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