Atmel's ARM(R)-based Microcontroller - Low Power for Portable Systems White Paper R is the registered trademark of Atmel Corporation, 2325 Orchard Parkway, San Jose, CA 95131 Rev. 1743A-04/01 Abstract The driving force behind the semiconductor industry today is the digital nomad. The digital nomad is an active, productive and creative person who is not constrained by conventional boundaries of space and time, work and leisure. He or she simply needs to be connected - to all the digital information services available - anywhere, any time. This is creating a demand for portable electronic devices on an unprecedented scale. The mobile telephone is the highest-profile example, but this is becoming the voicedriven interface for many other activities. Most significant is voice/keypad Web access, for information, Web phone calls, voice-driven e-mail, e-commerce and more. As road vehicles accumulate intelligence, for navigation, hands-off convoy driving, accident prevention, etc, a related growth market for on-board electronic systems comes into focus. These lifestyle accessories for the digital nomad are designed for mass consumption. They must be small, attractive, lightweight, easy-to-use and cheap. They have a short useful life - they are soon obsoleted by superior successors. In particular, the inconvenience of re-charging or carrying bulky power packs must be kept to a minimum. The information processing element behind these information appliances is the microcontroller. Embedded microcontrollers are the key building block in any system that responds to instructions or transforms information, from a pager to a satellite telephone. To satisfy the demanding requirements of the hand-held systems market, notably minimum battery size and weight along with maximum battery life, the lowest possible power consumption is mandatory. 2 White Paper 1743A-04/01 White Paper Atmel's AT91 Series Microcontrollers - the Low Power Solution Atmel has responded to this challenge in the design of its AT91 series microcontrollers, which embed the 32-bit ARM Thumb processor core. The bandwidth inherent in a 32bit processor is more than adequate for most tasks required by hand-held systems. The key to making this bandwidth available for portable systems is to minimize power consumption. The ARM7TDMI core is a market-leader in MIPS/Watt. Its advanced RISC architecture gives it a minimal yet highly efficient instruction set. This allows for a simple structure and small core size. In particular the instruction decode mechanism is highly compact and implemented in standard low-power logic. This architectural approach minimizes both the number of power-consuming elements, and the power consumption per element. Figure 1. The power saving modules of the AT91R40807 architecture TMS TDO TDI TCK NRST Reset Embedded ICE D0-D15 ARM7TDMI Core RAM 136K bytes MCKI Clock P25/MCKO ASB Controller P12/FIQ P9/IRQ0 P10/IRQ1 P11/IRQ2 P13/SCK0 P14/TXD0 P15/RXD0 P20/SCK1 P21/TXD1/NTRI P22/RXD1 P I O EBI: External Bus Interface ASB EBI User Interface AIC: Advanced Interrupt Controller 2 PDC Channels TC: Timer Counter P0/TCLK0 P3/TCLK1 P6/TCLK2 TC0 P1/TIOA0 P2/TIOB0 TC1 P4/TIOA1 P5/TIOB1 TC2 P7/TIOA2 P8/TIOB2 WD: Watchdog Timer NWDOVF APB USART1 P26/NCS2 P27/NCS3 P28/A20/CS7 P29/A21/CS6 P30/A22/CS5 P31/A23/CS4 P I O AMBA Bridge USART0 A1-A19 A0/NLB NRD/NOE NWR0/NWE NWR1/NUB NWAIT NCS0 NCS1 2 PDC Channels PS: Power Saving P16 P17 P18 P19 P23 P24/BMS Chip ID PIO: Parallel I/O Controller Modules designed for power saving The dual-bus structure surrounding the ARM core carries this concept to its peripherals. The ASB system bus is designed for maximum data throughput between the core and 3 1743A-04/01 memory blocks (both on- and off-chip). The AMBA Bridge links this to a lower-speed APB, which minimizes power consumption in data transfers to and from the peripherals. The AT91 series surrounds the ARM core with an SRAM workspace, External Bus Interface and a set of general-purpose peripherals. These include an Advanced Interrupt Controller, USARTs, Timer/Counters, SPI, Watchdog Timer, Real Time Clock, etc. Figure 1 shows the peripherals for the entry-level AT91R40807. The modular design enables power consumption to be minimized in every aspect of the operation of the device, under all scenarios of use. Fully Static Operation All AT91 devices are fully static, driven by a single system clock. The operational power consumption is proportional to the clock frequency. Power can be saved at device level by stopping the clock externally, or running it at the slowest rate required to complete a task in the time interval available. A common practice is to accomplish each task at full clock speed, and then stop the clock until the device is next required. With the external clock disabled, an AT91 device consumes between 30 and 60 A. Low Voltage Operation The AT91 devices normally operate at 3.3V, but they can be powered down to 1.8V and still deliver the performance required for most power-sensitive applications. The I/Os continue to operate at 3.3V - all internal level shifting is transparent to the environment of the system. This gives a power saving of around 35%. Power-saving SRAM The large on-chip SRAM (up to 136K bytes in the case of the AT91R40807) significantly reduces system power consumption compared with a two-ship solution. For