Design of extremely low power consumption system based on AT91SAM7L

The main factors affecting system power consumption

For a digital system, its power consumption roughly satisfies the following formula: P=CV2f, where C is the load capacitance of the system, V is the power supply voltage, and f is the operating frequency of the system. It can be seen that the power consumption is proportional to the square of the power supply voltage, so the power supply voltage has the greatest impact on the power consumption of the system, followed by the operating frequency, and then the load capacitance. Load capacitance is generally uncontrollable for designers, so when designing a low-power system, it should be considered to reduce the power supply voltage and use a low-frequency clock as much as possible without affecting the system performance. The following is a specific discussion of this issue with ATMEL's AT91SAM7L.

Design of an extremely low-power system based on AT91SAM7L

AT91SAM7L is an industrial-grade 32-bit RISC controller with the advantages of high performance and ultra-low power consumption. This series of products is embedded with power switches that control multiple power islands (Power islands), as well as programmable voltage regulators to reduce power consumption in working and standby modes, providing favorable conditions for designing low-power systems.

Figure 1 is the principle framework of a calculator using AT91SAM7L as the CPU, which also includes the AT91SAM7L power system structure diagram.

⑴ Power supply voltage

AT91SAM7L operates in single voltage mode, with a voltage as low as 1.8V. In operating mode, the typical current consumption is 0.5mA/MHz when executing code from flash memory. The power consumption of different standby modes can be controlled in a variety of ways, such as power switches, adjustable voltage regulators, and using sampling techniques in voltage monitors, power-on resets, and undervoltage detection circuits instead of continuous measurement. In power-off mode, the typical current consumption of AT91SAM7L is only 100nA.

⑵ Low power mode and fast wake-up function

In power-off mode, only the fast wake-up (Fast Wake-up, FWUP) pin is powered, so that the system can be woken up by pressing a button. This function can be used to control the power on/off of applications such as calculators. In backup mode, only the power controller, zero-power power-on reset (POR) and 32kHz crystal oscillator circuit are running. RTC, 2kb backup SRAM, BOD, charge pump, LCD regulator and LCD controller can be turned on/off independently. In wait mode, the 2MHz RC oscillator will quickly wake up the system and achieve fast external event management. Figure 2 is the transition and state diagram between various modes of AT91SAM7L.

⑶ Power reduction technology

AT91SAM7L adopts innovative power reduction technology for working and standby modes. In working mode, the operating voltage and operating frequency, peripheral clock activity can be set in a programmable way, and DMA can be used to replace the CPU to complete data transmission, thereby optimizing power consumption.