使用定時器和計數器創建高效的基于MCU的設計

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　　定時器和計數器可能是MCU設計中最普遍的外圍設備。任何應用程序都可以使用定時器或計數器來提高性能，降低功耗，或者簡化設計，用簡單的定時器或計數器中斷替換重復或循環的CPU操作。您可能沒有使用一些最近的計時器/計數器單位，但是，一些先進的功能現在可以提高您的設計更。本文將快速回顧一些新的定時器/計數器功能，您可以使用，以改善您的設計與說明性的例子，從流行的MCU家庭，特別強調功能的自主操作和電機控制。

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　　Timer and counter modes： from simple to advanced

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　　Timers and counters started out with some very simple modes of operation to replace common-program loops for counting external events， for timing internal and external operations and for collecting various statistics on key MCU operations. Some of the most familiar counting modes are illustrated by the features in the Maxim MAXQ612 MCU and some examples are shown in Figure 1 below. The table at the bottom of the figure shows three common autonomous operation modes for Timer B， the MAXQ612 timer/counter block： Auto-reload， capture， and pp/down counting. The figure in the upper right shows the block diagram for the auto-reload mode. The clock input for Timer B can come from a clock prescaler that can divide the system clock with eight different settings， or from an external pin. A control bit， TRB， enables or disables Timer B operation. The timer-value register （TVB） counts up when clocked and when it reaches the value stored in the Timer B reload register （TBR） and interrupt is generated and TVB is reset to zero. This provides a simple method for creating delays without using up valuable CPU cycles to create a delay. An external pin can be used to optionally reset TVB to zero as well， making this mode useful for creating a timeout if an expected input does not show up in the expected amount of time.

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