1
2	Real Time Clock Driver for Linux
3	================================
4
5All PCs (even Alpha machines) have a Real Time Clock built into them.
6Usually they are built into the chipset of the computer, but some may
7actually have a Motorola MC146818 (or clone) on the board. This is the
8clock that keeps the date and time while your computer is turned off.
9
10However it can also be used to generate signals from a slow 2Hz to a
11relatively fast 8192Hz, in increments of powers of two. These signals
12are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
13for...) It can also function as a 24hr alarm, raising IRQ 8 when the
14alarm goes off. The alarm can also be programmed to only check any
15subset of the three programmable values, meaning that it could be set to
16ring on the 30th second of the 30th minute of every hour, for example.
17The clock can also be set to generate an interrupt upon every clock
18update, thus generating a 1Hz signal.
19
20The interrupts are reported via /dev/rtc (major 10, minor 135, read only
21character device) in the form of an unsigned long. The low byte contains
22the type of interrupt (update-done, alarm-rang, or periodic) that was
23raised, and the remaining bytes contain the number of interrupts since
24the last read.  Status information is reported through the pseudo-file
25/proc/driver/rtc if the /proc filesystem was enabled.  The driver has
26built in locking so that only one process is allowed to have the /dev/rtc
27interface open at a time.
28
29A user process can monitor these interrupts by doing a read(2) or a
30select(2) on /dev/rtc -- either will block/stop the user process until
31the next interrupt is received. This is useful for things like
32reasonably high frequency data acquisition where one doesn't want to
33burn up 100% CPU by polling gettimeofday etc. etc.
34
35At high frequencies, or under high loads, the user process should check
36the number of interrupts received since the last read to determine if
37there has been any interrupt "pileup" so to speak. Just for reference, a
38typical 486-33 running a tight read loop on /dev/rtc will start to suffer
39occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
40frequencies above 1024Hz. So you really should check the high bytes
41of the value you read, especially at frequencies above that of the
42normal timer interrupt, which is 100Hz.
43
44Programming and/or enabling interrupt frequencies greater than 64Hz is
45only allowed by root. This is perhaps a bit conservative, but we don't want
46an evil user generating lots of IRQs on a slow 386sx-16, where it might have
47a negative impact on performance.  Note that the interrupt handler is only
48a few lines of code to minimize any possibility of this effect.
49
50Also, if the kernel time is synchronized with an external source, the
51kernel will write the time back to the CMOS clock every 11 minutes. In
52the process of doing this, the kernel briefly turns off RTC periodic
53interrupts, so be aware of this if you are doing serious work. If you
54don't synchronize the kernel time with an external source (via ntp or
55whatever) then the kernel will keep its hands off the RTC, allowing you
56exclusive access to the device for your applications.
57
58The alarm and/or interrupt frequency are programmed into the RTC via
59various ioctl(2) calls as listed in ./include/linux/rtc.h
60Rather than write 50 pages describing the ioctl() and so on, it is
61perhaps more useful to include a small test program that demonstrates
62how to use them, and demonstrates the features of the driver. This is
63probably a lot more useful to people interested in writing applications
64that 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
92int main(void) {
93
94int i, fd, retval, irqcount = 0;
95unsigned long tmp, data;
96struct rtc_time rtc_tm;
97
98fd = open ("/dev/rtc", O_RDONLY);
99
100if (fd ==  -1) {
101	perror("/dev/rtc");
102	exit(errno);
103}
104
105fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
106
107/* Turn on update interrupts (one per second) */
108retval = ioctl(fd, RTC_UIE_ON, 0);
109if (retval == -1) {
110	perror("ioctl");
111	exit(errno);
112}
113
114fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading /dev/rtc:");
115fflush(stderr);
116for (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
128fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
129fflush(stderr);
130for (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 */
154retval = ioctl(fd, RTC_UIE_OFF, 0);
155if (retval == -1) {
156	perror("ioctl");
157	exit(errno);
158}
159
160/* Read the RTC time/date */
161retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
162if (retval == -1) {
163	perror("ioctl");
164	exit(errno);
165}
166
167fprintf(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 */
172rtc_tm.tm_sec += 5;
173if (rtc_tm.tm_sec >= 60) {
174	rtc_tm.tm_sec %= 60;
175	rtc_tm.tm_min++;
176}
177if  (rtc_tm.tm_min == 60) {
178	rtc_tm.tm_min = 0;
179	rtc_tm.tm_hour++;
180}
181if  (rtc_tm.tm_hour == 24)
182	rtc_tm.tm_hour = 0;
183
184retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
185if (retval == -1) {
186	perror("ioctl");
187	exit(errno);
188}
189
190/* Read the current alarm settings */
191retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
192if (retval == -1) {
193	perror("ioctl");
194	exit(errno);
195}
196
197fprintf(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 */
201retval = ioctl(fd, RTC_AIE_ON, 0);
202if (retval == -1) {
203	perror("ioctl");
204	exit(errno);
205}
206
207fprintf(stderr, "Waiting 5 seconds for alarm...");
208fflush(stderr);
209/* This blocks until the alarm ring causes an interrupt */
210retval = read(fd, &data, sizeof(unsigned long));
211if (retval == -1) {
212	perror("read");
213	exit(errno);
214}
215irqcount++;
216fprintf(stderr, " okay. Alarm rang.\n");
217
218/* Disable alarm interrupts */
219retval = ioctl(fd, RTC_AIE_OFF, 0);
220if (retval == -1) {
221	perror("ioctl");
222	exit(errno);
223}
224
225/* Read periodic IRQ rate */
226retval = ioctl(fd, RTC_IRQP_READ, &tmp);
227if (retval == -1) {
228	perror("ioctl");
229	exit(errno);
230}
231fprintf(stderr, "\nPeriodic IRQ rate was %ldHz.\n", tmp);
232
233fprintf(stderr, "Counting 20 interrupts at:");
234fflush(stderr);
235
236/* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
237for (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
275fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
276fprintf(stderr, "\nTyping \"cat /proc/interrupts\" will show %d more events on IRQ 8.\n\n",
277								 irqcount);
278
279close(fd);
280return 0;
281
282} /* end main */
283