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docs/developers/hardware_ref/power/decoupling.md

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+## 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)
+
+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
+
+--8<--
+includes/glossary.md
+--8<--