rtc — real-time clock
#include <linux/rtc.h>
int ioctl( |
fd, |
| RTC_request, | |
param); |
This is the interface to drivers for real-time clocks (RTCs).
Most computers have one or more hardware clocks which record the current "wall clock" time. These are called "Real Time Clocks" (RTCs). One of these usually has battery backup power so that it tracks the time even while the computer is turned off. RTCs often provide alarms and other interrupts.
All i386 PCs, and ACPI based systems, have an RTC that is compatible with the Motorola MC146818 chip on the original PC/AT. Today such an RTC is usually integrated into the mainboard's chipset (south bridge), and uses a replaceable coin-sized backup battery.
Non-PC systems, such as embedded systems built around system-on-chip processors, use other implementations. They usually won't offer the same functionality as the RTC from a PC/AT.
RTCs should not be confused with the system clock, which is a software clock maintained by the kernel and used to implement gettimeofday(2) and time(2), as well as setting timestamps on files, etc. The system clock reports seconds and microseconds since a start point, defined to be the POSIX Epoch: 1970-01-01 00:00:00 +0000 (UTC). (One common implementation counts timer interrupts, once per "jiffy", at a frequency of 100, 250, or 1000 Hz.) That is, it is supposed to report wall clock time, which RTCs also do.
A key difference between an RTC and the system clock is that RTCs run even when the system is in a low power state (including "off"), and the system clock can't. Until it is initialized, the system clock can only report time since system boot ... not since the POSIX Epoch. So at boot time, and after resuming from a system low power state, the system clock will often be set to the current wall clock time using an RTC. Systems without an RTC need to set the system clock using another clock, maybe across the network or by entering that data manually.
RTCs can be read and written with hwclock(8), or directly with the ioctl requests listed below.
Besides tracking the date and time, many RTCs can also generate interrupts
on every clock update (i.e., once per second);
at periodic intervals with a frequency that can be set to any power-of-2 multiple in the range 2 Hz to 8192 Hz;
on reaching a previously specified alarm time.
Each of those interrupt sources can be enabled or disabled separately. On many systems, the alarm interrupt can be configured as a system wakeup event, which can resume the system from a low power state such as Suspend-to-RAM (STR, called S3 in ACPI systems), Hibernation (called S4 in ACPI systems), or even "off" (called S5 in ACPI systems). On some systems, the battery backed RTC can't issue interrupts, but another one can.
The /dev/rtc (or
/dev/rtc0, /dev/rtc1, etc.) device can be opened
only once (until it is closed) and it is read-only. On
read(2) and select(2) the calling
process is blocked until the next interrupt from that RTC
is received. Following the interrupt, the process can read
a long integer, of which the least significant byte
contains a bit mask encoding the types of interrupt that
occurred, while the remaining 3 bytes contain the number of
interrupts since the last read(2).
The following ioctl(2) requests are defined on file descriptors connected to RTC devices:
RTC_RD_TIMEReturns this RTC's time in the following structure:
struct rtc_time { int tm_sec;int tm_min;int tm_hour;int tm_mday;int tm_mon;int tm_year;int tm_wday;int tm_yday;int tm_isdst;};
The fields in this structure have the same meaning
and ranges as for the tm structure described in gmtime(3). A
pointer to this structure should be passed as the
third ioctl(2)
argument.
RTC_SET_TIMESets this RTC's time to the time specified by the
rtc_time structure
pointed to by the third ioctl(2) argument.
To set the RTC's time the process must be privileged
(i.e., have the CAP_SYS_TIME capability).
RTC_ALM_READ, RTC_ALM_SETRead and set the alarm time, for RTCs that support
alarms. The alarm interrupt must be separately
enabled or disabled using the RTC_AIE_ON, RTC_AIE_OFF requests. The third
ioctl(2) argument
is a pointer to an rtc_time structure. Only the
tm_sec,
tm_min, and
tm_hour
fields of this structure are used.
RTC_IRQP_READ, RTC_IRQP_SETRead and set the frequency for periodic
interrupts, for RTCs that support periodic
interrupts. The periodic interrupt must be separately
enabled or disabled using the RTC_PIE_ON, RTC_PIE_OFF requests. The third
ioctl(2) argument
is an unsigned long
* or an unsigned long,
respectively. The value is the frequency in
interrupts per second. The set of allowable
frequencies is the multiples of two in the range 2 to
8192. Only a privileged process (i.e., one having the
CAP_SYS_RESOURCE
capability) can set frequencies above the value
specified in /proc/sys/dev/rtc/max-user-freq.
