Yet Another Way to Verify the Clock Source on a Dev Board

In a previous post, we showed two reliable ways to determine the frequency of the clock source onboard a microcontroller development board (Dev. Board). These two methods were:

• Visual magnification of the physical component in order to read the engraved or printed markings, and
• Changing the microcontroller fuses to output the clock signal on a digital pin and measuring it with an oscilloscope.

The first method is by far the easiest and fastest way to obtain results. However, it does not help us achieve our goal if the component is counterfeit or falsely labeled. Unfortunately, this is a common issue with many electronic components.

The second method provides results with high confidence, but it also carries inherent risks. First, the fuses must be set correctly so that the external clock frequency can be determined. For example, the ATmega328P includes an internal clock source, so the user must ensure that the correct fuse is changed to output the external clock rather than the internal one, which is often the default.

Second, there are fuse settings that can effectively “brick” a microcontroller. These settings can prevent further changes or even block uploading sketches. Although this is not always permanent—since the fuses can be reset with a process called High Voltage Parallel Programming (HVPP)—it does require a special programmer and 12 V on the RESET pin.

Given the above, we wanted to demonstrate yet another method to determine the external clock frequency. Two main pieces of equipment are needed:

• An oscilloscope with an FFT math function, and
• Near-field probes

The FFT math function on an oscilloscope is a powerful tool. Given any waveform, the FFT can break it down into its corresponding frequency components. In our context, this allows us to quickly see any peak frequency near or around the expected value. In this tutorial, we are using the Nano Flip and probing the onboard external crystal with a 16 MHz frequency. Therefore, we should expect to see a peak decoded by the FFT near that frequency.

As for the near-field probes, also known as sniffer probes, high accuracy is not required in our case. These probes are often called “sniffer” probes because they provide an indication of which parts on a PCB (trace or component) are emitting EM noise. In fact, one can easily make a probe by coiling wire and connecting it to the scope. Alternatively, inexpensive probe kits are widely available.

For our application in this tutorial, accuracy is not critical. We already know which component we are measuring (the crystal) and the expected frequency range. This allows us to use only the large probe and set the FFT horizontal scale around 16 MHz.

In other contexts, these probes are often used the opposite way—for pre-compliance FCC/ISED testing. Typically, one would start with a large probe and a wide frequency range to identify potential compliance issues. Then, smaller probes and a narrower frequency range are used to locate the exact source. Larger probes are more sensitive to EM radiation overall, while smaller probes provide better localization on specific components or traces. However, for our purpose, the large probe is sufficient.

The below image shows the large near-field probe held above the Nano Flip, centred around the external crystal. It can easily be seen that since the probe is as larger as the entire Dev Board, that had we not known to target the crystal in particular, we’d be clueless which component is emitting a 16 MHz frequency. We would have to use smaller loops to narrow down the source.

Finally, let’s look at the results. The FFT on the oscilloscope clearly shows a strong peak at 16 MHz, as seen below.

This confirms that the onboard crystal is indeed 16 MHz and not any other frequency. Using near-field probes allows us to quickly and easily locate sources of EM radiation. In this case, our goal was simply to confirm the clock frequency we already expected, but these probes are also very useful for pre-compliance testing. As a bonus, we can change the settings of the horizontal axis (scale and offset) so to see the16 MHz fundamental clock frequency, but also the first harmonic at 32 MHz. Often both of these will be easily spotted with near field probes.

There you have it—yet another effective way to verify the clock source on a development board.