2 .\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
4 .\" This is free documentation; you can redistribute it and/or
5 .\" modify it under the terms of the GNU General Public License as
6 .\" published by the Free Software Foundation; either version 2 of
7 .\" the License, or (at your option) any later version.
9 .\" The GNU General Public License's references to "object code"
10 .\" and "executables" are to be interpreted as the output of any
11 .\" document formatting or typesetting system, including
12 .\" intermediate and printed output.
14 .\" This manual is distributed in the hope that it will be useful,
15 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
16 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
17 .\" GNU General Public License for more details.
19 .\" You should have received a copy of the GNU General Public
20 .\" License along with this manual; if not, write to the Free
21 .\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
24 .\" $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $
26 .\" 2006-02-08 Various additions by mtk
27 .\" 2006-11-26 cleanup, cover the generic rtc framework; David Brownell
29 .TH RTC 4 2010-02-25 "Linux" "Linux Programmer's Manual"
31 rtc \- real-time clock
33 #include <linux/rtc.h>
35 .BI "int ioctl(" fd ", RTC_" request ", " param ");"
37 This is the interface to drivers for real-time clocks (RTCs).
39 Most computers have one or more hardware clocks which record the
40 current "wall clock" time.
41 These are called "Real Time Clocks" (RTCs).
42 One of these usually has battery backup power so that it tracks the time
43 even while the computer is turned off.
44 RTCs often provide alarms and other interrupts.
46 All i386 PCs, and ACPI-based systems, have an RTC that is compatible with
47 the Motorola MC146818 chip on the original PC/AT.
48 Today such an RTC is usually integrated into the mainboard's chipset
49 (south bridge), and uses a replaceable coin-sized backup battery.
51 Non-PC systems, such as embedded systems built around system-on-chip
52 processors, use other implementations.
53 They usually won't offer the same functionality as the RTC from a PC/AT.
54 .SS RTC vs System Clock
55 RTCs should not be confused with the system clock, which is
56 a software clock maintained by the kernel and used to implement
60 as well as setting timestamps on files, etc.
61 The system clock reports seconds and microseconds since a start point,
62 defined to be the POSIX Epoch: 1970-01-01 00:00:00 +0000 (UTC).
63 (One common implementation counts timer interrupts, once
64 per "jiffy", at a frequency of 100, 250, or 1000 Hz.)
65 That is, it is supposed to report wall clock time, which RTCs also do.
67 A key difference between an RTC and the system clock is that RTCs
68 run even when the system is in a low power state (including "off"),
69 and the system clock can't.
70 Until it is initialized, the system clock can only report time since
71 system boot ... not since the POSIX Epoch.
72 So at boot time, and after resuming from a system low power state, the
73 system clock will often be set to the current wall clock time using an RTC.
74 Systems without an RTC need to set the system clock using another clock,
75 maybe across the network or by entering that data manually.
77 RTCs can be read and written with
79 or directly with the ioctl requests listed below.
81 Besides tracking the date and time, many RTCs can also generate
84 on every clock update (i.e., once per second);
86 at periodic intervals with a frequency that can be set to
87 any power-of-2 multiple in the range 2 Hz to 8192 Hz;
89 on reaching a previously specified alarm time.
91 Each of those interrupt sources can be enabled or disabled separately.
92 On many systems, the alarm interrupt can be configured as a system wakeup
93 event, which can resume the system from a low power state such as
94 Suspend-to-RAM (STR, called S3 in ACPI systems),
95 Hibernation (called S4 in ACPI systems),
96 or even "off" (called S5 in ACPI systems).
97 On some systems, the battery backed RTC can't issue
98 interrupts, but another one can.
106 device can be opened only once (until it is closed) and it is read-only.
111 the calling process is blocked until the next interrupt from that RTC
113 Following the interrupt, the process can read a long integer, of which
114 the least significant byte contains a bit mask encoding
115 the types of interrupt that occurred,
116 while the remaining 3 bytes contain the number of interrupts since the
119 .SS ioctl(2) interface
122 requests are defined on file descriptors connected to RTC devices:
125 Returns this RTC's time in the following structure:
136 int tm_wday; /* unused */
137 int tm_yday; /* unused */
138 int tm_isdst; /* unused */
143 The fields in this structure have the same meaning and ranges as for the
145 structure described in
147 A pointer to this structure should be passed as the third
152 Sets this RTC's time to the time specified by the
154 structure pointed to by the third
158 RTC's time the process must be privileged (i.e., have the
162 .BR RTC_ALM_READ ", " RTC_ALM_SET
163 Read and set the alarm time, for RTCs that support alarms.
164 The alarm interrupt must be separately enabled or disabled using the
165 .BR RTC_AIE_ON ", " RTC_AIE_OFF
169 argument is a pointer to an
177 fields of this structure are used.
