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 */
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