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UART(4)                FreeBSD Kernel Interfaces Manual                UART(4)

     uart - driver for Universal Asynchronous Receiver/Transmitter (UART)

     device uart

     device puc
     device uart

     device scc
     device uart

     In /boot/device.hints:

     With flags encoded as:
     0x00010   device is potential system console
     0x00080   use this port for remote kernel debugging
     0x00100   set RX FIFO trigger level to ``low'' (NS8250 only)
     0x00200   set RX FIFO trigger level to ``medium low'' (NS8250 only)
     0x00400   set RX FIFO trigger level to ``medium high'' (default, NS8250
     0x00800   set RX FIFO trigger level to ``high'' (NS8250 only)

     The uart 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 uart 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
     puc(4), or scc(4) device drivers.  However, the serial interfaces of
     those devices that are managed by the puc(4), or scc(4) driver are each
     independently controlled by the uart driver.  As such, the puc(4), or
     scc(4) driver provides umbrella functionality for the uart driver and
     hides the complexities that are inherent when elementary components are
     packaged together.

     The uart 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.

     At the heart of the uart driver is the core component.  It contains the
     bus attachments and the low-level interrupt handler.

     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.

     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.

     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.

     The uart driver supports the following classes of UARTs:

     +o   NS8250: standard hardware based on the 8250, 16450, 16550, 16650,
         16750 or the 16950 UARTs.
     +o   SCC: serial communications controllers supported by the scc(4) device

Pulse Per Second (PPS) Timing Interface
     The uart driver can capture PPS timing information as defined in RFC
     2783.  The API, accessed via ioctl(8), is available on the tty device.
     To use the PPS capture feature with ntpd(8), symlink the tty callout
     device /dev/cuau? to /dev/pps0.

     The hw.uart.pps_mode tunable configures the PPS capture mode for all uart
     devices; it can be set in loader.conf(5).  The dev.uart.0.pps_mode sysctl
     configures the PPS capture mode for a specific uart device; it can be set
     in loader.conf(5) or sysctl.conf(5).

     The following capture modes are available:
         0x00  Capture disabled.
         0x01  Capture pulses on the CTS line.
         0x02  Capture pulses on the DCD line.

     The following values may be ORed with the capture mode to configure
     capture processing options:
         0x10  Invert the pulse (RS-232 logic low = ASSERT, high = CLEAR).
         0x20  Attempt to capture narrow pulses.

     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 ntpd(8).

     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

     /dev/ttyu?           for callin ports
     /dev/ttyu?.lock      corresponding callin initial-state and lock-state

     /dev/cuau?           for callout ports
     /dev/cuau?.lock      corresponding callout initial-state and lock-state

     puc(4), scc(4)

     The uart device driver first appeared in FreeBSD 5.2.

     The uart device driver and this manual page were written by Marcel
     Moolenaar <>.

FreeBSD 11.0-PRERELEASE        December 9, 2015        FreeBSD 11.0-PRERELEASE


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