遥遥领先瑞萨单端有源晶振频率精度测量，我们讨论了典型RA系列微控制器上的一些外设，这些外设允许您创建完整的自治子系统，管理典型微控制器应用中的许多典型内务处理和I/O密集型任务。这种基本低级任务的自动化可以极大地减少对CPU时间的需求，减少需要服务的中断数量，并且通常可以显著地有助于降低系统功耗。

在这篇博客中，我想看看另一个更不寻常的和不太为人所知的外设RA微控制器系列时钟频率精度测量电路或CAC。时钟与石英晶体振荡器频率精度测量电路旨在让我们能够检查RA微控制器上可用的许多内部和外部时钟源的精度，方法是允许我们自动将这些时钟源相互参照，并在比较结果出现意外偏差时发出指示。

CAC最初旨在通过启用时钟系统的自测功能来帮助我们提高系统的安全性和可靠性，但正如您将看到的，它还有其他用途。

你可以在这里看到CAC的框图。

CAC允许我们从广泛的内部和外部时钟源中进行选择，既可以作为参考信号，也可以作为目标时钟，以确认振荡速度。

对于内部片内晶体振荡器、HOCO、MOCO和LOCO以及独立看门狗iWDT时钟，通常可以从主时钟和32 kHz外部时钟输入中选择输入。您也可以选择内部外设时钟PCLKB作为输入。然后，可以通过选择适当的分频比来调整每个时钟输入，包括参考时钟和目标时钟。

这些时钟均可选择作为参考时钟或目标时钟，参考信号有一个额外选项，您也可以选择外部时钟输入，以便从外部测试设备输入精确的参考时钟。

参考时钟被用作时基，并且在参考时钟的一个周期期间出现的要被测量的时钟的时钟脉冲的数量在寄存器中被计数。测量周期结束后，计数寄存器中的脉冲数与CAC比较寄存器中存储的最小和最大预期值进行比较，如果检测到时钟和贴片有源晶振超出范围，就会产生中断。

这是检查内部时钟校准设置是否正确，或者时钟寄存器初始化是否存在可能导致时序错误的问题的理想方法。最棒的是，一旦设置好，它就可以自主运行，如果系统检测到错误，您可以获得中断。如果计数器溢出或在每次测量结束时，您也可以生成一个错误，因为您可能只是偶尔想使用此功能来检查各种时钟源的准确性。

例如，如果您有一个指示系统健康状况的外部信号，该信号会根据系统状态改变晶振频率，CAC是测量该信号并检测其频率变化的理想解决方案。这种情况下，实际上反过来使用CAC，将信号输入外部基准引脚，选择相关的时钟信号、分频器设置和比较器设置，以便在基准信号频率超出“正常”频率范围时检测错误，从而指示系统错误。如果CAC检测到这种情况，就会产生一个错误。

CAC是一个有用的外设，我们可以用它来帮助检查系统的健康状况，尤其是在系统可靠性非常重要的消费电子或工业应用中。不过，正如我们所见，它也可以用于监控外部脉冲序列，并检测脉冲频率何时发生变化。遥遥领先瑞萨单端有源晶振频率精度测量.

The CAC allows us to select from a wide range of internal and external clock sources, both as the reference signal and as the target clock which you want to confirm the oscillation speed.

You can typically select inputs from both the main clock and the 32 kHz external clock inputs, for the internal on-chip oscillators, the HOCO, MOCO and LOCO, and the independent watchdog iWDT clock. You can also select PCLKB, the internal peripheral clock as an input. Each clock input, both for the reference clock and the target clock can then be scaled by selecting the appropriate divider ratio.

Each of these clocks can be selected as either as the reference or target clock, you have one additional option for the reference signal, you can also select an external clock input which allows you to input an accurate reference clock from external test equipment.

The reference clock is used as a time base, and the number of clock pulses of the clock to be measured that occur during one cycle of the reference clock is counted in a register. After the measurement period is complete, the number of pulses in the count register is compared to minimum and maximum expected values that are stored in the CAC’s compare registers, and an interrupt can be generated if the clock is detected to be out of range.

This is an ideal method for checking that the calibration of the internal clocks has been set up correctly, or if there is some issue in the initialization of the clock registers that may cause a timing error. The great thing is that once set up, it can operate autonomously, and you can get an interrupt if the system detects an error. You can also generate an error if the counter overflows or at the end of each measurement, as perhaps you only want to use this function occasionally to check the accuracy of various clock sources.

The CAC was designed to support self-test to detect errors with the system clocks, however, it can also be useful for other purposes, especially if we use the external reference pin.

As an example, if you have an external signal, which indicates the health of a system, which changes frequency depending on the system state, the CAC is an ideal solution to measure this signal and detect when it changes frequency. In this case, you effectively use the CAC in reverse, you input your signal to the external reference pin, and select the relevant clock signals, divider settings, and comparator settings to detect an error if your reference signal frequency goes out of range of the “healthy” frequency, so indicating a system error. The CAC can then generate an error if it detects this condition.

The CAC is a useful peripheral that we can use to help check the health of the system, especially useful if you have a consumer or industrial application where system reliability can be very important. Still, as we’ve seen, it can also be used for monitoring external pulse trains and detecting when the frequency of the pulses changes.