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docs/developers/hardware_ref/uart.md

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+The Allwinner V3s provides 3x UARTs (Universal Asynchronous Receivers
+/ Transmitters): UART0 with only RX and TX signals, and UART1 and
+UART2 with additional RTS and CTS hardware flow control signals.
+
+Like most SoCs, the Allwinner V3s provides a serial console as a
+control terminal for debug and/or normal operation. By default, it is
+mapped to UART0, and it is used by the BROM (Boot Rom), the U-Boot
+bootloader and by the Linux kernel to output messages during the boot
+process, and later by the Linux kernel to log messages during normal
+operation. Depending on the configuration, it can be used too for
+loging into the system over an UART.
+
+The Console schematic only requires a minimum of external components:
+
+![Console Schematics](/assets/images/UART_Schematics.png)
+
+Besides the 3-pin 1.27 mm (0.05") pitch header J1 that will not be
+mounted on standard products, there is only a single series resistor
+**R3**.
+
+What is the purpose of this resistor?
+
+ - As explained previously for the SD Card clock signal, this may be
+   to prevent ringing. But given the relatively slow signal speed
+   (115200 bps), it is not the case here
+
+ - If it were placed on the RX input signal, this could prevent frying
+   the input pin if a large voltage (+5V, for example) is applied to
+   it by dissipating the excessive voltage as heat in the resistor. It
+   is not the case here, as the resistor is placed on the TX output
+   signal, but we could have added one, if only we had some space left
+   on the board... But as the V3s SoC can tolerate input voltages up
+   to +3.6V on its pins, we should be safe!
+
+ - In fact, the resistor is on the output TX signal to prevent
+   short-circuits if the serial cable is reversed and the 2 TX outputs
+   are connected together, one driving the signal low, while the other
+   is driving it high: again in this case, the voltage difference
+   between the 2 outputs will be burned as heat in the resistor,
+   saving the internal output buffers!
+
+!!! Warning
+
+    There is no ESD protection TVS diodes on the UART: this interface
+    is not supposed to be mounted in the final user device, and PCB
+    space is really constrained in this area, so they are omitted.
+
+--8<--
+includes/glossary.md
+--8<--