Introduction

This project implements a wake-on-rtc solution for the Raspberry Pi and other small computers based on the DS3231 real-time-clock.

Overview

The solution consists of two parts: a circuit doing the actual power switching and secondly some programs to control the rtc.

The circuit reacts to three events

• a push of the power-on button: this will switch power on and boot the pi
• an alarm of the rtc: this will also turn power on
• a gpio-interrupt from the pi after shutdown: this will switch off power

The software-part takes care to program the alarm of the rtc before shutdown, so the system will boot at the requested time.

Hardware

On the hardware side, you will need a Pi (or compatible computer), a DS3231 and various components for the circuit. See doc/circuit.md for details. The total cost of all parts (execpt Pi) should be around 25€.

Note that the DS3231 is not a perfect solution for wake-on-rtc, since the alarms in the clock will fire on every match of day-of-month, hour, minute and seconds. So it is not possible to set the alarm to say May 6, but only to day six of the next month.

This limitation should be no problem on normal operation, but it is important to keep in mind that the time-horizon of the clock is only about a month.

Software

The software consists of a program to control the rtc and a system daemon which takes care of clearing alarms after boot and setting the alarm (next boot time) at shutdown. For installation, see the next section.

Installation

Clone the archive and then run the install-command from the tools-directory:

git clone https://github.com/bablokb/pi-wake-on-rtc.git
cd pi-wake-on-rtc
sudo tools/install

Besides changing some system configuration files, this command will mainly install three components:

• rtcctl, the control program for the real-time-clock
• wake-on-rtc.service, the systemd-service
• an udev-rule to update the system-time from the rtc at boot

Configuration

System configuration

The install-command will automatically configure the system. Nevertheless, please check the modified files /boot/config.txt and /etc/modules, especially if you don't start from a vanilla Raspbian.

Make sure you don't load the standard DS3231-driver from /boot/config.txt: you should remove or comment out the following line:

dtoverlay=i2c-rtc,ds3231

Initializing the real-time-clock

If your real-time-clock is not yet set, you have to initilize it once. Make sure that you have an internet connection so the ntp-daemon updates the system time. Check the system time and the time of the rtc:

date
sudo rtcctl show date

If the system time is correct and the time matches, you are done. Otherwise initialize the rtc once with:

sudo rtcctl init

Note that the standard linux tool hwclock will not work, since we don't expose the standard rtc-device interface of the rtc.

The install-command will add an udev-rule which updates the system time at every boot from the rtc. Also, at shutdown the current system time is written back to the rtc, but this is configurable.

Service configuration

The systemd-service wake-on-rtc.service is the central software component to manage the rtc. At boot time it will clear any existing alarms. At shutdown, it will set the alarm of the rtc to the next boot time.

The configuration of the service is in /etc/wake-on-rtc.conf. For a detailed explanation of all options, you should read the comments in the file. The most important options are in the sections [boot] and ¸[halt]`:

[boot]
hook_cmd: my_special_boot_script
auto_halt: 0

[halt]
next_boot: next_boot.sh
lead_time: 2
set_hwclock: 1

If you provide hook_cmd, the script will be called either with the parameter "normal" or "alarm", the latter only in the case the system was powered on due to a rtc-alarm. Use this script if you need to take special measures depending on the power-on mode.

With next_boot in the section [halt] you provide a program or script which returns the next boot time of the system as a single line of output. Valid formats are

• YYYY-mm-dd HH:MM:SS
• mm/dd/YYYY HH:MM:SS
• dd.mm.YYYY HH:MM:SS

You can leave away the seconds part and also use only two-digit years. If the next boot time is unknown, the program should just return a zero.

lead_time is interpreted in minutes and will be substracted from the boot-time returned by the next_boot script.

auto_halt is a feature to work around the limitation of the DS3231. If the wake-on-rtc service detects at boot time that the requested boot-time is actually not reached, it will shutdown automatically again.

To enable the feature, you have to set the value of auto_halt to a positve integer interpreted as minutes. If the next boot time is more than the given number of minutes in the future, the system shuts down again automatically. Setting the value of auto_halt to less than about 15 does not make much sense.

Usage

The command /usr/local/sbin/rtcctl is the main user-interface to the real time clock. With this command, you can read and write the RTC-time, set the alarm times, turn the alarms on, off or clear an alarm that has fired:

[root@pi2:~] # rtcctl help

Available commands (date and time are synonyms):
     help                                - dump list of available commands
     init                                - initialize RTC
     show  [date|time|alarm1|alarm2|sys]
                                         - display given type or all
     dump  [control|status|alarm1|alarm2]
                                         - display registers (hex/binary format)
     set   date|time|alarm1|alarm2|sys   - set date/time, alarm1, alarm2 times
                                           Format: dd.mm.YYYY [HH:MM[:SS]] or
                                                   mm/dd.YYYY [HH:MM[:SS]]
                                           (does not turn alarm on!)
     on    alarm1|alarm2                 - turn alarm1/alarm2 on
     off   alarm1|alarm2                 - turn alarm1/alarm2 off
     clear alarm1|alarm2                 - clear alarm1/alarm2-flag