NAME¶

uart — driver for Universal Asynchronous Receiver/Transmitter (UART) devices

SYNOPSIS¶

device uart

device puc
device uart

device scc
device uart

In /boot/device.hints:
hint.uart.0.disabled="1"
hint.uart.0.baud="38400"
hint.uart.0.port="0x3f8"
hint.uart.0.flags="0x10"

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 only)

0x00800

set RX FIFO trigger level to ``high'' (NS8250 only)

DESCRIPTION¶

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.

HARDWARE¶

The uart driver supports the following classes of UARTs:

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

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:

The following values may be ORed with the capture mode to configure capture processing options:

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 volts.

Special Devices¶

The uart driver also supports an initial-state and a lock-state control device for each of the callin and the callout "data" 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 stty(1) in the normal way on the initial-state devices to program initial termios states suitable for your setup.

The lock termios state acts as flags to disable changing the termios state. E.g., to lock a flag variable such as CRTSCTS, use stty crtscts 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 “stty 115200” on the initial-state device and “stty 1” on the lock-state device.

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.

DEPRECATION NOTICE¶

The PC Card attachment of this driver is scheduled for removal prior to the release of FreeBSD 13.0

FILES¶

/dev/ttyu?

for callin ports

/dev/ttyu?.init

 

/dev/ttyu?.lock

corresponding callin initial-state and lock-state devices

/dev/cuau?

for callout ports

/dev/cuau?.init

 

/dev/cuau?.lock

corresponding callout initial-state and lock-state devices

SEE ALSO¶

puc(4), scc(4)

HISTORY¶

The uart device driver first appeared in FreeBSD 5.2.

AUTHORS¶

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