(This file contains the value 64 by default.)
RTC_AIE_ON, RTC_AIE_OFFEnable or disable the alarm interrupt, for RTCs that support alarms. The third ioctl(2) argument is ignored.
RTC_UIE_ON, RTC_UIE_OFFEnable or disable the interrupt on every clock update, for RTCs that support this once-per-second interrupt. The third ioctl(2) argument is ignored.
RTC_PIE_ON, RTC_PIE_OFFEnable or disable the periodic interrupt, for RTCs
that support these periodic interrupts. The third
ioctl(2) argument
is ignored. Only a privileged process (i.e., one
having the CAP_SYS_RESOURCE capability) can
enable the periodic interrupt if the frequency is
currently set above the value specified in
/proc/sys/dev/rtc/max-user-freq.
RTC_EPOCH_READ, RTC_EPOCH_SETMany RTCs encode the year in an 8-bit register
which is either interpreted as an 8-bit binary number
or as a BCD number. In both cases, the number is
interpreted relative to this RTC's Epoch. The RTC's
Epoch is initialized to 1900 on most systems but on
Alpha and MIPS it might also be initialized to 1952,
1980, or 2000, depending on the value of an RTC
register for the year. With some RTCs, these
operations can be used to read or to set the RTC's
Epoch, respectively. The third ioctl(2) argument
is a unsigned long
* or a unsigned long,
respectively, and the value returned (or assigned) is
the Epoch. To set the RTC's Epoch the process must be
privileged (i.e., have the CAP_SYS_TIME capability).
RTC_WKALM_RD, RTC_WKALM_SETSome RTCs support a more powerful alarm interface, using these ioctls to read or write the RTC's alarm time (respectively) with this structure:
struct rtc_wkalrm { unsigned char enabled;unsigned char pending;struct rtc_time time;};
The enabled flag is used to
enable or disable the alarm interrupt, or to read its
current status; when using these calls, RTC_AIE_ON and RTC_AIE_OFF are not used. The
pending flag
is used by RTC_WKALM_RD
to report a pending interrupt (so it's mostly useless
on Linux, except when talking to the RTC managed by
EFI firmware). The time field is as used
with RTC_ALM_READ and
RTC_ALM_SET except that
the tm_mday,
tm_mon, and
tm_year
fields are also valid. A pointer to this structure
should be passed as the third ioctl(2)
argument.
/dev/rtc, /dev/rtc0, /dev/rtc1, etc: RTC special character
device files.
/proc/driver/rtc status of the (first) RTC.:
When the kernel's system time is synchronized with an external reference using adjtimex(2) it will update a designated RTC periodically every 11 minutes. To do so, the kernel has to briefly turn off periodic interrupts; this might affect programs using that RTC.
An RTC's Epoch has nothing to do with the POSIX Epoch which is only used for the system clock.
If the year according to the RTC's Epoch and the year register is less than 1970 it is assumed to be 100 years later, that is, between 2000 and 2069.
Some RTCs support "wildcard" values in alarm fields, to support scenarios like periodic alarms at fifteen minutes after every hour, or on the first day of each month. Such usage is nonportable; portable user space code only expects a single alarm interrupt, and will either disable or reinitialize the alarm after receiving it.
Some RTCs support periodic interrupts with periods that are multiples of a second rather than fractions of a second; multiple alarms; programmable output clock signals; nonvolatile memory; and other hardware capabilities that are not currently exposed by this API.
date(1), adjtimex(2), gettimeofday(2), settimeofday(2), stime(2), time(2), gmtime(3), time(7), hwclock(8), /usr/src/linux/Documentation/rtc.txt
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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rtc.4 Copyright 2002 Urs Thuermann (ursisnogud.escape.de) This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License's references to "object code" and "executables" are to be interpreted as the output of any document formatting or typesetting system, including intermediate and printed output. This manual is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA. $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $ 2006-02-08 Various additions by mtk 2006-11-26 cleanup, cover the generic rtc framework; David Brownell |