fdfbb3f5b1
option to invert the polarity in software. Also add an option to capture very narrow pulses by using the hardware's MSR delta-bit capability of latching line state changes. This effectively reverts the mistake I made in r286595 which was based on empirical measurements made on hardware using TTL-level signaling, in which the logic levels are inverted from RS-232. Thus, this re-syncs the polarity with the requirements of RFC 2783, which is writen in terms of RS-232 signaling. Narrow-pulse mode uses the ability of most ns8250 and similar chips to provide a delta indication in the modem status register. The hardware is able to notice and latch the change when the pulse width is shorter than interrupt latency, which results in the signal no longer being asserted by time the interrupt service code runs. When running in this mode we get notified only that "a pulse happened" so the driver synthesizes both an ASSERT and a CLEAR event (with the same timestamp for each). When the pulse width is about equal to the interrupt latency the driver may intermittantly see both edges of the pulse. To prevent generating spurious events, the driver implements a half-second lockout period after generating an event before it will generate another. Differential Revision: https://reviews.freebsd.org/D4477
256 lines
7.9 KiB
Groff
256 lines
7.9 KiB
Groff
.\"
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.\" Copyright (c) 2003 Marcel Moolenaar
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\"
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 9, 2015
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.Dt UART 4
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.Os
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.Sh NAME
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.Nm uart
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.Nd driver for Universal Asynchronous Receiver/Transmitter (UART) devices
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.Sh SYNOPSIS
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.Cd "device uart"
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.Pp
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.Cd "device puc"
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.Cd "device uart"
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.Pp
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.Cd "device scc"
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.Cd "device uart"
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.Pp
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In
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.Pa /boot/device.hints :
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.Cd hint.uart.0.disabled="1"
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.Cd hint.uart.0.baud="38400"
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.Cd hint.uart.0.port="0x3f8"
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.Cd hint.uart.0.flags="0x10"
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.Pp
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With
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.Ar flags
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encoded as:
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.Bl -tag -compact -width 0x000000
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.It 0x00010
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device is potential system console
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.It 0x00080
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use this port for remote kernel debugging
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.It 0x00100
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set RX FIFO trigger level to ``low'' (NS8250 only)
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.It 0x00200
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set RX FIFO trigger level to ``medium low'' (NS8250 only)
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.It 0x00400
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set RX FIFO trigger level to ``medium high'' (default, NS8250 only)
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.It 0x00800
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set RX FIFO trigger level to ``high'' (NS8250 only)
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.El
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.\"
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.Sh DESCRIPTION
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The
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.Nm
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device driver provides support for various classes of UARTs implementing the
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EIA RS-232C (CCITT V.24) serial communications interface.
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Each such interface is controlled by a separate and independent instance of
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the
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.Nm
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driver.
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The primary support for devices that contain multiple serial interfaces or
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that contain other functionality besides one or more serial interfaces is
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provided by the
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.Xr puc 4 ,
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or
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.Xr scc 4
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device drivers.
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However, the serial interfaces of those devices that are managed by the
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.Xr puc 4 ,
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or
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.Xr scc 4
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driver are each independently controlled by the
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.Nm
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driver.
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As such, the
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.Xr puc 4 ,
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or
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.Xr scc 4
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driver provides umbrella functionality for the
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.Nm
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driver and hides the complexities that are inherent when elementary components
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are packaged together.
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.Pp
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The
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.Nm
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driver has a modular design to allow it to be used on differing hardware and
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for various purposes.
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In the following sections the components are discussed in detail.
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Options are described in the section that covers the component to which each
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option applies.
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.\"
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.Ss CORE COMPONENT
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At the heart of the
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.Nm
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driver is the core component.
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It contains the bus attachments and the low-level interrupt handler.
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.\"
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.Ss HARDWARE DRIVERS
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The core component and the kernel interfaces talk to the hardware through the
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hardware interface.
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This interface serves as an abstraction of the hardware and allows varying
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UARTs to be used for serial communications.
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.\"
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.Ss SYSTEM DEVICES
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System devices are UARTs that have a special purpose by way of hardware
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design or software setup.
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For example, Sun UltraSparc machines use UARTs as their keyboard interface.
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Such an UART cannot be used for general purpose communications.
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Likewise, when the kernel is configured for a serial console, the
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corresponding UART will in turn be a system device so that the kernel can
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output boot messages early on in the boot process.
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.\"
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.Ss KERNEL INTERFACES
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The last but not least of the components is the kernel interface.