example, an AT91M40800 operating at 40 MHz with an external 12 ns SRAM consumes 120 mA. The AT91R40807 under the same conditions consumes only 50 mA. Power Reduction during Interrupts and Data Transfer Two architectural features play significant roles in reducing power consumption during operation: the Advanced Interrupt Controller and the Peripheral Data Controller channels associated with the on-chip peripherals. The 8-level priority Advanced Interrupt Controller intercepts all internal and external interrupt requests, prioritizes them, associates them with the corresponding interrupt vector and passes them to the core when they are scheduled for handling. It reduces the number of core instructions required to reach the interrupt handler to only one. In realtime interrupt driven systems (the majority of hand-held devices are in this category) this represents a major saving of processor cycles, and hence of power. The integral Peripheral Data Controller provides a direct memory access (DMA) function between on-chip peripherals such as the USART, SPI or DAC and the system memory (on- or off-chip). The PDC does not use any processor resources, allowing the processor to be put into idle mode during data transfers. With a capacity of up to 64K contiguous bytes from the same start address, it significantly reduces power consumption during data transfer. 4 White Paper 1743A-04/01 White Paper The Way Ahead - Enable Logic and Self-timed Powerdown The programmable Power Management Module features enable logic for each peripheral (as well as for the core). See Figure 2. This permits the application developer to switch off the clock to each peripheral device, and to the core, when it is not in use. The core is re-activated by an enabled interrupt, and resumes operation after only a few clock cycles. Only the elements required for each activity are consuming power at any given instant. This minimizes power consumption during operation with very little programming effort and no intervention on the external clock source. Power consumption is reduced to between 150 and 250 A when all internal clocks are off. Figure 2. AT91 Power Management Enable Logic MCKI Power Management Enable Logic Internal Clocks ARM7TDMI Core SPI USARTs Counter/Timers Parallel I/O Watchdog Timer Reset 5 1743A-04/01 The AT91M55800A has a Real Time Clock with an independent battery power supply that provides shut-down and wake-up logic for the system power source. This permits the AT91 (apart from the RTC) to be powered down under program control, and awakened either by an external source such as a push-button or radio receiver, or by an alarm set in the RTC. See Figure 3. Only the RTC is active in shut-down mode, which may be for extended periods of time. This gives power consumption of just a few A in shut-down mode. Figure 3. AT91 Real Time Clock Shut-down/Wake-up System Power Supply DC/DC Switching Regulator SHUTDOWN VDDCORE AT91 VDDIO GND VDDBU Battery Real Time Clock GND SHUTDOWN WAKEUP Towards System Level Integration Atmel's AT91 series of standard products aim to provide an off-the-shelf solution to immediate requirements for low-power, high-performance microcontrollers. They are also an element in Atmel's corporate strategy of system level integration. The constituent modules of an AT91 device can be incorporated into an application-specific or customer-specific system-on-chip. This can include whatever peripherals are required, plus on-chip memory (Atmel is an industry leader in NVM), DSP cores, analog interfaces and application-specific IP. The power saving features of the AT91 are all made available to the system-on-chip. SLI gives the ultimate in power reduction - the entire device is implemented in a single IC. This minimizes silicon area, eliminates board interconnections and enables total system power management to be optimized. In this way the digital nomad can be equipped with advanced communication and information processing devices that are practically invisible, except as fashion accessories. 6 White Paper 1743A-04/01 Atmel Headquarters Atmel Operations Corporate Headquarters Atmel Colorado Springs 2325 Orchard Parkway San Jose, CA 95131 TEL (408) 441-0311 FAX (408) 487-2600 Europe Atmel SarL Route des Arsenaux 41 Casa Postale 80 CH-1705 Fribourg Switzerland TEL (41) 26-426-5555 FAX (41) 26-426-5500 Asia Atmel Asia, Ltd. Room 1219 Chinachem Golden Plaza 77 Mody Road Tsimhatsui East Kowloon Hong Kong TEL (852) 2721-9778 FAX (852) 2722-1369 Japan 1150 E. Cheyenne Mtn. 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The Company assumes no responsibility for any errors which may appear in this document, reserves the right to change devices or specifications detailed herein at any time without notice, and does not make any commitment to update the information contained herein. No licenses to patents or other intellectual property of Atmel are granted by the Company in connection with the sale of Atmel products, expressly or by implication. Atmel's products are not authorized for use as critical components in life support devices or systems. ATMEL (R) and the Atmel logo are the registered trademarks of Atmel Corporation. ARM, ARM7TDMI are trademarks or registered trademarks of ARM Ltd. Other terms and product names may be the trademark of others. Printed on recycled paper. 1743A-04/01/0M