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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="FunKey S - the world's smallest foldable handheld console Ultra-compact, foldable, powerful and comfortable, the FunKey S is the perfect handheld for playing retrogames anywhere, anytime."><meta name=author content="Michel Stempin"><link href=https://doc.funkey-project.com/developer_guide/hardware_reference/uart/ rel=canonical><link href=../usb/ rel=prev><link href=../magnetic_switch/ rel=next><link rel=icon href=../../../assets/favicon.png><meta name=generator content="mkdocs-1.6.0, mkdocs-material-9.5.21"><title>UART - FunKey Project Documentation</title><link rel=stylesheet href=../../../assets/stylesheets/main.66ac8b77.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet 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aria-labelledby=__nav_2_label aria-expanded=false> <label class=md-nav__title for=__nav_2> <span class="md-nav__icon md-icon"></span> User's Manual </label> <ul class=md-nav__list data-md-scrollfix> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_2_1> <label class=md-nav__link for=__nav_2_1 id=__nav_2_1_label tabindex=0> <span class=md-ellipsis> Tutorials </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_2_1_label aria-expanded=false> <label class=md-nav__title for=__nav_2_1> <span class="md-nav__icon md-icon"></span> Tutorials </label> <ul class=md-nav__list data-md-scrollfix> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_2_1_1> <label class=md-nav__link for=__nav_2_1_1 id=__nav_2_1_1_label tabindex=0> <span class=md-ellipsis> Software </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=3 aria-labelledby=__nav_2_1_1_label aria-expanded=false> <label class=md-nav__title for=__nav_2_1_1> <span class="md-nav__icon md-icon"></span> Software </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/add_roms/ class=md-nav__link> <span class=md-ellipsis> Add New ROMs </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/add_opk/ class=md-nav__link> <span class=md-ellipsis> Add OPK Files </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/gba_bios/ class=md-nav__link> <span class=md-ellipsis> Install the GBA/PS1 BIOS </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/aspect_ratio/ class=md-nav__link> <span class=md-ellipsis> Change Aspect Ratio </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/snapshot/ class=md-nav__link> <span class=md-ellipsis> Take a Snapshot </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/firmware_update/ class=md-nav__link> <span class=md-ellipsis> Firmware Upgrade </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/gb_palette/ class=md-nav__link> <span class=md-ellipsis> Change Game Boy Palette </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/software/gmenu2x_wallpaper/ class=md-nav__link> <span class=md-ellipsis> Change GMenu2X Wallpaper </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_2_1_2> <label class=md-nav__link for=__nav_2_1_2 id=__nav_2_1_2_label tabindex=0> <span class=md-ellipsis> Hardware </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=3 aria-labelledby=__nav_2_1_2_label aria-expanded=false> <label class=md-nav__title for=__nav_2_1_2> <span class="md-nav__icon md-icon"></span> Hardware </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../user_manual/tutorials/hardware/teardown/ class=md-nav__link> <span class=md-ellipsis> Teardown </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/hardware/flash_change_sd_card/ class=md-nav__link> <span class=md-ellipsis> Flash & Change micro-SD Card </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/hardware/change_buttons/ class=md-nav__link> <span class=md-ellipsis> Change the Buttons </span> </a> </li> <li class=md-nav__item> <a href=../../../user_manual/tutorials/hardware/fix_unstuck_screen/ class=md-nav__link> <span class=md-ellipsis> </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--active md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3 checked> <label class=md-nav__link for=__nav_3 id=__nav_3_label tabindex=0> <span class=md-ellipsis> Developer's Guide </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_3_label aria-expanded=true> <label class=md-nav__title for=__nav_3> <span class="md-nav__icon md-icon"></span> Developer's Guide </label> <ul class=md-nav__list data-md-scrollfix> <li class="md-nav__item md-nav__item--active md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3_1 checked> <div class="md-nav__link md-nav__container"> <a href=../ class="md-nav__link "> <span class=md-ellipsis> Hardware Reference </span> </a> <label class="md-nav__link " for=__nav_3_1 id=__nav_3_1_label tabindex=0> <span class="md-nav__icon md-icon"></span> </label> </div> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_3_1_label aria-expanded=true> <label class=md-nav__title for=__nav_3_1> <span class="md-nav__icon md-icon"></span> Hardware Reference </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../architecture/ class=md-nav__link> <span class=md-ellipsis> Architecture </span> </a> </li> <li class=md-nav__item> <a