## Capacitor Usage

So far, we already encountered capacitors for many different usages:

### Load Capacitors

We have seen load capacitors used with the 2 crystals in the
discussion about [CPU][1].

A quartz crystal always provides both series and parallel resonance,
the series resonance being a few kilohertz lower than the parallel
one.

Crystals below 30 MHz like ours are generally operated between series
and parallel resonance, which means that the crystal appears as an
inductive reactance in operation, this inductance forming a
**parallel resonant circuit** with externally connected parallel
"load" capacitance. Any small additional capacitance added in parallel
with the crystal pulls the frequency lower in the range between the
series and parallel resonance frequencies, insuring crystal startup
and stable operation.

For modern circuits, these load capacitors have a typical small value
< 20 pF.

### Bulk Capacitors

Bulk capacitors are used to prevent a power supply from dropping too
far during the periods when current is not available. At the same
time, they help to reduce the power supply voltage ripples by
smoothing their output voltage.

Many such capacitors are used at both the input and output of the
numerous linear and switched mode power supplies in the [PMIC
discussion][2].

The main bulk capacitor value is generally high (some µF), but there
may be smaller parallel capacitors added for stability.

### Coupling Capacitors

As you probably know, capacitors are made of 2 parallel conductive
electrodes separated by a (thin) isolating dielectric material (even
if these electrodes are rolled or layered to reduce the component
size). Thus by construction, no DC (Direct Current) can flow from one
electrode to the other, but by influence using the electric field, AC
(Alternative Current) still can go through. This is how coupling
capacitors are used to link 2 circuits while removing any DC bias
voltage on one side or the other of the capacitor.

We use such a coupling capacitor in the [Audio schematic
description][3] for feeding the audio power amplifier from the CPU
audio output.

### Filter Capacitors

We have seen many examples where capacitors are used within passive
filter circuits along with resistors or inductors, mainly to remove
unwanted frequencies from a power supply or a signal.

### Decoupling (Bypass) Capacitors

We use some decoupling capacitors in the [buttons circuit][4].

Active components such as transistors and chips are connected to their
power supplies through conductors featuring a (small) common impedance
made up of complex (resistive, capacitive and inductive)
value. Because of these parasitic components, a device that suddenly
draws some current in spikes will generate a drop in its voltage power
supply. If many devices are sharing the same power supply and
impedance, the state of one device will be coupled to the other ones
through the common impedance of the power supply conductors and may
affect their operation.

In order to decouple the devices, capacitors placed as close as
possible to the device power supply input pins are used, which act as
local energy storage. These capacitors are also named "bypass
capacitors" as they shunt transient energy from the power supplies
past the device to be decoupled, right to the GND return path.

There may be different capacitors values placed on the same power
supply pins in order to filter transients at different frequencies:
the bigger the capacitor value, the lower the frequency. A typical
value is 100 nF, and values from 1 µF to 10 µF are used for lower
frequencies and / or higher current draws, while lower values of a few
nF are used for filtering higher frequencies.

In essence, decoupling capacitors are not very different in their
function from bulk capacitors: the only difference is one of scale,
both of current and of transient duration. Bulk capacitors deal with
large currents and periods of 10s of ms, whereas decoupling capacitors
are used for much lower currents and much briefer periods (typically
10s of ns for TTL or CMOS devices) .

## Schematics

The last part of the FunKey schematics merely contains only decoupling
capacitors:

![Decoupling Schematics](/assets/images/Decoupling_Schematics.png){: .lightbox}

One exception is the Allwinner V3s CPU HPR/HPL circuit which features
an RC-to-ground circuit between the amplifier and the preamplifier
input with the resistor **R27** and capacitors **C79** and **C81**, as
recommended in the [V3s hardware design guide][5].

The only other remarkable point left in this schematic is the resistor
divider **R25**/**R28** which provides a reference voltage at half the
DRAM power supply voltage level, which is used for the integrated DDR2
DRAM merged drivers and dynamic on-chip termination already discussed
at the end of the previous [CPU schematic description][6].

[1]: /developers/hardware/cpu
[2]: /developers/hardware/power/pmic
[3]: /developers/hardware/audio
[4]: /developers/hardware/buttons
[5]: https://github.com/Squonk42/V3s_Documentation/blob/master/V3s%20hardware%20design%20guide%20V1.0_20150519%20EN%20Non%20Official.pdf
[6]: /developers/hardware/cpu

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