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This component ultimately determines how the UART is made visible to the
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kernel in particular and to users in general.
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The default kernel interface is the TTY interface.
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This allows the UART to be used for terminals, modems and serial line IP
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applications.
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System devices, with the notable exception of serial consoles, generally
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have specialized kernel interfaces.
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.\"
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.Sh HARDWARE
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The
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.Nm
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driver supports the following classes of UARTs:
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.Pp
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.Bl -bullet -compact
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.It
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NS8250: standard hardware based on the 8250, 16450, 16550, 16650, 16750 or
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the 16950 UARTs.
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.It
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SCC: serial communications controllers supported by the
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.Xr scc 4
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device driver.
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.El
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.\"
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.Sh Pulse Per Second (PPS) Timing Interface
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The
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.Nm
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driver can capture PPS timing information as defined in RFC 2783.
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The API, accessed via
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.Xr ioctl 8 ,
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is available on the tty device.
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To use the PPS capture feature with
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.Xr ntpd 8 ,
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symlink the tty callout device
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.Va /dev/cuau?
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to
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.Va /dev/pps0.
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.Pp
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The
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.Va hw.uart.pps_mode
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tunable configures the PPS capture mode for all uart devices;
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it can be set in
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.Xr loader.conf 5 .
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The
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.Va dev.uart.0.pps_mode
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sysctl configures the PPS capture mode for a specific uart device;
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it can be set in
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.Xr loader.conf 5
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or
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.Xr sysctl.conf 5 .
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.Pp
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The following capture modes are available:
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.Bl -tag -compact -offset "mmmm" -width "mmmm"
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.It 0x00
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Capture disabled.
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.It 0x01
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Capture pulses on the CTS line.
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.It 0x02
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Capture pulses on the DCD line.
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.El
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.Pp
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The following values may be ORed with the capture mode to configure
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capture processing options:
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.Bl -tag -compact -offset "mmmm" -width "mmmm"
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.It 0x10
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Invert the pulse (RS-232 logic low = ASSERT, high = CLEAR).
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.It 0x20
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Attempt to capture narrow pulses.
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.El
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.Pp
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Add the narrow pulse option when the incoming PPS pulse width is small
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enough to prevent reliable capture in normal mode.
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In narrow mode the driver uses the hardware's ability to latch a line
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state change; not all hardware has this capability.
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The hardware latch provides a reliable indication that a pulse occurred,
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but prevents distinguishing between the CLEAR and ASSERT edges of the pulse.
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For each detected pulse, the driver synthesizes both an ASSERT and a CLEAR
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event, using the same timestamp for each.
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To prevent spurious events when the hardware is intermittently able to
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see both edges of a pulse, the driver will not generate a new pair of
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events within a half second of the prior pair.
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Both normal and narrow pulse modes work with
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.Xr ntpd 8 .
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.Pp
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Add the invert option when the connection to the uart device uses TTL
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level signals, or when the PPS source emits inverted pulses.
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RFC 2783 defines an ASSERT event as a higher-voltage line level, and a CLEAR
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event as a lower-voltage line level, in the context of the RS-232 protocol.
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The modem control signals on a TTL-level connection are typically
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inverted from the RS-232 levels.
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For example, carrier presence is indicated by a high signal on an RS-232
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DCD line, and by a low signal on a TTL DCD line.
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This is due to the use of inverting line driver buffers to convert between
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TTL and RS-232 line levels in most hardware designs.
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Generally speaking, a connection to a DB-9 style connector is an RS-232
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level signal at up to 12 volts.
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A connection to header pins or an edge-connector on an embedded board
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is typically a TTL signal at 3.3 or 5 volts.
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.Sh FILES
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.Bl -tag -width ".Pa /dev/ttyu?.init" -compact
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.It Pa /dev/ttyu?
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for callin ports
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.It Pa /dev/ttyu?.init
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.It Pa /dev/ttyu?.lock
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corresponding callin initial-state and lock-state devices
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.Pp
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.It Pa /dev/cuau?
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for callout ports
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.It Pa /dev/cuau?.init
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.It Pa /dev/cuau?.lock
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corresponding callout initial-state and lock-state devices
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.El
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.Sh SEE ALSO
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.Xr puc 4 ,
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.Xr scc 4
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.\"
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.Sh HISTORY
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The
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.Nm
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device driver first appeared in
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.Fx 5.2 .
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.Sh AUTHORS
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The
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.Nm
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device driver and this manual page were written by
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.An Marcel Moolenaar Aq Mt marcel@xcllnt.net .
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