Universal Cores Threaten to Keep Proprietary MPUs at ARM's Length

By Reuben Townsend

If cost no longer rules out the use of an ARM, MIPS or ColdFire® 32-bit microprocessor (MPU) in the vast majority of applications, then what does? Reuben Townsend, Technical Sales Manager, Future Electronics Europe (UK) explains.

The 32-bit microprocessor market has been transformed in recent years, with the boundaries between ARM, MIPS and ColdFire devices diminishing enormously in terms of performance and functionality. In addition, unit costs for 32- bit devices, even in small volumes, are now often below $5, so the price differences between device types are small.

Given that ARM is a universal core, many observers have questioned whether this similarity in prices will impact on the choice of core for devices currently based on Freescale’s ColdFire® or MIPSbased proprietary solutions.

The reality is that each has its advantages and drawbacks and there are areas in which a proprietary core will remain preferable. The ColdFire family, for instance, offers a rich and diverse peripheral mix, robust on-chip security and performance enhancing features such as cache and maths acceleration.

By contrast, MIPS technology offers a lot of processing power for the money – more than 500MIPS at the high end. Finally, ARM offers low power consumption and high speed memory configurations.

It should be noted, however, that the differences are far less marked than the similarities. ARM and MIPS both offer support for a wide range of peripherals; and the ‘performance per dollar’ value of ARM and ColdFire is far from poor. It is true that ColdFire offers excellent hardware security but security can also be designed into ARM and MIPS software.

All these examples show how competitive the 32-bit market has become. This article will therefore give a detailed overview of the 32-bit MPU market by comparing and contrasting the ARM, ColdFire and MIPS architectures. All products mentioned, including devices from NXP, Freescale and AMD, are supplied and supported by Future Electronics.

 

 

ARM Processors

ARM is a family of processors maintained and promoted by ARM Holdings Limited. ARM does not manufacture its own processors. Instead, it designs the CPU cores for customers based on the ARM core; they charge the customers licensing fees on the design, and then lets them manufacture the chip wherever they choose.

ARM’s low core power consumption has made it a popular choice for portable applications. Most mobile phones and Personal Digital Assistants (PDAs) contain an ARM-based CPU. Also, because every ARM processor shares the same instruction set, one ARM device’s software is almost always compatible with that of another.

Most ARM cores indicate their key features in their name. ‘D’ stands for Debugging support, ‘M’ for enhanced Multiply (usually 64-bit), and ‘I’ for ICEbreaker™ on-chip support for in-circuit emulation.

Another common letter is ‘T’ which stands for ‘Thumb’. This is a specialized instruction set for RISC processor cores that consists of a subset of 16-bit instructions. These 16-bit instructions are a kind of shorthand for commonly used 32-bit ARM instructions; they require less memory. (Note: not all 32-bit instructions are, of course, expressible in this way.)

The ARM7TDMI mid-range 32-bit core, as shown in Figure 1, is a popular ARM device for general embedded use (rather than for specialized applications such as telecoms, video and audio, for which other ARM cores are more suited). Many vendors’ devices include peripherals such as SRAM/Flash controllers, UARTs, SPI interfaces, USB, Ethernet, CAN, general purpose I/O, Analog-to-Digital Converters (ADCs), Digital-to-Analog Converters (DACs), Pulse Width Modulation (PWM)/ timers and interrupts.

The ARM memory interface has been designed to allow its performance potential to be realized without incurring high costs in the memory system. Speed critical control signals are pipelined to allow system control functions to be implemented in standard low power logic, and these control signals can be used to exploit the fast burst and page access modes offered by industry standard dynamic RAMs.

In the real world, of course, while this provides for a low cost memory system, it also means you have to be careful about memory access when designing with an ARM7 core. Besides offering a substantial list of peripherals, NXP, for instance, has modified and improved memory access on its ARM7 chips.

ARM7 code is capable of running at up to 70MHz with single cycle execution of many instructions. The on-chip Flash memory, however, has an access time of typically 30ns, which limits processor execution to a peak of 33MHz. Clearly, running from Flash would leave a lot of processor performance unused.

