Yes, I think RTC is needed for reliable operation. I have found out the solution by myself.
If anyone is interested hereby I share my SD card image. It has some extra features.
– Based on latest Raspbian Jessie Lite
– Supports Raspberry Pi Zero
– Easier wifi configuration
You do not need a wired LAN connection to configure wifi. The wifi configuration file is moved to FAT partititon accessible by Windows.
Just write the image on an SD card then open in a file manager and edit the wpasupplicant configuration file directly.
If your wifi stick is supported by default then Pi will automatically connect to your wifi network after power up.
– Raspberry on board watchdog is activated and handled by OSPI software for higher reliability
If OSPI software gets stuck for more than 15 sec then Raspberry will restart. The operation can be tested by issuing this command:
sudo /etc/init.d/OpenSprinkler.sh stop
– Control of relay on legacy Opensprinkler Pi boards.
To assign the relay to a station just configure the station as “RF” and set “0” as RF code.
– 1s resolution of timers
If you set the timer value by directly input numbers instead of using +/- signs then 1 sec resolution is accepted and stored at any long intervals
– Scheduled adjustments and update of RTC.
Maintainment of RTC
Download, unzip and write the image to an SD card. You can use win32diskimager tool for writing. Minimum SD card size is 2GB.
MD5 hash: 4094777bcf7577c56626a93eff32da8e
Open the SD card in the file browser and edit the wpa_supplicant.conf file
You have to edit the SSID and Wifi PSK then save the file