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2 Real Time Clock (RTC) Drivers for Linux 2 Real Time Clock (RTC) Drivers for Linux
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4 4
5 When Linux developers talk about a "Real Time 5 When Linux developers talk about a "Real Time Clock", they usually mean
6 something that tracks wall clock time and is b 6 something that tracks wall clock time and is battery backed so that it
7 works even with system power off. Such clocks 7 works even with system power off. Such clocks will normally not track
8 the local time zone or daylight savings time - 8 the local time zone or daylight savings time -- unless they dual boot
9 with MS-Windows -- but will instead be set to 9 with MS-Windows -- but will instead be set to Coordinated Universal Time
10 (UTC, formerly "Greenwich Mean Time"). 10 (UTC, formerly "Greenwich Mean Time").
11 11
12 The newest non-PC hardware tends to just count 12 The newest non-PC hardware tends to just count seconds, like the time(2)
13 system call reports, but RTCs also very common 13 system call reports, but RTCs also very commonly represent time using
14 the Gregorian calendar and 24 hour time, as re 14 the Gregorian calendar and 24 hour time, as reported by gmtime(3).
15 15
16 Linux has two largely-compatible userspace RTC 16 Linux has two largely-compatible userspace RTC API families you may
17 need to know about: 17 need to know about:
18 18
19 * /dev/rtc ... is the RTC provided by PC 19 * /dev/rtc ... is the RTC provided by PC compatible systems,
20 so it's not very portable to non-x86 s 20 so it's not very portable to non-x86 systems.
21 21
22 * /dev/rtc0, /dev/rtc1 ... are part of a 22 * /dev/rtc0, /dev/rtc1 ... are part of a framework that's
23 supported by a wide variety of RTC chi 23 supported by a wide variety of RTC chips on all systems.
24 24
25 Programmers need to understand that the PC/AT 25 Programmers need to understand that the PC/AT functionality is not
26 always available, and some systems can do much 26 always available, and some systems can do much more. That is, the
27 RTCs use the same API to make requests in both 27 RTCs use the same API to make requests in both RTC frameworks (using
28 different filenames of course), but the hardwa 28 different filenames of course), but the hardware may not offer the
29 same functionality. For example, not every RT 29 same functionality. For example, not every RTC is hooked up to an
30 IRQ, so they can't all issue alarms; and where 30 IRQ, so they can't all issue alarms; and where standard PC RTCs can
31 only issue an alarm up to 24 hours in the futu 31 only issue an alarm up to 24 hours in the future, other hardware may
32 be able to schedule one any time in the upcomi 32 be able to schedule one any time in the upcoming century.
33 33
34 34
35 Old PC/AT-Compatible driver: /dev/rtc 35 Old PC/AT-Compatible driver: /dev/rtc
36 -------------------------------------- 36 --------------------------------------
37 37
38 All PCs (even Alpha machines) have a Real Time 38 All PCs (even Alpha machines) have a Real Time Clock built into them.
39 Usually they are built into the chipset of the 39 Usually they are built into the chipset of the computer, but some may
40 actually have a Motorola MC146818 (or clone) o 40 actually have a Motorola MC146818 (or clone) on the board. This is the
41 clock that keeps the date and time while your 41 clock that keeps the date and time while your computer is turned off.
42 42
43 ACPI has standardized that MC146818 functional 43 ACPI has standardized that MC146818 functionality, and extended it in
44 a few ways (enabling longer alarm periods, and 44 a few ways (enabling longer alarm periods, and wake-from-hibernate).
45 That functionality is NOT exposed in the old d 45 That functionality is NOT exposed in the old driver.
46 46
47 However it can also be used to generate signal 47 However it can also be used to generate signals from a slow 2Hz to a
48 relatively fast 8192Hz, in increments of power 48 relatively fast 8192Hz, in increments of powers of two. These signals
49 are reported by interrupt number 8. (Oh! So *t 49 are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
50 for...) It can also function as a 24hr alarm, 50 for...) It can also function as a 24hr alarm, raising IRQ 8 when the
51 alarm goes off. The alarm can also be programm 51 alarm goes off. The alarm can also be programmed to only check any
52 subset of the three programmable values, meani 52 subset of the three programmable values, meaning that it could be set to
53 ring on the 30th second of the 30th minute of 53 ring on the 30th second of the 30th minute of every hour, for example.