179 .BR RTC_IRQP_READ ", " RTC_IRQP_SET
180 Read and set the frequency for periodic interrupts,
181 for RTCs that support periodic interrupts.
182 The periodic interrupt must be separately enabled or disabled using the
183 .BR RTC_PIE_ON ", " RTC_PIE_OFF
188 .I "unsigned long\ *"
190 .IR "unsigned long" ,
192 The value is the frequency in interrupts per second.
193 The set of allowable frequencies is the multiples of two
194 in the range 2 to 8192.
195 Only a privileged process (i.e., one having the
197 capability) can set frequencies above the value specified in
198 .IR /proc/sys/dev/rtc/max-user-freq .
199 (This file contains the value 64 by default.)
201 .BR RTC_AIE_ON ", " RTC_AIE_OFF
202 Enable or disable the alarm interrupt, for RTCs that support alarms.
207 .BR RTC_UIE_ON ", " RTC_UIE_OFF
208 Enable or disable the interrupt on every clock update,
209 for RTCs that support this once-per-second interrupt.
214 .BR RTC_PIE_ON ", " RTC_PIE_OFF
215 Enable or disable the periodic interrupt,
216 for RTCs that support these periodic interrupts.
220 Only a privileged process (i.e., one having the
222 capability) can enable the periodic interrupt if the frequency is
223 currently set above the value specified in
224 .IR /proc/sys/dev/rtc/max-user-freq .
226 .BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
227 Many RTCs encode the year in an 8-bit register which is either
228 interpreted as an 8-bit binary number or as a BCD number.
230 the number is interpreted relative to this RTC's Epoch.
232 initialized to 1900 on most systems but on Alpha and MIPS it might
233 also be initialized to 1952, 1980, or 2000, depending on the value of
234 an RTC register for the year.
236 these operations can be used to read or to set the RTC's Epoch,
241 .I "unsigned long\ *"
243 .IR "unsigned long" ,
244 respectively, and the value returned (or assigned) is the Epoch.
245 To set the RTC's Epoch the process must be privileged (i.e., have the
249 .BR RTC_WKALM_RD ", " RTC_WKALM_SET
250 Some RTCs support a more powerful alarm interface, using these ioctls
251 to read or write the RTC's alarm time (respectively) with this structure:
257 unsigned char enabled;
258 unsigned char pending;
259 struct rtc_time time;
267 flag is used to enable or disable the alarm interrupt,
268 or to read its current status; when using these calls,
269 .BR RTC_AIE_ON " and " RTC_AIE_OFF
275 to report a pending interrupt
276 (so it's mostly useless on Linux, except when talking
277 to the RTC managed by EFI firmware).
280 field is as used with
289 fields are also valid.
290 A pointer to this structure should be passed as the third
297 etc: RTC special character device files.
299 .IR /proc/driver/rtc :
300 status of the (first) RTC.
302 When the kernel's system time is synchronized with an external
305 it will update a designated RTC periodically every 11 minutes.
306 To do so, the kernel has to briefly turn off periodic interrupts;
307 this might affect programs using that RTC.
309 An RTC's Epoch has nothing to do with the POSIX Epoch which is only
310 used for the system clock.
312 If the year according to the RTC's Epoch and the year register is
313 less than 1970 it is assumed to be 100 years later, that is, between 2000
316 Some RTCs support "wildcard" values in alarm fields, to support
317 scenarios like periodic alarms at fifteen minutes after every hour,
318 or on the first day of each month.
319 Such usage is nonportable;
320 portable user space code only expects a single alarm interrupt, and
321 will either disable or reinitialize the alarm after receiving it.
323 Some RTCs support periodic interrupts with periods that are multiples
324 of a second rather than fractions of a second;
326 programmable output clock signals;
329 capabilities that are not currently exposed by this API.
333 .BR gettimeofday (2),
334 .BR settimeofday (2),
340 /usr/src/linux/Documentation/rtc.txt