Microcontrollers Improve Power

文章摘要：Hurry Up and Wait In many applications, the time out of Stop mode is not entirely speed-dependent. Frequently, a device will have to access a peripheral with a fixed response time, such as an A/D converter or thermostat. In such a case, the microcontroller will have a burst of activity, typically to

Microcontrollers Improve Power,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com
Hurry Up and Wait In many applications, the time out of Stop mode is not entirely speed-dependent. Frequently, a device will have to access a peripheral with a fixed response time, such as an A/D converter or thermostat. In such a case, the microcontroller will have a burst of activity, typically to initiate a process, followed by a period of little or no activity. In such a case, a combination of power conservation techniques can be effective.

A practical example can illustrate the advantage of a high-speed microcontroller with PMM in such a system. Suppose that the DS87C520 is interfaced to a DS1620 digital thermometer and thermostat. This device is addressed serially using a standard 8051 serial port operating in mode 0. A host processor will occasionally wake the DS87C520 from Stop mode using an external interrupt and request that it read the temperature from the DS1620. After the data has been retrieved, the DS87C520 will store it in internal memory to be transmitted later. The DS1620 functions similarly to many A/D converters: a command is issued to start a conversion, then there is a delay while the conversion is completed, then the data is shifted out. In the case of the DS1620, conversion time is approximately 1 second. The device is polled to determine when the conversion is complete. The DS87C520 is well suited to such a task because it can perform the initialization and computation functions quickly. The device can then place itself in PMM while waiting for the conversion to complete. In a conventional 8051, Idle mode would be used to place the conventional 8051 in a low-power state once the conversion was started. The use of this mode allows an internal 16-bit timer to measure the conversion period. Operating at 16MHz, the conventional 8051 could require exiting Idle mode as many as 32 times before the conversion was complete.

Table 1. Energy Consumed vs. Processor Speed for a 500 Machine Cycle Task

Clock Frequency	Machine Cycle Period	Machine Cycles Required	Total Time	ICC	Current Time Product
10MHz	400ns	500	200s	12.41mA	2.48As
30MHz	133ns	500	66.5s	34.66mA	2.30As

One further improvement can be made in this example. Because the DS1620 is addressed as a synchronous device, high precision timing operations are not required. As a result, the microcontroller can operate from the ring oscillator while initiating and when reading the results of the conversion. This results in further power savings by eliminating the dead time needed to stabilize an external crystal.

Figure 8 illustrates the operation of two 8051 systems implementing the "hurry up and wait" schemes mentioned above. As can be seen from the figure, there is a significant power savings during program execution following the exit from Stop Mode. In addition to the power saved by using PMM2 instead of Idle mode, the elimination of the crystal warm-up period means that the routine can return to Stop mode more quickly. Running from the ring oscillator during the 1s conversion delay slows the processor speed even more, allowing for greater power savings.

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