Difference between revisions of "RTC"

Revision as of 17:13, 20 June 2013

On this page, you will find useful informations to use the Real Time Clock of your boards (if chip is mounted).

Hardware

APF9328/APF27

No permanent RTC is present on the APF9328/APF27 modules, but you can have one (as an option) on the development boards or add it yourself if you are an electrician (it is not so complex to add a DS1374 (with integrated quartz) on the I2C bus). Do not forget to provide the two power-supply (VCC and Vbackup)). Currently only Maxim's DS1374 has been used but any I2C RTC, supported by Linux, should work the same way.

APF51/APF28

APF51 and APF28 modules have an onboard PMIC with an integrated RTC.

Driver installation

APF9328/APF27

DS1374 is by default included in standard Armadeus Linux kernel. To check if your board (Linux) has correctly detected the RTC:

# dmesg | grep ds1374
ds1374-legacy 0-0068: chip found, driver version 1.0
ds1374-legacy 0-0068: rtc core: registered ds1374-legacy as rtc0
ds1374-legacy 0-0068: setting system clock to 1970-01-01 00:00:07 UTC (7)

APF51

Driver is by default installed and launched. To check if your board (Linux) has correctly detected the RTC:

# dmesg | grep rtc
wm831x-rtc wm831x-rtc: rtc core: registered wm831x as rtc0
wm831x-rtc wm831x-rtc: setting system clock to 2011-05-03 13:27:26 UTC (1304429246)

APF28

On APF28 the RTC is integrated under the i.MX28 processor. See the reference manual chapter 22 "Real-Time Clock Alarm Watchdog Persistent Bits".

There isn't button-lithium backup battery under APF28dev board, if we unplug the power, clock is not saved and then we could ask : what the interest of RTC if not backuped ?

The interest is when we power the whole board with a battery and we poweroff the apf28 with the command :

$ poweroff

The processor will be powered of ... except the RTC. Then we could power on the board pushing power switch and keep the clock up to date.

Warning: To enable poweroff on APF28dev the board must be powered by battery (J10 pin 4) and the USB-powering must be disconnected. To disconnect it, just unsolder the self L7 (behind the usb J3 connector)

Note: Currently, the clock doesn't work in poweroff it must be fixed. The date and hour are keeped but not incremented

Clock

To read the hardware value of clock use hwclock command on file /dev/rtc0:

Keep clock

To power off the board type :

# poweroff
The system is going down NOW!
Sent SIGTERM to all processes
Sent SIGKILL to all processes
P
Communications disconnect (Back at localhost)
----------------------------------------------------
(/home/fabien/) C-Kermit>

And to wake-up the board push the button 'ON'. The date and hour will be keeped when you type 'date' or 'hwclock'.

Usage

Note: We recommand to store time in UTC format in the RTC; that will ease the timezone/summer time handling.

• From Linux use the command hwclock to read, write, synchronize the RTC (here we pre-suppose that RTC is storing time in UTC format).

 # hwclock --help
 # hwclock -r -u      to read hardware clock and print result (localtime)
 # hwclock -s -u      to set the system time from the hardware clock
 # hwclock -w -u      to set the hardware clock to the current system time (UTC)

Warning: Security "features" prevent to change RTC time more than 8 times per hour on the APF51 !!

• and date to read and update system date and time while running:

 # date --help        for more information
 # date -R            to read hardware clock and print result
 # date 013122302011           to sets the system date to january 31th 2011 22h30 (localtime)
 # date 2011.01.31-22:30:00    also do the job

• Then, when booting you will see something like this:

ds1374 0-0068: setting the system clock to 2011-xxxx
or
wm831x-rtc wm831x-rtc: setting system clock to 2011-xxxx

• TimeZone/Summer time handling can be configured in /etc/TZ:

 # cat /etc/TZ
 CET-1DST,M3.5.0/2:00,M10.5.0/3:00

• you can update RTC from a NTP (Network Time Protocol) server if your board is connected to internet or a NTP server

 # ntpd -nqp 217.147.208.1
 ntpd: setting clock to Mon Apr 23 22:36:39 DST 2012 (offset 1335216961.855897s)
 # hwclock -wu  # to set the hardware clock to the current system time (UTC)

• if Alarm/wakeup is supported by the RTC driver then the alarm/wakeup time may be set like that for a 60 sec delay with RTC0

 # sec=`date '+%s'`
 # let "sec=$sec+60"
 # echo $sec > /sys/class/rtc/rtc0/wakealarm

• if the time specified in wakealarm is earlier than the current time, then the Alarm/wakeup is disabled

Links

• Maxim's DS1374 datasheet