时钟信号clock signal

In electronics and especially synchronous digital circuits, a clock signal (historically also known as logic beat) is an electronic logic signal (voltage or current) which oscillates between a high and a low state at a constant frequency and is used like a metronome to synchronize actions of digital circuits.
在电子学，尤其是同步数字电路中，时钟信号（历史上也称为逻辑节拍）是一种电子逻辑信号（电压或电流），它以恒定频率在高低状态之间振荡，并像节拍器一样用于同步数字电路的动作。

We can define a clock signal as a particular type of signal that oscillates between a high and a low state. The signal acts like a metronome, which the digital circuit follows in time to coordinate its sequence of actions. Digital circuits rely on clock signals to know when and how to execute the functions that are programmed.
我们可以将时钟信号定义为在高电平和低电平之间振荡的特定类型的信号。信号的作用就像一个节拍器，数字电路按照节拍器的时间来协调其动作顺序。数字电路依靠时钟信号来知道何时以及如何执行被编程的功能。

If the clock in a design is like the heart of an animal, then clock signals are the heartbeats that keep the system in motion.
如果设计中的时钟就像动物的心脏，那么时钟信号就是保持系统运动的心跳。

Crystal Oscillator晶体振荡器

Crystals have a sinusoidal output and are typically used if the target IC has an integrated oscillator and on-chip phase locked loops (PLLs) for internal timing. When a crystal and oscillation circuit are combined in the same package, it is commonly referred to as a crystal oscillator. This quartz piezo-electric oscillator outputs a usable oscillating signal, most commonly a square wave with 50% duty cycle. Usually, this clock signal is fixed at a constant frequency and synchronization may become active at either the rising or falling edge of each clock cycle.

晶体具有正弦波输出，如果目标IC具有集成振荡器和用于内部定时的片上锁相环（PLL），则通常使用晶体。当一个晶体和振荡电路组合在同一封装中时，它通常被称为晶体振荡器。这个石英压电振荡器输出一个可用的振荡信号，最常见的方波与50%的占空比。通常，这个时钟信号固定在一个恒定的频率上，同步可能在每个时钟周期的上升沿或下降沿激活。

How are clock signals produced? 时钟信号是如何产生的？

There are different ways a clock signal can to be produced, but basically they all start off with the crystal resonator. A crystal resonator is commonly referred to as a crystal. In order to operate, crystals are combined with an amplifier circuit to apply voltage to an electrode near or on the crystal.
我们可以通过不同方式产生时钟信号，但基本上都开始于晶体谐振器。晶体谐振器通常被称为晶体。为了工作，晶体与放大器电路相结合，向晶体附近或晶体上的电极施加电压。

The quartz crystal is a tiny slit of quartz with each of the two surfaces metalized and attached with an electrical connection. It’s important the physical size and shape of the quartz crystal are precisely cut because this determines the frequency of oscillations produced from the crystal. Once the crystal is cut and shaped, it cannot be used at any other frequency. Quartz crystals are more commonly used since the frequency generated from quartz crystals are more resistant to changes in temperature. If an internal RC resonator was used instead, changes in temperature would affect the behavior of the oscillator, leading to changes in the output frequency.
石英晶片是石英晶体的一个微薄的片体，其两个表面经由镀银工艺金属化，并通过电极连接。精确切割石英晶体的物理尺寸和形状非常重要，因为这决定了晶体产生的振荡频率。一旦晶体被切割成型，它就不能在任何其他频率下使用。石英晶体之所以更常用，是因为石英晶体产生的频率更能抵抗温度的变化。如果使用内部RC谐振器，温度的变化将影响振荡器的行为，导致输出频率的变化。