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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 inter-
     face.  Each such interface	is controlled by a separate and	independent
     instance of the uart driver.  The primary support for devices that	con-
     tain 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 com-
     plexities that are	inherent when elementary components are	packaged

     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 compo-
     nents 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 ker-
     nel 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 inter-

     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(2), 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 cap-
     ture 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	indi-
     cated 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	11.2			April 26, 2017			  FreeBSD 11.2


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