path: root/static/freebsd/man4/uart.4 3.html

<table class="head">
  <tr>
    <td class="head-ltitle">UART(4)</td>
    <td class="head-vol">Device Drivers Manual</td>
    <td class="head-rtitle">UART(4)</td>
  </tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">uart</code> &#x2014; <span class="Nd">Universal
    Asynchronous Receiver/Transmitter serial driver</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="Cd">device uart</code></p>
<p class="Pp">
  <br/>
  <code class="Cd">device puc</code>
  <br/>
  <code class="Cd">device uart</code></p>
<p class="Pp">
  <br/>
  <code class="Cd">device scc</code>
  <br/>
  <code class="Cd">device uart</code></p>
<p class="Pp">In <span class="Pa">/boot/device.hints</span>:
  <br/>
  <code class="Cd">hint.uart.0.disabled=&quot;1&quot;</code>
  <br/>
  <code class="Cd">hint.uart.0.baud=&quot;38400&quot;</code>
  <br/>
  <code class="Cd">hint.uart.0.port=&quot;0x3f8&quot;</code>
  <br/>
  <code class="Cd">hint.uart.0.flags=&quot;0x10&quot;</code></p>
<p class="Pp">With <var class="Ar">flags</var> encoded as:</p>
<dl class="Bl-tag Bl-compact">
  <dt>0x00010</dt>
  <dd>device is potential system console</dd>
  <dt>0x00080</dt>
  <dd>use this port for remote kernel debugging</dd>
  <dt>0x00100</dt>
  <dd>set RX FIFO trigger level to ``low'' (NS8250 only)</dd>
  <dt>0x00200</dt>
  <dd>set RX FIFO trigger level to ``medium low'' (NS8250 only)</dd>
  <dt>0x00400</dt>
  <dd>set RX FIFO trigger level to ``medium high'' (default, NS8250 only)</dd>
  <dt>0x00800</dt>
  <dd>set RX FIFO trigger level to ``high'' (NS8250 only)</dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The <code class="Nm">uart</code> device driver provides support
    for various classes of UARTs implementing the EIA RS-232C (CCITT V.24)
    serial communications interface. Each such interface is controlled by a
    separate and independent instance of the <code class="Nm">uart</code>
    driver. The primary support for devices that contain multiple serial
    interfaces or that contain other functionality besides one or more serial
    interfaces is provided by the <a class="Xr">puc(4)</a>, or
    <a class="Xr">scc(4)</a> device drivers. However, the serial interfaces of
    those devices that are managed by the <a class="Xr">puc(4)</a>, or
    <a class="Xr">scc(4)</a> driver are each independently controlled by the
    <code class="Nm">uart</code> driver. As such, the <a class="Xr">puc(4)</a>,
    or <a class="Xr">scc(4)</a> driver provides umbrella functionality for the
    <code class="Nm">uart</code> driver and hides the complexities that are
    inherent when elementary components are packaged together.</p>
<p class="Pp">The <code class="Nm">uart</code> driver has a modular design to
    allow it to be used on differing hardware and for various purposes. In the
    following sections the components are discussed in detail. Options are
    described in the section that covers the component to which each option
    applies.</p>
<section class="Ss">
<h2 class="Ss" id="CORE_COMPONENT"><a class="permalink" href="#CORE_COMPONENT">CORE
  COMPONENT</a></h2>
<p class="Pp">At the heart of the <code class="Nm">uart</code> driver is the
    core component. It contains the bus attachments and the low-level interrupt
    handler.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="HARDWARE_DRIVERS"><a class="permalink" href="#HARDWARE_DRIVERS">HARDWARE
  DRIVERS</a></h2>
<p class="Pp">The core component and the kernel interfaces talk to the hardware
    through the hardware interface. This interface serves as an abstraction of
    the hardware and allows varying UARTs to be used for serial
  communications.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="SYSTEM_DEVICES"><a class="permalink" href="#SYSTEM_DEVICES">SYSTEM
  DEVICES</a></h2>
<p class="Pp">System devices are UARTs that have a special purpose by way of
    hardware design or software setup. For example, Sun UltraSparc machines use
    UARTs as their keyboard interface. Such an UART cannot be used for general
    purpose communications. Likewise, when the kernel is configured for a serial
    console, the corresponding UART will in turn be a system device so that the
    kernel can output boot messages early on in the boot process.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="KERNEL_INTERFACES"><a class="permalink" href="#KERNEL_INTERFACES">KERNEL
  INTERFACES</a></h2>
<p class="Pp">The last but not least of the components is the kernel interface.
