Rabbit 2000™ Microprocessor - UTN

For extreme low-power operation it should be taken into account that some memory chips drawsubstantial current at zero frequency. For example, a Samsung static RAM (part numberKM684000BPL-7L) was found to draw 1 mA at 5 V when chip select and output enable were heldenabled and all the other signals were held at fixed levels (a long read). When the microprocessoris operating at the slowest frequency (32 kHz clock), the memory cycle is about 64 µs and thememory chip spends most of its time with the chip enable and output enable on. The current drawduring a long read cycle is not specified in most memory data sheets. The Samsung chip, accordingthe data sheet, typically draws about 4 mA per megahertz when it is operating. However, itappears that current consumption curve flattens out at about 250 kHz because of the constant 1mA draw during a long read.In order to take full advantage of the Rabbit’s ultra slow sleepy execution modes, a memory thatdoes not consume power during a static read is required. Advanced Micro Devices has a line of 3V flash memories (AM29LV010, AM29LV040) that power down automatically whenever theaddress (and control) lines do not change for a period of time slightly longer than the access time.These memories will consume on the order of 30 µA when operated at a data rate of 1/64 MHz.Prior to version 7.10, Dynamic C did not allow debugging with flash chips having sector sizesgreater than 4096 bytes, nor did the flash drivers provided in the Dynamic C libraries support suchflash chips. To use a large sector flash in your product design if you are using a version ofDynamic C prior to 7.10, you can debug your application in RAM by using the “Compile toRAM” compiler option, or use a board with small sector flash for development only.The Rabbit low-power sleepy mode of operation is achieved by switching the main clock to the32.768 kHz clock and then disabling the main oscillator. In this mode, the Rabbit executes about 3instructions every millisecond. Adding memory wait states can further slow the processor to about500 instructions per second or one every 2 ms. At these speeds the power consumed by the microprocessor,exclusive of the 32.768 kHz oscillator, is very low, in the area of 50 µA to 100 µA. TheRabbit will generally test for some external event and leave sleepy mode when that event isdetected. The 32.768 kHz oscillator is a major consumer of power, requiring approximately 80 µAat 3.3 V. This drops dramatically to about 18 µA at 2.2 V. For the lowest standby power it may bedesirable to use an external oscillator to generate the 32.768 kHz clock. The Intersil (formerlyHarris) part HA7210 can be used to construct a 32.768 kHz oscillator that consumes approximately5 µA at 3.3 V.For the very lowest power consumption the processor can execute a long string of mul instructionswith the de and bc registers set to zero. Few if any internal registers change during the executionof a string of mul zero by zero, and a memory cycle takes place only once in every 12clocks. By combining all these techniques it may be possible to get the sleepy current under 50µA.48 Rabbit 2000 Microprocessor