1 =======================================
2 Real Time Clock (RTC) Drivers for Linux
3 =======================================
4
5 When Linux developers talk about a "Real Time
6 something that tracks wall clock time and is b
7 works even with system power off. Such clocks
8 the local time zone or daylight savings time -
9 with MS-Windows -- but will instead be set to
10 (UTC, formerly "Greenwich Mean Time").
11
12 The newest non-PC hardware tends to just count
13 system call reports, but RTCs also very common
14 the Gregorian calendar and 24 hour time, as re
15
16 Linux has two largely-compatible userspace RTC
17 need to know about:
18
19 * /dev/rtc ... is the RTC provided by PC
20 so it's not very portable to non-x86 s
21
22 * /dev/rtc0, /dev/rtc1 ... are part of a
23 supported by a wide variety of RTC chi
24
25 Programmers need to understand that the PC/AT
26 always available, and some systems can do much
27 RTCs use the same API to make requests in both
28 different filenames of course), but the hardwa
29 same functionality. For example, not every RT
30 IRQ, so they can't all issue alarms; and where
31 only issue an alarm up to 24 hours in the futu
32 be able to schedule one any time in the upcomi
33
34
35 Old PC/AT-Compatible driver: /dev/rtc
36 --------------------------------------
37
38 All PCs (even Alpha machines) have a Real Time
39 Usually they are built into the chipset of the
40 actually have a Motorola MC146818 (or clone) o
41 clock that keeps the date and time while your
42
43 ACPI has standardized that MC146818 functional
44 a few ways (enabling longer alarm periods, and
45 That functionality is NOT exposed in the old d
46
47 However it can also be used to generate signal
48 relatively fast 8192Hz, in increments of power
49 are reported by interrupt number 8. (Oh! So *t
50 for...) It can also function as a 24hr alarm,
51 alarm goes off. The alarm can also be programm
52 subset of the three programmable values, meani
53 ring on the 30th second of the 30th minute of
54 The clock can also be set to generate an inter
55 update, thus generating a 1Hz signal.
56
57 The interrupts are reported via /dev/rtc (majo
58 character device) in the form of an unsigned l
59 the type of interrupt (update-done, alarm-rang
60 raised, and the remaining bytes contain the nu
61 the last read. Status information is reported
62 /proc/driver/rtc if the /proc filesystem was e
63 built in locking so that only one process is a
64 interface open at a time.
65
66 A user process can monitor these interrupts by
67 select(2) on /dev/rtc -- either will block/sto
68 the next interrupt is received. This is useful
69 reasonably high frequency data acquisition whe
70 burn up 100% CPU by polling gettimeofday etc.
71
72 At high frequencies, or under high loads, the
73 the number of interrupts received since the la
74 there has been any interrupt "pileup" so to sp
75 typical 486-33 running a tight read loop on /d
76 occasional interrupt pileup (i.e. > 1 IRQ even
77 frequencies above 1024Hz. So you really should
78 of the value you read, especially at frequenci
79 normal timer interrupt, which is 100Hz.
80
81 Programming and/or enabling interrupt frequenc
82 only allowed by root. This is perhaps a bit co
83 an evil user generating lots of IRQs on a slow
84 a negative impact on performance. This 64Hz li
85 a different value to /proc/sys/dev/rtc/max-use
86 interrupt handler is only a few lines of code
87 of this effect.
88
89 Also, if the kernel time is synchronized with
90 kernel will write the time back to the CMOS cl
91 the process of doing this, the kernel briefly
92 interrupts, so be aware of this if you are doi
93 don't synchronize the kernel time with an exte
94 whatever) then the kernel will keep its hands
95 exclusive access to the device for your applic
96
97 The alarm and/or interrupt frequency are progr
98 various ioctl(2) calls as listed in ./include/
99 Rather than write 50 pages describing the ioct
100 perhaps more useful to include a small test pr
101 how to use them, and demonstrates the features
102 probably a lot more useful to people intereste
103 that will be using this driver. See the code
104
105 (The original /dev/rtc driver was written by P
106
107
108 New portable "RTC Class" drivers: /dev/rtcN
109 --------------------------------------------
110
111 Because Linux supports many non-ACPI and non-P
112 have more than one RTC style clock, it needed
113 than expecting a single battery-backed MC14681
114 Accordingly, a new "RTC Class" framework has b
115 three different userspace interfaces:
116
117 * /dev/rtcN ... much the same as the old
118
119 * /sys/class/rtc/rtcN ... sysfs attribut
120 access to some RTC attributes.
121
122 * /proc/driver/rtc ... the system clock
123 using a procfs interface. If there is
124 rtc0 is used by default. More informat
125 here than through sysfs.
126
127 The RTC Class framework supports a wide variet
128 integrated into embeddable system-on-chip (SOC
129 using I2C, SPI, or some other bus to communica
130 even support for PC-style RTCs ... including t
131 through ACPI.
132
133 The new framework also removes the "one RTC pe
134 example, maybe the low-power battery-backed RT
135 a high functionality RTC is integrated into th
136 the system clock from the discrete RTC, but us
137 other tasks, because of its greater functional
138
139 Check out tools/testing/selftests/rtc/rtctest.
140 ioctl interface.
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