Need Help With the Schematic

Hi guys,

As I mentioned in a previous post, I am a high school teacher trying to do an “EDU” version of the hardware for student soldering.

After studying some schematics on the internet, the following questions came up:

  1. I looked at photos of the official FX hardware, it appears that they used a 3-pin ceramic resonator in place of a crystal. What is the justification for this? Surely any crystal would be much better than the ceramic they used…

  2. Also on the official hardware, I find that there is no voltage regulation. This is very naughty right? Because a number of things will be operating out of spec. The MCU is rated at 8MHz maximum under 2.7V. Also the external SPI flash and the OLED will be subject to extreme voltages. It’s a bit worrisome for me in particular, because I might wanna use cheaper CH1116-based OLEDs and flash from different vendors.

  3. I am under the impression that the piezo element is very-high-impedance under DC and therefore the 4.7uF isolation capacitor is not really necessary? Also has anyone tried using electromagnetic buzzers or just plain speakers, and how do those compare to piezo in terms of sound quality?

  4. The “MAKE LOUD” solder bridge in the original production schematic, how does that work to increase loudness? The schematic is a bit unclear and I am quite confused.

  5. Is there a system-wide way in firmware to disable the buzzer (i.e. mute) or do I need to add a mute switch if mute functionality is desired?

  6. The two “user LEDs”, LED1 and LED2, what purpose do they serve? I’m trying to determine the optimal placement for these LEDs. On the official board they are placed near the USB connector. I watched some review videos on YouTube but didn’t see any of them light up, so still not sure what these 2 LEDs do.

  7. What are some scenarios in which the on-board reset button needs to be operated by the user, seeing that Arduboy has a hard-wired-in power switch already?

Thanks I would appreciate your help!

Great notes some have been addressed before some are new!

  1. Resonator is better in an environment with vibration and shock. Also removes 2 capacitors, as they are integrated, saving board space.

  2. “Very” is relative. It’s slightly out of spec, as noticed many times before, but the voltage to mhz is on a curve and we’re actually in spec for about the first half of the battery voltage. We’re effectively overclocking the chip a few mhz over spec. I asked the folks at Atmel about this and they can officially only support the datasheet but over drinks they said it’s just fine.

The chips seem to run just fine 16mhz at 3v3 even.

The OLED actually is supposed to be getting a full 5v for it’s rail so it’s being “overclocked” as well.

Practically none of this matters, I’ve produced over 30,000 units like this without a problem.

  1. Correct isolation cap is unnecessary has been since changed to a resistor actually. You can use a speaker or buzzer they work but in some applications that use differential signaling they will not be a sharp as the piezo creates it’s mechanical forces on the edge transition not the peak.

  2. Make loud attaches to the second ceramic electrode on the piezo, it barely makes a noticeable difference so it’s omitted.

  3. There is no way to mute you’ll have to add a switch.

  4. the user leds are RX and TX activity lights for the serial over usb connection

  5. If the device becomes unrecoverable, i.e. software auto reset is not working the reset button will force the hardware into bootloader mode. So it’s basically only there if you brick it which is to some degree theoretically impossible with the FX because the menu it loads into is actually the bootloader, which is write protected.


Huge thanks!

If you’re trying to teach your students proper electronics design, using datasheet absolute maximum ratings and “max/min” calculations, then the Arduboy is not a good example.

The Arduboy used a two element piezoelectric disc. You will likely use a standard single sided disc, so “MAKE LOUD” is not relevant.

Yes. For any sketches that use the common sound libraries or follow the Arduboy2 library guidelines (which is the vast majority), mute is controlled by a flag in system EEPROM that can be set using a button sequence during power up. Some sketches implement a menu selection for this, as well.


Thanks for the additional explanation. I had not known that 3-electrode piezos could be used this way. They are usually in a self-resonant configuration to achieve super loud sound, say, in a residential smoke alarm. Not sure if connecting up the extra electrode was the intended use case. I guess it didn’t turn out to be effective, but it was an innovative idea.

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