Comparing embedded processors -

Comparing embedded processors

Before you buy an embedded processor take this quick look at some high-end choices compiled by an independent analyst.

Comparing general-purpose processors is always tricky, because it's difficult to make an apples-to-apples comparison. Performance is more complicated than just MHz or Dhrystone MIPS. Some vendors quote maximum power, others quote typical, and still others don't include I/O power. Some processors include useful system peripherals; others don't.

For this analysis, we chose seven general-purpose processors with clock speeds between 400 and 600MHz. Four are MIPS-based RISC processors: PMC-Sierra's popular RM7065C chip; Broadcom's low-end BCM1122; AMD's newest Alchemy Au1550; and a new entrant, Raza Microelectronics, with the XL5105. The Raza chip is pin-compatible with the RM7065C.

Because most of the PowerPC chips in this segment are aging and uncompetitive, we chose Freescale's forthcoming MPC8349, due to sample early this year. For variety, we included Intel's 80219, a general-purpose XScale (ARM-based) processor, and Via's Eden C3, a low-cost x86 chip for embedded systems.

Our first step was to choose speed grades to try to balance performance. For example, the XScale chip has a scalar (single-instruction) CPU with no level-two (L2) cache, so we used a 600MHz speed grade to boost performance. Conversely, the Broadcom chip is a four-way superscalar CPU with 128KB of integrated L2 cache, which should deliver similar performance at 400MHz.

The other processors are all at 500 or 533MHz. We would have preferred to bump the speed of the AMD chip to 600MHz, making up for its scalar architecture, but that device is not yet available at the faster speed.

The RM7065C, XL5105, and Eden C3 are all traditional standalone CPUs, whereas the other four processors integrate a memory controller, PCI interface, and other common peripherals. To even the comparison, we added Marvell's new Discovery LT companion chip to the two MIPS processors. The LT costs about $39 (list) and burns 2.5W (typical). For Via, we used the company's own CN400 chip set, which adds $35 and 2.0W.

Figure 1: Price and power comparison

The graph in Figure 1 shows the resulting comparison of price and power consumption for this group of processors. At $20 (list), the XL5105 is the least expensive of the group, but when the cost of the Discovery LT is added, the Raza product ends up costing nearly $60. Via's Eden C3 is similarly priced. The lowest-cost solutions are instead the integrated processors that don't require an external support chip. At $30, the Au1550 looks to be the best deal, but its performance is slightly worse than the others in this group. The MPC8349 and XScale 80219 offer a bit more performance and still cost much less than the others in this group.

The BCM1122 costs significantly more than other integrated processors because it's a stripped-down version of the more expensive BCM1125; therefore Broadcom can't cut its price as deeply. PMC-Sierra has no such excuse for the high price of its RM7065C.

When comparing power dissipation, the ranking of these products is similar. In this case, AMD is far and away the best, at less than 1W (typical) for a highly integrated processor. Even taking into account its slightly lower performance, the Au1550 is clearly superior in performance per watt. Again, the integrated parts take the top three spots, and this time the BCM1122 ranks a more respectable fifth. The RM7065C again is last, although this time the margin is much closer. In fact, all these parts, except the Au1550, are between 3W and 5W once system-logic power is considered.

This data, taken from our new report “A Guide to High-Speed Embedded Processors,” shows that integrated processors are becoming popular not just because they reduce board area but because they deliver real savings in system cost and power. Designers looking for a processor in the 500MHz range should consider AMD's new Au1550 as well as Intel's XScale and Freescale's forthcoming MPC8349.

Linley Gwennap () is founder and principal analyst of The Linley Group and coauthor of A Guide to High-Speed Embedded Processors .

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