    This component ultimately determines how the UART is made visible to the
    kernel in particular and to users in general. The default kernel interface
    is the TTY interface. This allows the UART to be used for terminals, modems
    and serial line IP applications. System devices, with the notable exception
    of serial consoles, generally have specialized kernel interfaces.</p>
</section>
</section>
<section class="Sh">
<h1 class="Sh" id="HARDWARE"><a class="permalink" href="#HARDWARE">HARDWARE</a></h1>
<p class="Pp">The <code class="Nm">uart</code> driver supports the following
    classes of UARTs:</p>
<p class="Pp"></p>
<ul class="Bl-bullet Bl-compact">
  <li>NS8250: standard hardware based on the 8250, 16450, 16550, 16650, 16750 or
      the 16950 UARTs.</li>
  <li>SCC: serial communications controllers supported by the
      <a class="Xr">scc(4)</a> device driver.</li>
</ul>
</section>
<section class="Sh">
<h1 class="Sh" id="Pulse_Per_Second_(PPS)_Timing_Interface"><a class="permalink" href="#Pulse_Per_Second_(PPS)_Timing_Interface">Pulse
  Per Second (PPS) Timing Interface</a></h1>
<p class="Pp">The <code class="Nm">uart</code> driver can capture PPS timing
    information as defined in RFC 2783. The API, accessed via
    <a class="Xr">ioctl(2)</a>, is available on the tty device. To use the PPS
    capture feature with <a class="Xr">ntpd(8)</a>, symlink the tty callout
    device <var class="Va">/dev/cuau?</var> to
  <var class="Va">/dev/pps0.</var></p>
<p class="Pp">The <var class="Va">hw.uart.pps_mode</var> tunable configures the
    PPS capture mode for all uart devices; it can be set in
    <a class="Xr">loader.conf(5)</a>. The
    <var class="Va">dev.uart.0.pps_mode</var> sysctl configures the PPS capture
    mode for a specific uart device; it can be set in
    <a class="Xr">loader.conf(5)</a> or <a class="Xr">sysctl.conf(5)</a>.</p>
<p class="Pp">The following capture modes are available:</p>
<div class="Bd-indent">
<dl class="Bl-tag Bl-compact">
  <dt>0x00</dt>
  <dd>Capture disabled.</dd>
  <dt>0x01</dt>
  <dd>Capture pulses on the CTS line.</dd>
  <dt>0x02</dt>
  <dd>Capture pulses on the DCD line.</dd>
</dl>
</div>
<p class="Pp">The following values may be ORed with the capture mode to
    configure capture processing options:</p>
<div class="Bd-indent">
<dl class="Bl-tag Bl-compact">
  <dt>0x10</dt>
  <dd>Invert the pulse (RS-232 logic low = ASSERT, high = CLEAR).</dd>
  <dt>0x20</dt>
  <dd>Attempt to capture narrow pulses.</dd>
</dl>
</div>
<p class="Pp">Add the narrow pulse option when the incoming PPS pulse width is
    small enough to prevent reliable capture in normal mode. In narrow mode the
    driver uses the hardware's ability to latch a line state change; not all
    hardware has this capability. The hardware latch provides a reliable
    indication that a pulse occurred, but prevents distinguishing between the
    CLEAR and ASSERT edges of the pulse. For each detected pulse, the driver
    synthesizes both an ASSERT and a CLEAR event, using the same timestamp for
    each. To prevent spurious events when the hardware is intermittently able to
    see both edges of a pulse, the driver will not generate a new pair of events
    within a half second of the prior pair. Both normal and narrow pulse modes
    work with <a class="Xr">ntpd(8)</a>.</p>
<p class="Pp">Add the invert option when the connection to the uart device uses
    TTL level signals, or when the PPS source emits inverted pulses. RFC 2783
    defines an ASSERT event as a higher-voltage line level, and a CLEAR event as
    a lower-voltage line level, in the context of the RS-232 protocol. The modem
    control signals on a TTL-level connection are typically inverted from the
    RS-232 levels. For example, carrier presence is indicated by a high signal
    on an RS-232 DCD line, and by a low signal on a TTL DCD line. This is due to
    the use of inverting line driver buffers to convert between TTL and RS-232
    line levels in most hardware designs. Generally speaking, a connection to a
    DB-9 style connector is an RS-232 level signal at up to 12 volts. A
    connection to header pins or an edge-connector on an embedded board is
    typically a TTL signal at 3.3 or 5 volts.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="Special_Devices"><a class="permalink" href="#Special_Devices">Special
