A 4500 MIPS/W, 86 µA Resume-Standby, 11 µA Ultra-Standby Application Processor for 3G Cellular Phones

Makoto ISHIKAWA
Tatsuya KAMEI
Yuki KONDO
Masanao YAMAOKA
Yasuhisa SHIMAZAKI
Motokazu OZAWA
Saneaki TAMAKI
Mikio FURUYAMA
Tadashi HOSHI
Fumio ARAKAWA
Osamu NISHII
Kenji HIROSE
Shinichi YOSHIOKA
Toshihiro HATTORI

Publication
IEICE TRANSACTIONS on Electronics   Vol.E88-C    No.4    pp.528-535
Publication Date: 2005/04/01
Online ISSN: 
DOI: 10.1093/ietele/e88-c.4.528
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Low-Power LSI and Low-Power IP)
Category: Digital
Keyword: 
application processor,  cellular phone,  pipeline structure,  resume-standby,  low power,  low leakage,  

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Summary: 
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.