Real Time Clock Driver for Linux
        ================================

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.

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.

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
interface open at a time.

A user process can monitor these interrupts by doing a read(2) or a
select(2) on /dev/rtc -- either will block/stop the user process until
the next interrupt is received. This is useful for things like
reasonably high frequency data acquisition where one doesn't want to
burn up 100% CPU by polling gettimeofday etc. etc.

At high frequencies, or under high loads, the user process should check
the number of interrupts received since the last read to determine if
there has been any interrupt "pileup" so to speak. Just for reference, a
typical 486-33 running a tight read loop on /dev/rtc will start to suffer
occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
frequencies above 1024Hz. So you really should check the high bytes
of the value you read, especially at frequencies above that of the
normal timer interrupt, which is 100Hz.

Programming and/or enabling interrupt frequencies greater than 64Hz is
only allowed by root. This is perhaps a bit conservative, but we don't want
an evil user generating lots of IRQs on a slow 386sx-16, where it might have
a negative impact on performance.  Note that the interrupt handler is only
a few lines of code to minimize any possibility of this effect.

Also, if the kernel time is synchronized with an external source, the 
kernel will write the time back to the CMOS clock every 11 minutes. In 
the process of doing this, the kernel briefly turns off RTC periodic 
interrupts, so be aware of this if you are doing serious work. If you
don't synchronize the kernel time with an external source (via ntp or
whatever) then the kernel will keep its hands off the RTC, allowing you
exclusive access to the device for your applications.

The alarm and/or interrupt frequency are programmed into the RTC via
various ioctl(2) calls as listed in ./include/linux/rtc.h
Rather than write 50 pages describing the ioctl() and so on, it is
perhaps more useful to include a small test program that demonstrates
how to use them, and demonstrates the features of the driver. This is
probably a lot more useful to people interested in writing applications
that will be using this driver.

                                                Paul Gortmaker

-------------------- 8< ---------------- 8< -----------------------------

/*
 *      Real Time Clock Driver Test/Example Program
 *
 *      Compile with:
 *              gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
 *
 *      Copyright (C) 1996, Paul Gortmaker.
 *
 *      Released under the GNU General Public License, version 2,
 *      included herein by reference.
 *
 */

#include <stdio.h>
#include <linux/rtc.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

int main(void) {

int i, fd, retval, irqcount = 0;
unsigned long tmp, data;
struct rtc_time rtc_tm;

fd = open ("/dev/rtc", O_RDONLY);

if (fd ==  -1) {
        perror("/dev/rtc");
        exit(errno);
}

fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");

/* Turn on update interrupts (one per second) */
retval = ioctl(fd, RTC_UIE_ON, 0);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}

fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading /dev/rtc:");
fflush(stderr);
for (i=1; i<6; i++) {
        /* This read will block */
        retval = read(fd, &data, sizeof(unsigned long));
        if (retval == -1) {
                perror("read");
                exit(errno);
        }
        fprintf(stderr, " %d",i);
        fflush(stderr);
        irqcount++;
}

fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
fflush(stderr);
for (i=1; i<6; i++) {
        struct timeval tv = {5, 0};     /* 5 second timeout on select */
        fd_set readfds;

        FD_ZERO(&readfds);
        FD_SET(fd, &readfds);
        /* The select will wait until an RTC interrupt happens. */
        retval = select(fd+1, &readfds, NULL, NULL, &tv);
        if (retval == -1) {
                perror("select");
                exit(errno);
        }
        /* This read won't block unlike the select-less case above. */
        retval = read(fd, &data, sizeof(unsigned long));
        if (retval == -1) {
                perror("read");
                exit(errno);
        }
        fprintf(stderr, " %d",i);
        fflush(stderr);
        irqcount++;
}

/* Turn off update interrupts */
retval = ioctl(fd, RTC_UIE_OFF, 0);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}

/* Read the RTC time/date */
retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}

fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
        rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
        rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);

/* Set the alarm to 5 sec in the future, and check for rollover */
rtc_tm.tm_sec += 5;
if (rtc_tm.tm_sec >= 60) {
        rtc_tm.tm_sec %= 60;
        rtc_tm.tm_min++;
}
if  (rtc_tm.tm_min == 60) {
        rtc_tm.tm_min = 0;
        rtc_tm.tm_hour++;
}
if  (rtc_tm.tm_hour == 24)
        rtc_tm.tm_hour = 0;

retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}

/* Read the current alarm settings */
retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}

fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
        rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);

/* Enable alarm interrupts */
retval = ioctl(fd, RTC_AIE_ON, 0);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}

fprintf(stderr, "Waiting 5 seconds for alarm...");
fflush(stderr);
/* This blocks until the alarm ring causes an interrupt */
retval = read(fd, &data, sizeof(unsigned long));
if (retval == -1) {
        perror("read");
        exit(errno);
}
irqcount++;
fprintf(stderr, " okay. Alarm rang.\n");

/* Disable alarm interrupts */
retval = ioctl(fd, RTC_AIE_OFF, 0);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}

/* Read periodic IRQ rate */
retval = ioctl(fd, RTC_IRQP_READ, &tmp);
if (retval == -1) {
        perror("ioctl");
        exit(errno);
}
fprintf(stderr, "\nPeriodic IRQ rate was %ldHz.\n", tmp);

fprintf(stderr, "Counting 20 interrupts at:");
fflush(stderr);

/* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
for (tmp=2; tmp<=64; tmp*=2) {

        retval = ioctl(fd, RTC_IRQP_SET, tmp);
        if (retval == -1) {
                perror("ioctl");
                exit(errno);
        }

        fprintf(stderr, "\n%ldHz:\t", tmp);
        fflush(stderr);

        /* Enable periodic interrupts */
        retval = ioctl(fd, RTC_PIE_ON, 0);
        if (retval == -1) {
                perror("ioctl");
                exit(errno);
        }

        for (i=1; i<21; i++) {
                /* This blocks */
                retval = read(fd, &data, sizeof(unsigned long));
                if (retval == -1) {
                        perror("read");
                        exit(errno);
                }
                fprintf(stderr, " %d",i);
                fflush(stderr);
                irqcount++;
        }

        /* Disable periodic interrupts */
        retval = ioctl(fd, RTC_PIE_OFF, 0);
        if (retval == -1) {
                perror("ioctl");
                exit(errno);
        }
}

fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
fprintf(stderr, "\nTyping \"cat /proc/interrupts\" will show %d more events on IRQ 8.\n\n",
                                                                 irqcount);

close(fd);
return 0;

} /* end main */