DS1307 returns incorrect data ???

[sinoteq]

Sure is.
But in your first example you did not write hex and then you only get 15 and not 21.

I would start by just writing 0 to all locations and from there start to read the DS once every second. I would do this to make sure I have a routine that can read and display the data in a correct way. When I am sure this is working I would set the DS to a specific time and date and test again.

You need the # in the serout function to convert the data in the variable to ASCII numbers. Without this you do not get nothing, you get a ASCII character that is difficult to detect but you do get something. Yes unless you send NUL

DS_CHECK:				I2C_Adr_B=0
						I2CREAD SDA,SCL,RTC,I2C_Adr_B,[Seconds,Minutes,Hours]
						t=Seconds &%00001111
						Seconds.7=0
						n=Seconds>>4
						Seconds=(n*10)+t
						I2C_Adr_B=0
						t=Minutes &%00001111
						n=Minutes>>4
						Minutes=(n*10)+t
						Hours=Hours &%00001111
						System_Time=(Hours*3600)+(Minutes*60)+Seconds
						DS_COUNT=0
						RETURN

This is one way to use the DS, maybe not the best and most code efficient but it works. Please note that it is only working to 9hours 59min 59 sec because I have not implemented the "10-hour" bit from the Hours register. The equipment I am using will never be on for more than a few hours and I am using the DS as a timer and not as a real RTC. I reset the RTC to 0 at startup.