54 The clock can also be set to generate an inter 54 The clock can also be set to generate an interrupt upon every clock
55 update, thus generating a 1Hz signal. 55 update, thus generating a 1Hz signal.
56 56
57 The interrupts are reported via /dev/rtc (majo 57 The interrupts are reported via /dev/rtc (major 10, minor 135, read only
58 character device) in the form of an unsigned l 58 character device) in the form of an unsigned long. The low byte contains
59 the type of interrupt (update-done, alarm-rang 59 the type of interrupt (update-done, alarm-rang, or periodic) that was
60 raised, and the remaining bytes contain the nu 60 raised, and the remaining bytes contain the number of interrupts since
61 the last read. Status information is reported 61 the last read. Status information is reported through the pseudo-file
62 /proc/driver/rtc if the /proc filesystem was e 62 /proc/driver/rtc if the /proc filesystem was enabled. The driver has
63 built in locking so that only one process is a 63 built in locking so that only one process is allowed to have the /dev/rtc
64 interface open at a time. 64 interface open at a time.
65 65
66 A user process can monitor these interrupts by 66 A user process can monitor these interrupts by doing a read(2) or a
67 select(2) on /dev/rtc -- either will block/sto 67 select(2) on /dev/rtc -- either will block/stop the user process until
68 the next interrupt is received. This is useful 68 the next interrupt is received. This is useful for things like
69 reasonably high frequency data acquisition whe 69 reasonably high frequency data acquisition where one doesn't want to
70 burn up 100% CPU by polling gettimeofday etc. 70 burn up 100% CPU by polling gettimeofday etc. etc.
71 71
72 At high frequencies, or under high loads, the 72 At high frequencies, or under high loads, the user process should check
73 the number of interrupts received since the la 73 the number of interrupts received since the last read to determine if
74 there has been any interrupt "pileup" so to sp 74 there has been any interrupt "pileup" so to speak. Just for reference, a
75 typical 486-33 running a tight read loop on /d 75 typical 486-33 running a tight read loop on /dev/rtc will start to suffer
76 occasional interrupt pileup (i.e. > 1 IRQ even 76 occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
77 frequencies above 1024Hz. So you really should 77 frequencies above 1024Hz. So you really should check the high bytes
78 of the value you read, especially at frequenci 78 of the value you read, especially at frequencies above that of the
79 normal timer interrupt, which is 100Hz. 79 normal timer interrupt, which is 100Hz.
80 80
81 Programming and/or enabling interrupt frequenc 81 Programming and/or enabling interrupt frequencies greater than 64Hz is
82 only allowed by root. This is perhaps a bit co 82 only allowed by root. This is perhaps a bit conservative, but we don't want
83 an evil user generating lots of IRQs on a slow 83 an evil user generating lots of IRQs on a slow 386sx-16, where it might have
84 a negative impact on performance. This 64Hz li 84 a negative impact on performance. This 64Hz limit can be changed by writing
85 a different value to /proc/sys/dev/rtc/max-use 85 a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
86 interrupt handler is only a few lines of code 86 interrupt handler is only a few lines of code to minimize any possibility
87 of this effect. 87 of this effect.
88 88
89 Also, if the kernel time is synchronized with 89 Also, if the kernel time is synchronized with an external source, the
90 kernel will write the time back to the CMOS cl 90 kernel will write the time back to the CMOS clock every 11 minutes. In
91 the process of doing this, the kernel briefly 91 the process of doing this, the kernel briefly turns off RTC periodic
92 interrupts, so be aware of this if you are doi 92 interrupts, so be aware of this if you are doing serious work. If you
93 don't synchronize the kernel time with an exte 93 don't synchronize the kernel time with an external source (via ntp or
94 whatever) then the kernel will keep its hands 94 whatever) then the kernel will keep its hands off the RTC, allowing you
95 exclusive access to the device for your applic 95 exclusive access to the device for your applications.
