linux-imx

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Real Time Clock (RTC) Drivers for Linux

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When Linux developers talk about a "Real Time Clock", they usually mean

something that tracks wall clock time and is battery backed so that it

works even with system power off.  Such clocks will normally not track

the local time zone or daylight savings time -- unless they dual boot

with MS-Windows -- but will instead be set to Coordinated Universal Time

(UTC, formerly "Greenwich Mean Time").

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The newest non-PC hardware tends to just count seconds, like the time(2)

system call reports, but RTCs also very commonly represent time using

the Gregorian calendar and 24 hour time, as reported by gmtime(3).

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Linux has two largely-compatible userspace RTC API families you may

need to know about:

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    *   /dev/rtc ... is the RTC provided by PC compatible systems,

    so it's not very portable to non-x86 systems.

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    *   /dev/rtc0, /dev/rtc1 ... are part of a framework that's

    supported by a wide variety of RTC chips on all systems.

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Programmers need to understand that the PC/AT functionality is not

always available, and some systems can do much more.  That is, the

RTCs use the same API to make requests in both RTC frameworks (using

different filenames of course), but the hardware may not offer the

same functionality.  For example, not every RTC is hooked up to an

IRQ, so they can't all issue alarms; and where standard PC RTCs can

only issue an alarm up to 24 hours in the future, other hardware may

be able to schedule one any time in the upcoming century.

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Old PC/AT-Compatible driver:  /dev/rtc

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All PCs (even Alpha machines) have a Real Time Clock built into them.

Usually they are built into the chipset of the computer, but some may

actually have a Motorola MC146818 (or clone) on the board. This is the

clock that keeps the date and time while your computer is turned off.

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ACPI has standardized that MC146818 functionality, and extended it in

a few ways (enabling longer alarm periods, and wake-from-hibernate).

That functionality is NOT exposed in the old driver.

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However it can also be used to generate signals from a slow 2Hz to a

relatively fast 8192Hz, in increments of powers of two. These signals

are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is

for...) It can also function as a 24hr alarm, raising IRQ 8 when the

alarm goes off. The alarm can also be programmed to only check any

subset of the three programmable values, meaning that it could be set to

ring on the 30th second of the 30th minute of every hour, for example.

The clock can also be set to generate an interrupt upon every clock

update, thus generating a 1Hz signal.

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The interrupts are reported via /dev/rtc (major 10, minor 135, read only

character device) in the form of an unsigned long. The low byte contains

the type of interrupt (update-done, alarm-rang, or periodic) that was

raised, and the remaining bytes contain the number of interrupts since

the last read.  Status information is reported through the pseudo-file

/proc/driver/rtc if the /proc filesystem was enabled.  The driver has

built in locking so that only one process is allowed to have the /dev/rtc