href=https://htmlpreview.github.io/?https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/BOM/ibom.html class=md-nav__link> <span class=md-ellipsis> BOM </span> </a> </li> <li class=md-nav__item> <a href=../cpu/ class=md-nav__link> <span class=md-ellipsis> CPU </span> </a> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3_1_4> <label class=md-nav__link for=__nav_3_1_4 id=__nav_3_1_4_label tabindex=0> <span class=md-ellipsis> Power </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=3 aria-labelledby=__nav_3_1_4_label aria-expanded=false> <label class=md-nav__title for=__nav_3_1_4> <span class="md-nav__icon md-icon"></span> Power </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../power/power_supplies_topologies/ class=md-nav__link> <span class=md-ellipsis> Regulated DC Power Supply Topologies </span> </a> </li> <li class=md-nav__item> <a href=../power/multiple_voltages/ class=md-nav__link> <span class=md-ellipsis> Multiple Power Supply Voltages </span> </a> </li> <li class=md-nav__item> <a href=../power/pmic/ class=md-nav__link> <span class=md-ellipsis> PMIC </span> </a> </li> <li class=md-nav__item> <a href=../power/dram_power/ class=md-nav__link> <span class=md-ellipsis> DRAM Power </span> </a> </li> <li class=md-nav__item> <a href=../power/decoupling/ class=md-nav__link> <span class=md-ellipsis> Decoupling </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=../screen/ class=md-nav__link> <span class=md-ellipsis> Screen </span> </a> </li> <li class=md-nav__item> <a href=../audio/ class=md-nav__link> <span class=md-ellipsis> Audio </span> </a> </li> <li class=md-nav__item> <a href=../buttons/ class=md-nav__link> <span class=md-ellipsis> Buttons </span> </a> </li> <li class=md-nav__item> <a href=../sd-card/ class=md-nav__link> <span class=md-ellipsis> SD Card </span> </a> </li> <li class=md-nav__item> <a href=../usb/ class=md-nav__link> <span class=md-ellipsis> USB </span> </a> </li> <li class="md-nav__item md-nav__item--active"> <input class="md-nav__toggle md-toggle" type=checkbox id=__toc> <a href=./ class="md-nav__link md-nav__link--active"> <span class=md-ellipsis> UART </span> </a> </li> <li class=md-nav__item> <a href=../magnetic_switch/ class=md-nav__link> <span class=md-ellipsis> Magnetic Switch </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3_2> <div class="md-nav__link md-nav__container"> <a href=../../software_reference/ class="md-nav__link "> <span class=md-ellipsis> Software Reference </span> </a> <label class="md-nav__link " for=__nav_3_2 id=__nav_3_2_label tabindex=0> <span class="md-nav__icon md-icon"></span> </label> </div> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_3_2_label aria-expanded=false> <label class=md-nav__title for=__nav_3_2> <span class="md-nav__icon md-icon"></span> Software Reference </label> <ul class=md-nav__list data-md-scrollfix> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3_2_1> <div class="md-nav__link md-nav__container"> <a href=../../software_reference/boot_process/ class="md-nav__link "> <span class=md-ellipsis> Boot Process </span> </a> <label class="md-nav__link " for=__nav_3_2_1 id=__nav_3_2_1_label tabindex=0> <span class="md-nav__icon md-icon"></span> </label> </div> <nav class=md-nav data-md-level=3 aria-labelledby=__nav_3_2_1_label aria-expanded=false> <label class=md-nav__title for=__nav_3_2_1> <span class="md-nav__icon md-icon"></span> Boot Process </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../software_reference/boot_process/boot_rom/ class=md-nav__link> <span class=md-ellipsis> Boot ROM </span> </a> </li> <li class=md-nav__item> <a href=../../software_reference/boot_process/spl/ class=md-nav__link> <span class=md-ellipsis> SPL </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3_2_2> <div class="md-nav__link md-nav__container"> <a href=../../software_reference/sd_card_layout/ class="md-nav__link "> <span class=md-ellipsis> SD-Card Layout </span> </a> <label class="md-nav__link " for=__nav_3_2_2 id=__nav_3_2_2_label tabindex=0> <span class="md-nav__icon md-icon"></span> </label> </div> <nav class=md-nav data-md-level=3 aria-labelledby=__nav_3_2_2_label aria-expanded=false> <label class=md-nav__title for=__nav_3_2_2> <span class="md-nav__icon md-icon"></span> SD-Card Layout </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../software_reference/sd_card_layout/directory_structure/ class=md-nav__link> <span class=md-ellipsis> Directory Structure </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3_3> <label class=md-nav__link for=__nav_3_3 id=__nav_3_3_label tabindex=0> <span class=md-ellipsis> Tutorials </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_3_3_label aria-expanded=false> <label class=md-nav__title for=__nav_3_3> <span class="md-nav__icon md-icon"></span> Tutorials </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../tutorials/access_recovery/ class=md-nav__link> <span class=md-ellipsis> Access the Recovery Boot Mode </span> </a> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3_3_2> <div class="md-nav__link md-nav__container"> <a href=../../tutorials/build_system/ class="md-nav__link "> <span class=md-ellipsis> Build System </span> </a> <label class="md-nav__link " for=__nav_3_3_2 id=__nav_3_3_2_label tabindex=0> <span class="md-nav__icon md-icon"></span> </label> </div> <nav class=md-nav data-md-level=3 aria-labelledby=__nav_3_3_2_label aria-expanded=false> <label class=md-nav__title for=__nav_3_3_2> <span class="md-nav__icon md-icon"></span> Build System </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../tutorials/build_system/compilation_environments/ class=md-nav__link> <span class=md-ellipsis> Compilation Environments </span> </a> </li> <li class=md-nav__item> <a href=../../tutorials/build_system/get_sources/ class=md-nav__link> <span class=md-ellipsis> Get the Sources </span> </a> </li> <li class=md-nav__item> <a href=../../tutorials/build_system/compile_sdk/ class=md-nav__link> <span class=md-ellipsis> Compile SDK </span> </a> </li> <li class=md-nav__item> <a href=../../tutorials/build_system/compile_distribution/ class=md-nav__link> <span class=md-ellipsis> Compile Distribution </span> </a> </li> <li class=md-nav__item> <a href=../../tutorials/build_system/write_image/ class=md-nav__link> <span class=md-ellipsis> Write firmware to SD Card </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=../../tutorials/build_system/build_program_using_sdk/ class=md-nav__link> <span class=md-ellipsis> Build Programs using SDK </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=https://grabcad.com/library/funkey-s-1 class=md-nav__link> <span class=md-ellipsis> 3D files </span> </a> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_5> <label class=md-nav__link for=__nav_5 id=__nav_5_label tabindex=0> <span class=md-ellipsis> Miscellaneous </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_5_label aria-expanded=false> <label class=md-nav__title for=__nav_5> <span class="md-nav__icon md-icon"></span> Miscellaneous </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../miscellaneous/glossary/ class=md-nav__link> <span class=md-ellipsis> Glossary </span> </a> </li> </ul> </nav> </li> </ul> </nav> </div> </div> </div> <div class="md-sidebar md-sidebar--secondary" data-md-component=sidebar data-md-type=toc> <div class=md-sidebar__scrollwrap> <div class=md-sidebar__inner> <nav class="md-nav md-nav--secondary" aria-label="Table of contents"> </nav> </div> </div> </div> <div class=md-content data-md-component=content> <article class="md-content__inner md-typeset"> <h1>UART</h1> <p>The Allwinner V3s provides 3x <abbr title="Universal Asynchronous Receiver/Transmitters">UARTs</abbr> (Universal Asynchronous Receivers / Transmitters): UART0 with only RX and TX signals, and UART1 and UART2 with additional RTS and CTS hardware flow control signals.</p> <p>Like most <abbr title="System on Chips">SoCs</abbr>, 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 <abbr title="Universal Asynchronous Receiver/Transmitter">UART</abbr>.</p> <p>The Console schematic only requires a minimum of external components:</p> <p><img alt="Console Schematics" class=lightbox src=/assets/images/UART_Schematics.png></p> <p>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 <strong>R3</strong>.</p> <p>What is the purpose of this resistor?</p> <ul> <li> <p>As explained previously for the <abbr title="Secure Digital">SD</abbr> Card clock signal, this may be to prevent ringing. But given the relatively slow signal speed (115200 bps), it is not the case here</p> </li> <li> <p>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 <abbr title="System on Chip">SoC</abbr> can tolerate input voltages up to +3.6V on its pins, we should be safe!</p> </li> <li> <p>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!</p> </li> </ul> <div class="admonition warning"> <p class=admonition-title>Warning</p> <p>There is no <abbr title="Electro-Static Discharge">ESD</abbr> protection <abbr title="Transcient Voltage Suppressor">TVS</abbr> diodes on the <abbr title="Universal Asynchronous Receiver/Transmitter">UART</abbr>: this interface is not supposed to be mounted in the final user device, and <abbr title="Printed Circuit Board">PCB</abbr> space is really constrained in this area, so they are omitted.</p> </div> </article> </div> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> © 2020-2024 FunKey Project<br> Last updated on 2024-05-04 by DrUm78<br> <a href=http://creativecommons.org/licenses/by-sa/4.0/ rel=license> <img alt="Creative Commons License" style=border-width:0 src=https://i.creativecommons.org/l/by-sa/4.0/80x15.png> </a><br>This work is licensed under a <a href=http://creativecommons.org/licenses/by-sa/4.0/ rel=license>Creative Commons Attribution-ShareAlike 4.0 International License</a>. </div> Made with <a href=https://squidfunk.github.io/mkdocs-material/ target=_blank rel=noopener> Material for MkDocs </a> </div> <div class=md-social> <a href=https://www.facebook.com/FunKeyProjectOfficial target=_blank rel=noopener title=www.facebook.com class=md-social__link> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 6.5.2 by @fontawesome - 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