96 96
97 The alarm and/or interrupt frequency are progr 97 The alarm and/or interrupt frequency are programmed into the RTC via
98 various ioctl(2) calls as listed in ./include/ 98 various ioctl(2) calls as listed in ./include/linux/rtc.h
99 Rather than write 50 pages describing the ioct 99 Rather than write 50 pages describing the ioctl() and so on, it is
100 perhaps more useful to include a small test pr 100 perhaps more useful to include a small test program that demonstrates
101 how to use them, and demonstrates the features 101 how to use them, and demonstrates the features of the driver. This is
102 probably a lot more useful to people intereste 102 probably a lot more useful to people interested in writing applications
103 that will be using this driver. See the code 103 that will be using this driver. See the code at the end of this document.
104 104
105 (The original /dev/rtc driver was written by P 105 (The original /dev/rtc driver was written by Paul Gortmaker.)
106 106
107 107
108 New portable "RTC Class" drivers: /dev/rtcN 108 New portable "RTC Class" drivers: /dev/rtcN
109 -------------------------------------------- 109 --------------------------------------------
110 110
111 Because Linux supports many non-ACPI and non-P 111 Because Linux supports many non-ACPI and non-PC platforms, some of which
112 have more than one RTC style clock, it needed 112 have more than one RTC style clock, it needed a more portable solution
113 than expecting a single battery-backed MC14681 113 than expecting a single battery-backed MC146818 clone on every system.
114 Accordingly, a new "RTC Class" framework has b 114 Accordingly, a new "RTC Class" framework has been defined. It offers
115 three different userspace interfaces: 115 three different userspace interfaces:
116 116
117 * /dev/rtcN ... much the same as the old 117 * /dev/rtcN ... much the same as the older /dev/rtc interface
118 118
119 * /sys/class/rtc/rtcN ... sysfs attribut 119 * /sys/class/rtc/rtcN ... sysfs attributes support readonly
120 access to some RTC attributes. 120 access to some RTC attributes.
121 121
122 * /proc/driver/rtc ... the system clock 122 * /proc/driver/rtc ... the system clock RTC may expose itself
123 using a procfs interface. If there is 123 using a procfs interface. If there is no RTC for the system clock,
124 rtc0 is used by default. More informat 124 rtc0 is used by default. More information is (currently) shown
125 here than through sysfs. 125 here than through sysfs.
126 126
127 The RTC Class framework supports a wide variet 127 The RTC Class framework supports a wide variety of RTCs, ranging from those
128 integrated into embeddable system-on-chip (SOC 128 integrated into embeddable system-on-chip (SOC) processors to discrete chips
129 using I2C, SPI, or some other bus to communica 129 using I2C, SPI, or some other bus to communicate with the host CPU. There's
130 even support for PC-style RTCs ... including t 130 even support for PC-style RTCs ... including the features exposed on newer PCs
131 through ACPI. 131 through ACPI.
132 132
133 The new framework also removes the "one RTC pe 133 The new framework also removes the "one RTC per system" restriction. For
134 example, maybe the low-power battery-backed RT 134 example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
135 a high functionality RTC is integrated into th 135 a high functionality RTC is integrated into the SOC. That system might read
136 the system clock from the discrete RTC, but us 136 the system clock from the discrete RTC, but use the integrated one for all
137 other tasks, because of its greater functional 137 other tasks, because of its greater functionality.
138 138
139 Check out tools/testing/selftests/rtc/rtctest. 139 Check out tools/testing/selftests/rtc/rtctest.c for an example usage of the
140 ioctl interface. 140 ioctl interface.
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