Microcontrollers Improve Power

文章摘要：Improving Stop ModeStop mode is the lowest power state available to 8051 designers. In this mode, the internal crystal amplifier is stopped, halting operation of the device. Exiting from Stop mode is typically initiated by an external reset. Some variants also support exiting from Stop mode using ex

Microcontrollers Improve Power,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

Improving Stop Mode

Stop mode is the lowest power state available to 8051 designers. In this mode, the internal crystal amplifier is stopped, halting operation of the device. Exiting from Stop mode is typically initiated by an external reset. Some variants also support exiting from Stop mode using external interrupts.

One of the disadvantages associated with Stop mode is the power consumed during the "dead time" while the crystal is resuming operation. A crystal oscillator relies on the motion of a quartz crystal for its operation. Physical limitations require a finite amount of time for the crystal oscillation to achieve sufficient amplitude for device operation. This warm-up period is encountered regardless of whether the clock source is an external crystal and internal crystal amplifier, or whether an external crystal oscillator is used. This time can be on the order of 3ms to 12ms, depending on the characteristics of the crystal and associated amplifier.

The effect of the warm-up period on power consumption is that while the device is not performing any useful work during this period, it is still consuming power. This can become significant if the device is entering and exiting Stop mode frequently, or is exiting Stop mode to perform short tasks. In fact, if the task is very short (< 5ms), the crystal restart period can consume more power than the task itself. If a ring oscillator is used to perform a "quick start" from Stop mode, this delay can be avoided. This will greatly reduce the amount of power when out of Stop mode.

Figure 6 shows the operation of two systems exiting from Stop mode and performing a short task. One device incorporates an internal ring oscillator, and the other uses a traditional external crystal. The device without the ring oscillator must endure a crystal warm-up period. During this time the device continues to consume power, but no useful work is done. The second device is a DS87C520 high-speed microcontroller that incorporates an internal ring oscillator. This allows the device to resume operation immediately when exiting Stop mode. In this example, the routine to be executed is less than 4ms in duration at approximately 2MHz. As can be seen in the figure, energy consumption can be greatly reduced by using a ring oscillator to perform short tasks when exiting Stop mode.


Figure 6. Comparison of stop mode exit with and without ring.

In some applications, the stability of a crystal oscillator may be required shortly after exiting Stop mode. In this case, the ring oscillator can still be advantageous. Immediately upon exiting Stop mode, the device should restart the crystal oscillator. The device can then initialize any data or registers necessary while the crystal is still warming up. Most high-speed microcontrollers incorporate a status bit that indicates whether the crystal oscillator has stabilized or not. Once the initialization routine for the crystal oscillator code is complete, the software can poll the status bit to determine when the high-precision timing operation can commence.

上一页  [1] [2] [3] [4] [5] [6] [7] [8] [9]  下一页