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New page on real-time clock driver
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Michael Kerrisk
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.\" rtc.4
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.\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
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.\"
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, write to the Free
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.\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
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.\" USA.
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.\"
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.\" $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $
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.\"
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.\" 2006-02-08 Various additions by mtk
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.\"
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.TH RTC 4 "2005-12-05" "Linux" "Linux Programmer's Manual"
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.SH NAME
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rtc \- real-time clock
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.SH SYNOPSIS
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#include <linux/rtc.h>
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.SH DESCRIPTION
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This is the driver for the real-time clock (RTC).
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Most computers have a built-in hardware clock, usually called the
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real-time clock.
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This clock is normally battery powered so
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that it keeps the time even while the computer is switched off.
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It represents the current time as year, month, day of month, hour,
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minute, and second.
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The RTC is a chip that maintains the time and date and is able to
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generate interrupts at specified times. This chip typically used to
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be a Motorola MC146818, a Dallas DS12887, or similar,
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but today it is usually implemented in the mainboard's chipset.
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The RTC should not be confused with the system time which is an
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independent, interrupt-driven software clock maintained by the kernel.
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The software clock is maintained by an interrupt routine that
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typically has a frequency of 100, 250, or 1000 Hz.
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The software clock counts seconds and microsecond since the POSIX
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Epoch, i.e., Jan 1, 1970, 0:00 UTC.
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This clock does not involve any special hardware.
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The RTC can be read and set with
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.BR hwclock (8).
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The RTC is almost never used by the Linux kernel. Instead,
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the kernel uses the software clock time for
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.BR time (2),
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.BR gettimeofday (2),
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timestamps on files, etc.
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However, at boot time the kernel initializes its software clock by
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reading the RTC.
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Besides counting the date and time, the RTC can also generate
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interrupts
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.IP *
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on every clock update (i.e. once per second);
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.IP *
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at periodic intervals with a frequency that can be set to
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any power-of-2 multiple in the range 2 Hz to 8192 Hz;
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.IP *
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on reaching a previously specified alarm time.
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.PP
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Each of these interrupt sources can be enabled or disabled separately.
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The
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.B /dev/rtc
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device can be opened only once simultaneously and it is read-only. On
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.BR read (2)
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and
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.BR select (2)
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the calling process is blocked until the next interrupt from the RTC
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is received.
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Following the interrupt, the process can read a long integer, of which
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the least significant byte contains the type of interrupt that occurred,
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while the remaining 3 bytes contain the number of interrupts since the
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last
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.BR read (2).
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The following
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.BR ioctl (2)
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operations are provided:
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.TP
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.B RTC_RD_TIME
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Returns the RTC time in the following structure:
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.PP
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.RS
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.in +0.5i
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.nf
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struct rtc_time {
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int tm_sec;
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int tm_min;
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int tm_hour;
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int tm_mday;
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int tm_mon;
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int tm_year;
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int tm_wday; /* unused */
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int tm_yday; /* unused */
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int tm_isdst; /* unused */
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};
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.fi
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.in -0.5i
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.RE
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.IP
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The fields in this structure have the same meaning and ranges as for the
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.I tm
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structure described in
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.BR gmtime (3).
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A pointer to this structure should be passed as the third
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.BR ioctl ()
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argument.
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.TP
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.B RTC_SET_TIME
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Sets the RTC time to the time specified by the
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.I rtc_time
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structure pointed to by the third
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.BR ioctl ()
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argument.
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To set the
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RTC time the process must be privileged (i.e., have the
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.B CAP_SYS_TIME
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capability).
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.TP
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.BR RTC_ALM_READ ", " RTC_ALM_SET
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Read and set the alarm time.
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The third \fBioctl\fP() argument is a pointer to an
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.I rtc_time
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structure.
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Only the
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.IR tm_sec ,
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.IR tm_min ,
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and
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.IR tm_hour
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fields of this structure are used.
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.TP
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.BR RTC_IRQP_READ ", " RTC_IRQP_SET
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Read and set the frequency for periodic interrupts.
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The third \fBioctl\fP() argument is a
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.I "long\ *"
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or a
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.IR long ,
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respectively.
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The value is the frequency in interrupts per second.
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The set of allowable frequencies is the multiples of two
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in the range 2 to 8192.
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Only a privileged process (i.e., one having the
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.B CAP_SYS_RESOURCE
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capability) can set frequencies above the value specified in
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.IR /proc/sys/dev/rtc/max-user-freq .
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(This file contains the value 64 by default.)
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.TP
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.BR RTC_AIE_ON ", " RTC_AIE_OFF
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Enable or disable the alarm interrupt.
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The third \fBioctl\fP() argument is ignored.
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.TP
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.BR RTC_UIE_ON ", " RTC_UIE_OFF
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Enable or disable the interrupt on every clock update.
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The third \fBioctl\fP() argument is ignored.
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.TP
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.BR RTC_PIE_ON ", " RTC_PIE_OFF
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Enable or disable the periodic interrupt.
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The third \fBioctl\fP() argument is ignored.
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Only a privileged process (i.e., one having the
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.B CAP_SYS_RESOURCE
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capability) can enable the periodic interrupt if the frequency is
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currently set above the value specified in
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.IR /proc/sys/dev/rtc/max-user-freq .
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.TP
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.BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
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The RTC encodes the year in an 8-bit register which is either
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interpreted as an 8-bit binary number or as a BCD number.
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In both cases,
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the number is interpreted relative to the RTC Epoch.
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The RTC Epoch is
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initialized to 1900 on most systems but on Alpha and Mips it might
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also be initialized to 1952, 1980, or 2000, depending on the value of
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RTC register for the year.
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These operations can be used to read or to set the RTC Epoch, respectively.
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To set the RTC Epoch the process must be privileged (i.e., have the
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.B CAP_SYS_TIME
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capability).
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.SH FILES
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.IR /dev/rtc :
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the RTC special character device file.
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.IR /proc/driver/rtc :
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status of the RTC.
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.SH NOTES
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When the kernel's system time is synchronized with an external
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reference using
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.BR adjtimex (2)
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it will update the RTC periodically every 11 minutes. To
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do so, the kernel has to briefly turn off periodic interrupts;
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this might affect programs using the RTC.
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The RTC Epoch has nothing to do with the POSIX Epoch which is only
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used for the system clock.
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If the year according to the RTC Epoch and the RTC's year register is
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less than 1970 it is assumed to be 100 years later, i.e. between 2000
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and 2069.
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.SH "SEE ALSO"
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.BR hwclock (8),
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.BR date (1),
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.BR time (2),
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.BR stime (2),
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.BR gettimeofday (2),
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.BR settimeofday (2),
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.BR adjtimex (2),
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.BR gmtime (3),
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/usr/src/linux/Documentation/rtc.txt
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