SOPC?

Until recently any thoughts of implementing a Sytem On a Chip have been restricted to those considering ASIC design.

Over the last few years there have been major developments in FPGA logic density, speed, packaging etc. These have made implementing a 'system' of processor/s, IP blocks, user logic in an FPGA (System On a Programmable Chip) a possiblility . Indeed, with potentially large complex designs FPGA implementation can be more attractive as it carries few of the risks of ASIC implementation.
Even the recently introduced low-end/cheap FPGA families have the resources to implement a simple CPU core.

Anyone starting a design with an embedded processor with any amount of application specific logic ought to consider whether a SOPC might provide a better solution.

SOPC Advantages

FLEXIBILITY
The main SOPC advantage is flexibility. A RISC CPU core can be configured to be 16/32 bits, have data or instruction pipelining, hardware multipliers, custom instructions with hardware acceleration to increase system performance. Typically this can run at speeds of up to 100MHz (see below for real world examples).

Also, whatever peripherals that are needed can be included. Six UARTS, ten I2C channels, LVDS interfaces, an SDRAM controller etc, whatever the application demands. Typically a large portion of the FPGA resource is left for aplication logic. In the smallest device in Altera's most recent low cost FPGA family (Cyclone 1C3), a minimal 16 bit CPU will occupy approximately 50% of the device logic. In the largest devices even a complex 32 bit CPU core will occupy less than 5% of the device logic.

The flexibility of an FPGA SOPC implementation gives some protection against feature creep after a PCB has been designed, and opens the possibility of one PCB being used in multiple applications depending on its configuration.

OBSOLESCENCE
Implementing a system in a programmable device gives significant protection against obsolescence. FPGA families are supported by the manufacturers for years. Due to their nature a small number of devices can be used for a multitute of purposes, so FPGA vendors don't have to build and stock a large number of differently configured devices. This economy of scale means that devices that are over ten years old are still available. As FPGA use increases and devices win more design-ins this situation is likely to continue.

If an FPGA does go obsolete as the SOPC is a soft core it can be re-targeted at a new family of FPGA. No software need be re-written/modified etc.

SIMPLER BOARD DESIGN
As the interconnections between different modules in a system are mostly internal to the FPGA the number of high speed signals radiating is vastly reduced. Clock distribution and inter chip timing problems are also removed, internal PLLs can remove the need to distribute high speed clocks round a PCB, making EMC tests much more likely to be passed first time.

Grounding, trace termination problems and so on are also removed for any modules in the FPGA.

As a result a PCB can be smaller, consume less power, be easier to get through EMC tests and more likely to work first time.

MANUFACTURING
PCBs with one large device rather than several smaller ones can be smaller, easier and cheaper to kit, manufacture and test.
To help with board test dedicated test designs can be loaded into the FPGA to automate testing. This is not nearly as easily done with an ASIC.

MISCELLANEOUS
If significant application logic is needed a controlling uP core can be had almost for free. A 16 bit CPU core can take less than 2% of the logic in the largest devices.

Depending on the application the FPGA/SOPC configuration and firmware can be remotely updated (the Cyclone configuration devices can hold the programme/data for a CPU core and both the FPGA configuration and program can be updated by the FPGA).

SOPC Disadvantages

FPGA's tend not to be suitable for very low power applications and most modern devices require two or three power supplies and ground, usually supplied via multiple planes. Implementing a complex system in an FPGA is not a simple task, especially without previous experience of the family/design tools.

Incorporating a number of modules in a single package can force the use of a BGA package to provide the required number of IO pins. If BGA packages hadn't been planned for the PCB this may lead to a more expensive build and more difficult re-work.

If an application includes a requirement for an A/D convertor this must be implemented with an external IC. D/A convertors can be implemented with PWM output on a pin with a low pass filter, and a basic A/D convertor can use this output with a comparator, but any requirement for speed/accuracy usually dictates external components.

NIOS/SOPC Builder

As their FPGAs have become more capable of system on a chip implementation Altera have addressed certain problems that arise. They have introduced a configurable CPU soft core, NIOS, together with a tool, SOPC Builder, to manage the complexity of defining the CPU and integrating the software, with custom peripherals, user logic and so on.

NIOS
This is a 16/32 bit risc processor soft core which comes with many available peripheral components. The CPU can be configured to inlude a hardware multiplier, data and instruction caches, hardware debug ports, interrupt support, hardware divide etc depending on the application.
User logic is easily interfaced with the core. A Wizard is available to examine the interface code and attempt to deduce functionality, or this can be specified manually
The peripherals which are immediately available are..

• SPI interface
• UART
• AMD 29LV065 Flash
• LAN91C111 Ethernet interface (10/100 MAC)
• SDRAM controller
• DMA controller
• Interval Timer
• Parrallel IO interface
• User Logic IF
• TCP/IP Software stack

Also available as IP cores from Altera are...

• CAN 2.0 Network controller
• I2C master/slave interfaces
• T1 framer
• 10/100/1G Ethernet MAC
• PCMCIA Compact Flash Controller
• USB 2.0 Controller

The NIOS CPU is supported by the GNUPro C/C++ toolkit which is becoming an industry standard toolset. A C/C++ compiler, an assembler, linker, and GDB source and assembly level debugger (with Insight GUI front end) are included.

If we deliver a SOPC design based on a NIOS CPU a client can then use the GNU tools to write/debug software and re-programme application FLASH (if included) as if using normal embedded processor. (The NIOS GNUPro tools can be directly downloaded from Altera).

SOPC Builder
When a system has been configured then generated, the SOPC builder generates an RTL description of the CPU and peripherals and a Software Development Kit (SDK) configured for this core. The SDK includes header files defining the memory map and all peripherals, a library of example drivers for the peripherals included and a series of example source files and a make files which may be used for debugging.

A modelsim environment is also created to allow the complete SPOC to be modelled prior to implementation.

Once the CPU and interfaces to application logic are defined software development can start immediately, allowing simultaneous hardware and software development.

SPEED EXAMPLES
Some examples of SOPC configurations/devices and speeds are...

Cyclone (-7 speed grade): Includes 32-bit Nios with 4K iCache & dCache, hardware multiply, JTAG-based "OCI" comm/debug port, boot ROM, SDRAM controller, SRAM/Flash/Ethernet MAC interfaces, UART, two timers, PIO for the buttons, LEDs, remote reconfiguration, etc., Compact flash interface, "ASMI" configuration interface (to low-cost serial flash ROMs). Requested f-max=50Mhz, actual f-max: 63.8Mhz.

Stratix (-6 speed grade): Includes most of the same peripherals as above, but without ASMI memory, and with the addition of 64K of on-chip "M-RAM" memory). Requested f-max=50Mhz, compiled for "speed" in Quartus, actual f-max: 69.3Mhz

These are very feature rich examples and were simple push button FPGA compiles with no specific constraints applied, CPUs with fewer external components will run at over 100MHz.

Here are some reviews on experiences with NIOS/SOPC implementation.