Documentation/rtc.txt

   1
   2         Real Time Clock Driver for Linux
   3         ================================
   4
   5 All PCs (even Alpha machines) have a Real Time Clock built into them.
   6 Usually they are built into the chipset of the computer, but some may
   7 actually have a Motorola MC146818 (or clone) on the board. This is the
   8 clock that keeps the date and time while your computer is turned off.
   9
  10 However it can also be used to generate signals from a slow 2Hz to a
  11 relatively fast 8192Hz, in increments of powers of two. These signals
  12 are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
  13 for...) It can also function as a 24hr alarm, raising IRQ 8 when the
  14 alarm goes off. The alarm can also be programmed to only check any
  15 subset of the three programmable values, meaning that it could be set to
  16 ring on the 30th second of the 30th minute of every hour, for example.
  17 The clock can also be set to generate an interrupt upon every clock
  18 update, thus generating a 1Hz signal.
  19
  20 The interrupts are reported via /dev/rtc (major 10, minor 135, read only
  21 character device) in the form of an unsigned long. The low byte contains
  22 the type of interrupt (update-done, alarm-rang, or periodic) that was
  23 raised, and the remaining bytes contain the number of interrupts since
  24 the last read.  Status information is reported through the pseudo-file
  25 /proc/driver/rtc if the /proc filesystem was enabled.  The driver has
  26 built in locking so that only one process is allowed to have the /dev/rtc
  27 interface open at a time.
  28
  29 A user process can monitor these interrupts by doing a read(2) or a
  30 select(2) on /dev/rtc -- either will block/stop the user process until
  31 the next interrupt is received. This is useful for things like
  32 reasonably high frequency data acquisition where one doesn't want to
  33 burn up 100% CPU by polling gettimeofday etc. etc.
  34
  35 At high frequencies, or under high loads, the user process should check
  36 the number of interrupts received since the last read to determine if
  37 there has been any interrupt "pileup" so to speak. Just for reference, a
  38 typical 486-33 running a tight read loop on /dev/rtc will start to suffer
  39 occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
  40 frequencies above 1024Hz. So you really should check the high bytes
  41 of the value you read, especially at frequencies above that of the
  42 normal timer interrupt, which is 100Hz.
  43
  44 Programming and/or enabling interrupt frequencies greater than 64Hz is
  45 only allowed by root. This is perhaps a bit conservative, but we don't want
  46 an evil user generating lots of IRQs on a slow 386sx-16, where it might have
  47 a negative impact on performance.  Note that the interrupt handler is only
  48 a few lines of code to minimize any possibility of this effect.
  49
  50 Also, if the kernel time is synchronized with an external source, the
  51 kernel will write the time back to the CMOS clock every 11 minutes. In
  52 the process of doing this, the kernel briefly turns off RTC periodic
  53 interrupts, so be aware of this if you are doing serious work. If you
  54 don't synchronize the kernel time with an external source (via ntp or
  55 whatever) then the kernel will keep its hands off the RTC, allowing you
  56 exclusive access to the device for your applications.
  57
  58 The alarm and/or interrupt frequency are programmed into the RTC via
  59 various ioctl(2) calls as listed in ./include/linux/rtc.h
  60 Rather than write 50 pages describing the ioctl() and so on, it is
  61 perhaps more useful to include a small test program that demonstrates
  62 how to use them, and demonstrates the features of the driver. This is
  63 probably a lot more useful to people interested in writing applications
  64 that will be using this driver.
  65
  66                                                 Paul Gortmaker
  67
  68 -------------------- 8< ---------------- 8< -----------------------------
  69
  70 /*
  71  *      Real Time Clock Driver Test/Example Program
  72  *
  73  *      Compile with:
  74  *              gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
  75  *
  76  *      Copyright (C) 1996, Paul Gortmaker.
  77  *
  78  *      Released under the GNU General Public License, version 2,
  79  *      included herein by reference.
