System on a chip

The Raspberry Pi uses a system on a chip as an almost fully-contained micro computer (this SoC doesn't contain any kind of data storage which is common for microprocessor SoC).

A system on a chip or system on chip (SoC or SOC) is an integrated circuit (also known as an "IC" or "chip") that integrates all components of a computer or other electronic systems. It may contain digital, analog, mixed-signal, and often radio-frequency functions—all on a single substrate. SoCs are very common in the mobile computing market because of their low power-consumption.[1] A typical application is in the area of embedded systems.

SoC integrates a microcontroller (or microprocessor) with advanced peripherals like graphics processing unit (GPU), Wi-Fi module, or coprocessor. If the definition of a microcontroller is a system that integrates a microprocessor with peripheral circuits and memory, the SoC is to a microcontroller what a microcontroller is to processors, remembering that the SoC does not necessarily contain built-in memory.

In general, there are three distinguishable types of SoCs. SoCs built around a microcontroller, SoCs built around a microprocessor (this type can be found in mobile phones), and specialized SoCs designed for specific applications that do not fit into the above two categories. A separate category may be Programmable SoC (PSoC), where some of the internal elements are not predefined and can be programmable in a manner analogous to the FPGA or CPLD.

When it is not feasible to construct a SoC for a particular application, an alternative is a system in package (SiP) comprising a number of chips in a single package. When produced in large volumes, SoC is more cost-effective than SiP because its packaging is simpler.[2]

Another option, as seen for example in higher-end cell phones, is package on package stacking during board assembly. The SoC includes processors and numerous digital peripherals, and comes in a ball grid package with lower and upper connections. The lower balls connect to the board and various peripherals, with the upper balls in a ring holding the memory buses used to access NAND flash and DDR2 RAM. Memory packages could come from multiple vendors.

AMD Am286ZX/LX, SoC based on 80286

Structure

Microcontroller-based system on a chip

A typical SoC consists of:

A bus – either proprietary or industry-standard such as the AMBA bus from ARM Holdings – connects these blocks. DMA controllers route data directly between external interfaces and memory, bypassing the processor core and thereby increasing the data throughput of the SoC.

Design flow

System-on-a-chip design flow

A SoC consists of both the hardware, described above, and the software controlling the microcontroller, microprocessor or DSP cores, peripherals and interfaces. The design flow for a SoC aims to develop this hardware and software in parallel.

Most SoCs are developed from pre-qualified hardware blocks for the hardware elements described above, together with the software drivers that control their operation. Of particular importance are the protocol stacks that drive industry-standard interfaces like USB. The hardware blocks are put together using CAD tools; the software modules are integrated using a software-development environment.

Once the architecture of the SoC has been defined, any new hardware elements are written in an abstract language termed RTL which defines the circuit behaviour. These elements are connected together in the same RTL language to create the full SoC design.

Chips are verified for logical correctness before being sent to foundry. This process is called functional verification and it accounts for a significant portion of the time and energy expended in the chip design life cycle (although the often quoted figure of 70% is probably an exaggeration).[3] With the growing complexity of chips, hardware verification languages like SystemVerilog, SystemC, e, and OpenVera are being used. Bugs found in the verification stage are reported to the designer.

Traditionally, engineers have employed simulation acceleration, emulation and/or an FPGA prototype to verify and debug both hardware and software for SoC designs prior to tapeout. With high capacity and fast compilation time, acceleration and emulation are powerful technologies that provide wide visibility into systems. Both technologies, however, operate slowly, on the order of MHz, which may be significantly slower – up to 100 times slower – than the SoC's operating frequency. Acceleration and emulation boxes are also very large and expensive at over US$1 million. FPGA prototypes, in contrast, use FPGAs directly to enable engineers to validate and test at, or close to, a system’s full operating frequency with real-world stimuli. Tools such as Certus[4] are used to insert probes in the FPGA RTL that make signals available for observation. This is used to debug hardware, firmware and software interactions across multiple FPGAs with capabilities similar to a logic analyzer.

In parallel, the hardware elements are grouped and passed through a process of logic synthesis, during which performance constraints, such as operational frequency and expected signal delays, are applied. This generates a logical netlist which is a file describing the circuit as a collection of connected silicon gate elements from a library provided by the silicon manufacturer.

This netlist is used as the basis for the physical design (place and route) flow to convert the designers' intent into the polygonal design of the SoC. Throughout this conversion process, the design is analysed with static timing modelling, simulation and other tools to ensure that it meets the specified operational parameters such as frequency, power consumption and dissipation, functional integrity vs. the RTL and electrical integrity.

When all known bugs have been rectified and these have been re-verified and all physical design checks are done, the physical design files describing each layer of the chip are sent to the foundry's mask shop where a full set of glass lithographic masks will be etched. These are sent to the wafer fabrication plant to create the SoC dice before packaging and testing.

Fabrication

SoCs can be fabricated by several technologies, including:

SoC designs usually consume less power and have a lower cost and higher reliability than the multi-chip systems that they replace. And with fewer packages in the system, assembly costs are reduced as well.

However, like most VLSI designs, the total cost is higher for one large chip than for the same functionality distributed over several smaller chips, because of lower yields and higher non-recurring engineering costs.

Benchmarks

SoC research and development often compares many options. Benchmarks, such as COSMIC,[5] are developed to help such evaluations.

See also

References

  1. ↑ Pete Bennett, EE Times. "The why, where and what of low-power SoC design." December 2, 2004. Retrieved July 28, 2015.
  2. ↑ EE Times. "The Great Debate: SOC vs. SIP." March 21, 2005. Retrieved July 28, 2015.
  3. ↑ EE Times. "Is verification really 70 percent?." June 14, 2004. Retrieved July 28, 2015.
  4. ↑ Brian Bailey, EE Times. "Tektronix hopes to shake up ASIC prototyping." October 30, 2012. Retrieved July 28, 2015.
  5. ↑ "COSMIC Heterogeneous Multiprocessor Benchmark Suite"

Further reading

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