Understanding the 100nF Capacitors

I created these notes to help better understand the role of the components we are soldering on the MAKERbuino. They also contribute towards an understanding of the schematic. Please feel free to suggest changes.

The 100nF Capacitors

The First 100nF Capacitor (C1)

The first 100 nanofarad capacitor, labelled C1, sits between VCC and ground, right where VCC connects to the microcontroller’s power pins. We should immediately recognise this as a decoupling capacitor like the 100 microfarad one we already soldered for the LCD decoupling. The clue is its close proximity to the power pins of a power-hungry component or one that is sensitive to noise in the power supply - the microcontroller is both. The role of this capacitor is to smooth out power going into the microcontroller (it will absorb some of the spikes and supplement in for some of the dips in power).

The Second 100nF Capacitor (C6)

The second 100 nanofarad capacitor, labelled C6, sits directly in between the reset line and one of the pins of the UART interface (JP2). This pin provides the external UART terminal with the ability to reset the microcontroller (discussed in the notes for that component). To reset the microcontroller, this pin must be brought to a low voltage for at least 2.5 microseconds and then released back to high voltage. Now the problem is that a UART terminal will typically hold that pin low (as it’s actually the DTR control pin). This would keep the microcontroller in a reset state. So to turn this long low signal into a short low pulse, we use a capacitor. This works because a capacitor only allows current to flow ‘through’ it for as long as it is charging. When it is fully charged, no more current will flow.

In the current circuit, this will divert power coming through the 10K pull-up resistor into the DTR pin for a moment, which will cause the RESET pin on the microcontroller to experience a momentary low voltage.

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That’s correct, well done sir.

Decoupling capacitors are surprisingly easy to master, but people tend to make a fuss out of them.

There’s a neat text about it here:

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