  80  *
  81  */
  82
  83 #include <stdio.h>
  84 #include <linux/rtc.h>
  85 #include <sys/ioctl.h>
  86 #include <sys/time.h>
  87 #include <sys/types.h>
  88 #include <fcntl.h>
  89 #include <unistd.h>
  90 #include <errno.h>
  91
  92 int main(void) {
  93
  94 int i, fd, retval, irqcount = 0;
  95 unsigned long tmp, data;
  96 struct rtc_time rtc_tm;
  97
  98 fd = open ("/dev/rtc", O_RDONLY);
  99
 100 if (fd ==  -1) {
 101         perror("/dev/rtc");
 102         exit(errno);
 103 }
 104
 105 fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
 106
 107 /* Turn on update interrupts (one per second) */
 108 retval = ioctl(fd, RTC_UIE_ON, 0);
 109 if (retval == -1) {
 110         perror("ioctl");
 111         exit(errno);
 112 }
 113
 114 fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading /dev/rtc:");
 115 fflush(stderr);
 116 for (i=1; i<6; i++) {
 117         /* This read will block */
 118         retval = read(fd, &data, sizeof(unsigned long));
 119         if (retval == -1) {
 120                 perror("read");
 121                 exit(errno);
 122         }
 123         fprintf(stderr, " %d",i);
 124         fflush(stderr);
 125         irqcount++;
 126 }
 127
 128 fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
 129 fflush(stderr);
 130 for (i=1; i<6; i++) {
 131         struct timeval tv = {5, 0};     /* 5 second timeout on select */
 132         fd_set readfds;
 133
 134         FD_ZERO(&readfds);
 135         FD_SET(fd, &readfds);
 136         /* The select will wait until an RTC interrupt happens. */
 137         retval = select(fd+1, &readfds, NULL, NULL, &tv);
 138         if (retval == -1) {
 139                 perror("select");
 140                 exit(errno);
 141         }
 142         /* This read won't block unlike the select-less case above. */
 143         retval = read(fd, &data, sizeof(unsigned long));
 144         if (retval == -1) {
 145                 perror("read");
 146                 exit(errno);
 147         }
 148         fprintf(stderr, " %d",i);
 149         fflush(stderr);
 150         irqcount++;
 151 }
 152
 153 /* Turn off update interrupts */
 154 retval = ioctl(fd, RTC_UIE_OFF, 0);
 155 if (retval == -1) {
 156         perror("ioctl");
 157         exit(errno);
 158 }
 159
 160 /* Read the RTC time/date */
 161 retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
 162 if (retval == -1) {
 163         perror("ioctl");
 164         exit(errno);
 165 }
 166
 167 fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
 168         rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
 169         rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 170
 171 /* Set the alarm to 5 sec in the future, and check for rollover */
 172 rtc_tm.tm_sec += 5;
 173 if (rtc_tm.tm_sec >= 60) {
 174         rtc_tm.tm_sec %= 60;
 175         rtc_tm.tm_min++;
 176 }
 177 if  (rtc_tm.tm_min == 60) {
 178         rtc_tm.tm_min = 0;
 179         rtc_tm.tm_hour++;
 180 }
 181 if  (rtc_tm.tm_hour == 24)
 182         rtc_tm.tm_hour = 0;
 183
 184 retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
 185 if (retval == -1) {
 186         perror("ioctl");
 187         exit(errno);
 188 }
 189
 190 /* Read the current alarm settings */
 191 retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
 192 if (retval == -1) {
 193         perror("ioctl");
 194         exit(errno);
 195 }
 196
 197 fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
 198         rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
 199
 200 /* Enable alarm interrupts */
 201 retval = ioctl(fd, RTC_AIE_ON, 0);
 202 if (retval == -1) {
 203         perror("ioctl");
 204         exit(errno);
 205 }
 206
 207 fprintf(stderr, "Waiting 5 seconds for alarm...");
 208 fflush(stderr);
 209 /* This blocks until the alarm ring causes an interrupt */
 210 retval = read(fd, &data, sizeof(unsigned long));
 211 if (retval == -1) {
 212         perror("read");
 213         exit(errno);
 214 }
 215 irqcount++;
 216 fprintf(stderr, " okay. Alarm rang.\n");
 217
 218 /* Disable alarm interrupts */
 219 retval = ioctl(fd, RTC_AIE_OFF, 0);
 220 if (retval == -1) {
 221         perror("ioctl");
 222         exit(errno);
 223 }
 224
 225 /* Read periodic IRQ rate */
 226 retval = ioctl(fd, RTC_IRQP_READ, &tmp);
 227 if (retval == -1) {
 228         perror("ioctl");
 229         exit(errno);
 230 }
 231 fprintf(stderr, "\nPeriodic IRQ rate was %ldHz.\n", tmp);
 232
 233 fprintf(stderr, "Counting 20 interrupts at:");
 234 fflush(stderr);
 235
 236 /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
 237 for (tmp=2; tmp<=64; tmp*=2) {
 238
 239         retval = ioctl(fd, RTC_IRQP_SET, tmp);
 240         if (retval == -1) {
 241                 perror("ioctl");
 242                 exit(errno);
 243         }
 244
 245         fprintf(stderr, "\n%ldHz:\t", tmp);
 246         fflush(stderr);
 247
 248         /* Enable periodic interrupts */
 249         retval = ioctl(fd, RTC_PIE_ON, 0);
 250         if (retval == -1) {
 251                 perror("ioctl");
 252                 exit(errno);
 253         }
 254
 255         for (i=1; i<21; i++) {
 256                 /* This blocks */
 257                 retval = read(fd, &data, sizeof(unsigned long));
 258                 if (retval == -1) {
 259                         perror("read");
 260                         exit(errno);
 261                 }
 262                 fprintf(stderr, " %d",i);
 263                 fflush(stderr);
 264                 irqcount++;
 265         }
 266
 267         /* Disable periodic interrupts */
 268         retval = ioctl(fd, RTC_PIE_OFF, 0);
 269         if (retval == -1) {
 270                 perror("ioctl");
 271                 exit(errno);
 272         }
 273 }
 274
 275 fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
 276 fprintf(stderr, "\nTyping \"cat /proc/interrupts\" will show %d more events on IRQ 8.\n\n",
 277                                                                  irqcount);
 278
 279 close(fd);
 280 return 0;
 281
 282 } /* end main */