I don't check for seconds because there is the possibility that at second zero the server is busy. If it is not busy it will send data to thinghspeak at seconds zero. If it is busy, seconds will be not zero but, for example 20. If I check for senconds == 0 and in this second the server is busy, the data will not be sended to thingspeak because when the server will be free the seconds will be not zero.
So when
((minute1) % 15 == 0)
data is sended to thingspeak.
Then there is a flag, AcquisitionDone, that became true after sending the data to thingspeak.
The next time, if minute is still ==0, this
((minute1) % 15 == 0)
is true but this
(AcquisitionDone == false)
is false because now AcquisitionDone is true.
So no data is sended to thingspeak again.
When
((minute1) % 15 == 0)
became false, AcquisitionDone became false too so the next time that
((minute1) % 15 == 0)
will be true, AcquisitionDone is false and data is sended to thingspeak.
I wroted this some months ago for an SD datalogger because I have some of them and I want to have all data synchronized.
This send data only every 15 minutes and synchronize RTC with NTP only every 15 minutes too