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