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A 4500 MIPS/W, 86 µA Resume-Standby, 11 µA Ultra-Standby Application Processor for 3G Cellular Phones
IEICE TRANSACTIONS on Electronics
Publication Date: 2005/04/01
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Low-Power LSI and Low-Power IP)
application processor, cellular phone, pipeline structure, resume-standby, low power, low leakage,
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We have developed an application processor optimized for 3G cellular phones. It provides high energy efficiency by using various low power techniques. For low active power consumption, we use a hierarchical clock gating technique with a static clock gating controlled by software and a two-level dynamic clock gating controlled by hardware. This technique reduces clock power consumption by 35%. And we also apply a pointer-based pipeline to in the CPU core, which reduces the pipeline latch power by 25%. This processor contains 256 kB of on-chip user RAM (URAM) to reduce the external memory access power. The URAM read buffer (URB) enables high-throughput, low latency access to the URAM while keeping the CPU clock frequency high because the URAM read data is transferred to the URB in 256-bit widths at half the frequency of the CPU. The average miss penalty is 3.5 cycles at the CPU clock frequency, hit rate is 89% and the energy used for URAM reads is 8% less that what it would be for URAM without a URB. These techniques reduce the power consumption of the CPU core, and achieve 4500 MIPS/W at 1.0 V power supply (Dhrystone 2.1). For the low leakage requirements, we use internal power switches, and provides resume-standby (R-standby) and ultra-standby (U-standby) modes. Signals across a power boundary are transmitted through µI/O circuits to prevent invalid signal transmission. In the R-standby mode, the power supply to almost all the CPU core area, except for the URAM is cut off and the URAM is set to a retention mode. In the U-standby mode, the power supply to the URAM is also turned off for less leakage current. The leakage currents in the R-standby and in the U-standby modes are respectively only 98 and 12 µA. For quick recovery from the R-standby mode, the boot address register (BAR) and control register contents needed immediately after wake-up are saved by hardware into backup latches. The other contents are saved by software into URAM. It takes 2.8 ms to fully recover from R-standby.