I am in the early stages of designing a combo shield that encompasses a good portion of my project builds. I am looking for feedback before committing to the project in hopes that others may find the shield useful.
I envision a shield with:
RF24 socket, either low power or the higher power with antenna which I often use. I would include a 3.3v supply to avoid a common issue.
A UNIX time http://www.epochconverter.com as battery baked-up realtime hardware clock. Currently I fetch UNIX time from an NTP server and increment locally. This works well enough but other tasks can cause drift. I also wish to accurately increment UNIX time in milliseconds. Using a hardware UNIX time feature, I could accurately log data when network access was not possible. I despise currently available hardware chips. They only supply text strings that need converting to binary. I have only really needed to know date/time for an irrigation controller to meet local bylaws regarding odd/even days and time for watering. It's easy to compute local day, week day, date, hour, minute from UNIX time.
A SD card slot for logging data. What little experience I have had with SD and FAT 8.3 format makes me wish there was a better way to store binary data.
Remaining shield edge with six or so screw terminals for sensors.
Should I use SPI or I2C (wire) to fetch the UNIX time? SPI is my current plan. Should I use 5v or 3v3 logic? I do realize that using 3v3 will limit board compatibility. Selectable?