You want low power? How does 27 μW/MHz sound? That's what Cambridge Consultants is claiming for its new XAP5 processor. To put this in perspective, the ARM7—often used as a benchmark of low power—burns 60 μW/MHz. That means XAP5 beats ARM7 by a factor of two! (All number discussed here are for 130 nm.)
In case you're not impressed yet, let's compare that to DSPs. A quick glance at BDTI's benchmark page (PDF) shows that the stingiest DSPs burn around 127 μW/MHz—way more than the XAP5.
So how does the XAP5 pull off such low power? One answer is that the core is tiny—it occupies only 18k gates, or around 0.10 mm2. By way of comparison the ARM7 occupies 0.26 mm2. Again that's a difference of roughly 2X. No big surprise there—power scales more or less linearly with gate counts.
The other secret is that the XAP5 is fairly specialized. The core targets applications like ZigBee, Bluetooth, RFID, and NFC—all of which are low-power wireless applications. These applications don't need the full 32 bits offered by the ARM7, so the XAP5 keeps size and power down by going with a 16-bit design.
The XAP5 is also designed for direct execution from Flash. Thus it goes with a Von Neumann architecture instead of the more complex Harvard architecture favored by DSPs. The simpler architecture saves gates, but it also limits performance. However, the performance concern only comes into play with a SRAM memory system With slower Flash memory, the Von Neumann architecture is perfectly sufficient.
