and data caches. All the cores share a coherent 2MB L2 cache. Real-time capabilities are enabled by the fact that both the L1 instruction and L2 caches can be configured into high speed deterministic SRAMs, says SiFive.

Other features include a debug with trace and a “CLINT” unit for multi-core timer and software interrupts. There’s also a “PLIC” with support for up to 511 interrupts with 7x priority levels.

There are no GPUs or other coprocessors, but the open source design is intended to encourage third parties to develop them. Customers can build peripheral IP with an open source, native TileLink interface bus. This “high-performance scalable cache-coherent fabric” is available with bridge adapters for legacy bus protocols such as AXI4, AHB-Lite, and APB, says SiFive.

The U540 is the first silicon release of SiFive’s Freedom Unleashed family of high-end processors, which are compatible with Unix-based operating systems and optimized for Linux. In July 2016, the San Mateo, Calif. based fabless chip designer announced the first Linux-ready Freedom Unleashed design with the octa-core Freedom U500. Instead, SiFive switched to the similar, but quad-core, U540.

Aside from being open source and customizable, one of the main benefits of RISC-V is that it is fully modern, purpose built, and unburdened with legacy code. Over the last year, SiFive customers like Microsemi and Arduino have released various RISC-V based MCU chips. SiFive’s initial, 320MHz FE310 MCU based on its original Freedom E300 design, appeared in Nov. 2016 in an Arduino ready HiFive1 dev kit. In May of last year, SiFive and Arduino unveiled a wireless-enabled Arduino Cinque board based on SiFive’s HiFive, featuring a RISC-V FE310 SoC and an ESP32 wireless module.

Further information

The HiFive Unleashed is available on Crowd Supply for $999 for a June 30 shipment, or $1,250 if you want the few remaining first-run boards that ship by Mar. 31. Crowd Supply will handle all logistics and fulfillment, and will ship from Portland, Oregon to US destinations for free, or for $40 worldwide. More information may be found at the HiFive Unleashed Crowd Supply page, as well as SiFive’s HiFive Unleashed product page.

SiFive demonstrated the HiFive Unleashed at FOSDEM over the last few days, and will be hosting its first hackathon for the board on Mar.12-14, 2018 in Portland, Oregon, in conjunction with the Embedded Linux Conference.

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