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59 lines
2.7 KiB
Markdown
59 lines
2.7 KiB
Markdown
In the **FunKey S** device, the USB interface has 2 functions:
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- provide an external power supply source for both powering the
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device and charging the built-in LiPo battery
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- provide a data interface to transfer firmware upgrades,
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configuration files, game emulators and game ROMs
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The first function only requires the +5V USB power and GND pins. The
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second function requires to wire the additional differential data
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lines D+ and D-. As we only need to operate as an USB device and
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although the V3s is able to work as either an USB host or USB device
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using the USB OTG protocol, we don't need the ID pin to determine by
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the cable wiring which role we must take.
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{: align=left }
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The main part is of course the [Micro USB edge-mounted
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connector:fontawesome-solid-external-link-alt:][1] **J2**, which we
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chose in order to "mask out" its already low-profile height into the
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PCB thickness. And with its "harpoon-like" through-hole legs, it
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should avoid tearing it off the board if the user don't pull the chord
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straight.
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The USB schematic is the following:
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{.lightbox}
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Before connecting 2 devices using an USB cable, they may be at
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completely different absolute voltages, and during cable insertion,
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the shield will be in contact before the other pins, including
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GND. The **C6** capacitor between the Protective Earth (Shield) and
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GND is here to provide an AC path for sinking this difference in
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voltage and align the GND levels when plugin the cable.
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The resistor **R4** on the USB ID connector pin should probably not be
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mounted: as we act only as an USB device, this pin should be left
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floating.
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The capacitors **C7**, **C8**, **C10** and ferrite bead **L2** form a
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constant-k 3 pole CLC low pass filter to remove any spurious in/out on
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the USB power supply wire. The USB 2.0 specification limits the
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maximum bulk capacitance value to 10 µF in order to avoid power supply
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excessive droops when plugin in a device with a discharged large bulk
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capacitor.
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**D15** is a [NXP
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PRTR5V0U2X,215:fontawesome-solid-external-link-alt:][2] combined TVS
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protection diode for the VBUS pin and a set of clamping diodes that
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will limit the voltage on D+ and D- pins to stay between GND and VBUS
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levels to in order to protect the V3s USB driver from under /
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over-voltages.
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[1]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C40958_MICRO5P%E6%B2%89%E6%9D%BF%E5%BC%8F0.8%E5%9B%9B%E8%84%9A%E5%85%A8%E6%8F%92%E6%97%A0%E5%AF%BC%E4%BD%8D%E6%9C%89%E5%AD%94%E8%80%90%E9%AB%98%E6%B8%A9_2016-05-20.PDF
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[2]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C12333_PRTR5V0U2X%2C215_2017-10-31.PDF
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--8<--
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includes/glossary.md
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--8<--
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