  Devices</a></h1>
<p class="Pp">The <code class="Nm">uart</code> driver also supports an
    initial-state and a lock-state control device for each of the callin and the
    callout &quot;data&quot; devices. The termios settings of a data device are
    copied from those of the corresponding initial-state device on first opens
    and are not inherited from previous opens. Use <a class="Xr">stty(1)</a> in
    the normal way on the initial-state devices to program initial termios
    states suitable for your setup.</p>
<p class="Pp" id="stty">The lock termios state acts as flags to disable changing
    the termios state. E.g., to lock a flag variable such as CRTSCTS, use
    <a class="permalink" href="#stty"><i class="Em">stty crtscts</i></a> on the
    lock-state device. Speeds and special characters may be locked by setting
    the corresponding value in the lock-state device to any nonzero value. E.g.,
    to lock a speed to 115200, use &#x201C;<code class="Li">stty
    115200</code>&#x201D; on the initial-state device and
    &#x201C;<code class="Li">stty 1</code>&#x201D; on the lock-state device.</p>
<p class="Pp">Correct programs talking to correctly wired external devices work
    with almost arbitrary initial states and almost no locking, but other setups
    may benefit from changing some of the default initial state and locking the
    state. In particular, the initial states for non (POSIX) standard flags
    should be set to suit the devices attached and may need to be locked to
    prevent buggy programs from changing them. E.g., CRTSCTS should be locked on
    for devices that support RTS/CTS handshaking at all times and off for
    devices that do not support it at all. CLOCAL should be locked on for
    devices that do not support carrier. HUPCL may be locked off if you do not
    want to hang up for some reason. In general, very bad things happen if
    something is locked to the wrong state, and things should not be locked for
    devices that support more than one setting. The CLOCAL flag on callin ports
    should be locked off for logins to avoid certain security holes, but this
    needs to be done by getty if the callin port is used for anything else.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="Console_Tuneable"><a class="permalink" href="#Console_Tuneable">Console
  Tuneable</a></h1>
<p class="Pp">The <code class="Nm">uart</code> driver can be designated as a
    system console.</p>
<dl class="Bl-tag">
  <dt id="hw.uart.console"><var class="Va">hw.uart.console</var></dt>
  <dd>Contains a number of <span class="Pa">/etc/remote</span> like tag:value
      pairs, separated by commas.
    <dl class="Bl-tag">
      <dt id="bd"><a class="permalink" href="#bd"><code class="Cm">bd</code></a></dt>
      <dd>Busy Detect. Enable the so-called &#x201C;Busy Detect&#x201D; quirk
          for the device. For NS 16550-compatible devices, this will use
          heuristics to ensure that the UART is no longer busy before
          manipulating line control. DesignWare-based UARTs need this due to a
          design flaw in the UART.</dd>
      <dt id="br"><a class="permalink" href="#br"><code class="Cm">br</code></a></dt>
      <dd>Baudrate. The data rate (bits per second) used for communications on
          the serial port. When the device clock rate (see below) is set to 0,
          then the baud rate will be used with the divisor to compute the device
          clock rate the first time the device is initialized. Only the
          traditional baud rates are allowed. Rates larger than 19200 must be a
          multiple of 19200. Baud rates between 1200 and 19200 must be a
          multiple of 1200. Otherwise the baud rate must be a multiple of 75. A
          value of '0' instructs the <code class="Nm">uart</code> driver to not
          program the baud rate divisor and use the hardware as-is.</dd>
      <dt id="ch"><a class="permalink" href="#ch"><code class="Cm">ch</code></a></dt>
      <dd>Channel. Defaults to 0. Has no effect on UARTs with only one
        channel.</dd>
      <dt id="db"><a class="permalink" href="#db"><code class="Cm">db</code></a></dt>
      <dd>Data bits. Defaults to 8.</dd>
      <dt id="dt"><a class="permalink" href="#dt"><code class="Cm">dt</code></a></dt>
      <dd>Device type. Specify the uart class to use for this device.