In order to overcome these problems, NXP has designed a Memory Accelerator Module (MAM), as illustrated in Figure 2. On the NXP LPC2106 ARM7TDMI device, this splits the 128kbyte Flash into a pair of 128-bit-wide memory banks. This allows four instructions to be fetched by each Flash access, and is further accelerated by interleaving the code between the two banks

 

 

Freescale ColdFire MPUs

The Freescale ColdFire is an aggressively priced processor architecture that is designed to address the performance needs of a huge range of embedded systems. Its ancestry stretches back to the 68000 architecture introduced 20 years ago, but is today offered in Version 2, 3 and 4e (or V2/V3/V4e) evolutions. These are indicated in the second number of product family codes such as MCF52xx, 53xx and 54xx. Figure 3 shows the MCF523x family block diagram.

Many ColdFire processors offer around 100MIPS performance; the latest V4 devices offer 400MIPS.

ColdFire processors feature a variable length instruction word of 16, 32 or 48-bits to reduce memory and system costs. Most instructions (such as the signed multiply) become encoded in 16-bits; complex instructions in 32 or 48-bits. The advantage is that the code becomes very compact, reducing the amount of Flash memory required. Freescale tests on ColdFire and MAC71xx (ARM) show that ARM native code is 27% larger than ColdFire; in ARM’s Thumb mode, ARM code is only 7% smaller than ColdFire.

The static ColdFire core is also completely synthesizable, and will easily integrate with memories, system modules and peripherals. (Note: like ARM, the ColdFire core is synthesized and portable, and has been licensed to third parties.) ColdFire cores are backward compatible and allow future code migration to new parts, and offer a rich peripheral features list. They also tend to be very cost competitive with comparable ARM7/ARM9 variants.

It is also often assumed that ColdFire returns poorer power consumption figures than an ARM core. It is true that ARM offers very low core power consumption, but when the device power consumption is considered (using information supplied by Freescale and calculated from comparable device data sheets), a ColdFire part’s performance canbe little different from that of an ARM device.

This would help explain why ColdFire cores are finding their way into an increasing number of consumer applications including printers, home gateways, MP3 players and network access servers.

Of course, ColdFire’s obvious strength is peripheral support. Peripherals commonly found in ColdFre devices include Ethernet, DDR support, serial interfaces, USB OTG, PCI and LCD. ColdFire also offers excellent on-chip security options including an integrated hardware security engine available on ColdFire MCF547x/548x families, and encryption hardware accelerator modules on the MCF523x/ 527x families. This can prove vitally important in IP internet and Ethernet applications.

 

 

MIPS CPU Cores

As with ARM, the company steering MIPS – MIPS Technologies Inc. – licenses CPU cores to third-party manufacturers. Unlike ARM, however, there are many varieties of the instruction set. 32-bit and 64-bit MIPS implementations are available from a range of semiconductor vendors including AMD.

One of the most important benefits of MIPS is that all usage models are supported. This means there is a wide and varied selection of products, all of which are compatible, from multiple vendors.

You can therefore purchase stand alone processors, integrated processors, hard macro cores for implementation on high performance SoC (System-on-Chip) devices, synthesizable cores for SoC implementations at the lower end of the performance spectrum, and finally, a full architectural license to create your own synthesizable or hard core implementation directly from the instruction set architecture.

With all of these usage models supported, it allows anyone who wants to use the MIPS architecture to access the technology at any step, and then scale their product line up or down as the need arises. To ensure code compatibility between devices, the MIPS architecture has a certification program. One family of MIPS devices, AMD’s Alchemy Au1x00 series, is aimed at video and high intensity computation in applications such as media players, voice over-internet protocol, network infrastructure and video streaming.

The 32-bit Au1200 processor, as shown in Figure 4, supports MPEG1, MPEG2 and MPEG4, WM9, H.263, DivX 3-5 and H.264 in Common Intermediate Format resolution; has a DDR SDRAM controller; 333MHz, 400MHz and 500MHz versions; separate data and instruction caches; an LCD controller with 32-bit RGB support for up to 1024 x 768 resolution; USB2.0 host/ device functionality; and a camera interface module. Its 32-bit Au1550 cousin adds advanced peripherals including PCI, a security engine and a programmable serial controller.

One key advantage of MIPS’ strength in audiovisual applications is that it can cut the number of development environments a designer needs to use when bringing an application to market. In audio, for instance, one MIPS chip could be used for DSP audio algorithms and another for control.

 

 

Conclusions

There are no hard and fast rules for discriminating between ARM, ColdFire and MIPS. In some cases the application will dictate the selection; in others – particularly in relation to ARM and ColdFire – the designer’s personal preference may be crucial.

Designers have never had more choice in the 32-bit processor market, which is why Future Electronics– a supplier of ARM, ColdFire and MIPS devices – is a useful ally.