        <dl class="Bl-tag">
          <dt>ns8250</dt>
          <dd>Traditional PC UART National Semiconductor 16550 and compatible
              devices.</dd>
          <dt>pl011</dt>
          <dd>Common ARM UART, based on ARM Limited designs.</dd>
        </dl>
      </dd>
      <dt id="io"><a class="permalink" href="#io"><code class="Cm">io</code></a></dt>
      <dd>I/O port address. Specifies the address of a UART in an Intel
          processor's I/O space. Mutually exclusive with
        &#x2018;mm&#x2019;.</dd>
      <dt id="mm"><a class="permalink" href="#mm"><code class="Cm">mm</code></a></dt>
      <dd>Memory mapped I/O address. Specifies the physical address of a
          memory-mapped UART. Mutually exclusive with &#x2018;io&#x2019;.</dd>
      <dt id="pa"><a class="permalink" href="#pa"><code class="Cm">pa</code></a></dt>
      <dd>Parity. The type of parity to use when sending data to the host. This
          may be one of ``even'', ``odd'', ``none'', ``zero'' (always set bit 8
          to zero), ``one'' (always set bit 8 to 1). The default is even
        parity.</dd>
      <dt id="rs"><a class="permalink" href="#rs"><code class="Cm">rs</code></a></dt>
      <dd>Register shift. Number of bits to shift left the base register
        offset.</dd>
      <dt id="rw"><a class="permalink" href="#rw"><code class="Cm">rw</code></a></dt>
      <dd>Register width. Size of operations to read and write the registers of
          the device.</dd>
      <dt id="sb"><a class="permalink" href="#sb"><code class="Cm">sb</code></a></dt>
      <dd>Stopbits. Defaults to 1.</dd>
      <dt id="xo"><a class="permalink" href="#xo"><code class="Cm">xo</code></a></dt>
      <dd>Device clock (xtal oscillator). Base frequency of the oscillator to
          use for the device. When set to 0, and the baud rate is also set, the
          UART's initialization code will compute this the first time and use
          that after. Automatically computed values can be as large as 5% when
          the base frequency is a poor match to the traditional baud rates.</dd>
    </dl>
  </dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="FILES"><a class="permalink" href="#FILES">FILES</a></h1>
<dl class="Bl-tag Bl-compact">
  <dt><span class="Pa">/dev/ttyu?</span></dt>
  <dd>for callin ports</dd>
  <dt><span class="Pa">/dev/ttyu?.init</span></dt>
  <dd style="width: auto;">&#x00A0;</dd>
  <dt><span class="Pa">/dev/ttyu?.lock</span></dt>
  <dd>corresponding callin initial-state and lock-state devices
    <p class="Pp"></p>
  </dd>
  <dt><span class="Pa">/dev/cuau?</span></dt>
  <dd>for callout ports</dd>
  <dt><span class="Pa">/dev/cuau?.init</span></dt>
  <dd style="width: auto;">&#x00A0;</dd>
  <dt><span class="Pa">/dev/cuau?.lock</span></dt>
  <dd>corresponding callout initial-state and lock-state devices</dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="EXAMPLES"><a class="permalink" href="#EXAMPLES">EXAMPLES</a></h1>
<div class="Bd
  Bd-indent"><code class="Li">hw.uart.console=&quot;io:0x2f8,br=115200&quot;</code></div>
<p class="Pp">When the kernel is using a serial console port, it should use COM2
    instead of COM1 and set the baud rate to 115200.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
  ALSO</a></h1>
<p class="Pp"><a class="Xr">cu(1)</a>, <a class="Xr">puc(4)</a>,
    <a class="Xr">scc(4)</a>, <a class="Xr">termios(4)</a>,
    <a class="Xr">tty(4)</a>, <a class="Xr">ttys(5)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The <code class="Nm">uart</code> device driver first appeared in
    <span class="Ux">FreeBSD 5.2</span>.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp">The <code class="Nm">uart</code> device driver and this manual
    page were written by <span class="An">Marcel Moolenaar</span>
    &lt;<a class="Mt" href="mailto:marcel@xcllnt.net">marcel@xcllnt.net</a>&gt;.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="BUGS"><a class="permalink" href="#BUGS">BUGS</a></h1>
<p class="Pp">Before the Internet, serial ports were primarily used for inbound
    connections from terminals, either directly or through modems, these days
    serial ports are primarily used for outbound connections to devices, an
    evolution which unfortunately has spread the relevant documentation over
    three different manual pages: <a class="Xr">termios(4)</a>,
    <a class="Xr">uart(4)</a> and <a class="Xr">tty(4)</a>.</p>
<p class="Pp"></p>
</section>
</div>
<table class="foot">
  <tr>
    <td class="foot-date">December 5, 2024</td>
    <td class="foot-os">FreeBSD 15.0</td>
  </tr>
</table>