Looking at the code you posted earlier, this is what you cannot do. The code is copied from the earlier post, I've just added comments.
void SetAlarm(){
digitalWrite(RTC_CS, LOW);
//Set Alarm 2 to once per minute
SPI.transfer(0x8B); //first byte is always the address byte
SPI.transfer(0x80); //bytes 2-n are always data bytes. So this writes 0x80 to register 0x8B.
SPI.transfer(0x8C); //and since CS is still low, this is interpreted as a data byte and written to 0x8C.
SPI.transfer(0x80); //ditto, this gets written to 0x8D.
SPI.transfer(0x8D); //and this to 0x8E,
SPI.transfer(0x80); //and this to 0x8F,
//Enable Alarm Interrupt 2
SPI.transfer(0x8E); //and since CS has never gone high, this writes 0x8E to 0x90, which is the Crystal Aging Offset
SPI.transfer(0x1E); //and this attempts to write 0x1E to the Temperature MSB at 0x91, but that's a read-only register, so probably nothing happens.
digitalWrite(RTC_CS, HIGH);
}
Now, if you could try this way:
void SetAlarm(){
digitalWrite(RTC_CS, LOW); //start a write operation
//Set Alarm 2 to once per minute
SPI.transfer(0x8B); //send the address byte
SPI.transfer(0x80); //write 0x80 to register 0x8B
SPI.transfer(0x80); //and to 0x8C
SPI.transfer(0x80); //and to 0x8D
//Enable Alarm Interrupt 2
SPI.transfer(0x1E); //write 0x1E to the control register (0x8E) to enable Alarm 2 interrupts
digitalWrite(RTC_CS, HIGH); //end of the data write
}
We could do all that in one operation because we were writing to four contiguous registers. If they weren't contiguous, something like this would work (and this will still work even if they are contiguous):
void SetAlarm(){
digitalWrite(RTC_CS, LOW); //start the first write operation
//Set Alarm 2 to once per minute
SPI.transfer(0x8B); //send the address byte
SPI.transfer(0x80); //write 0x80 to register 0x8B
SPI.transfer(0x80); //and to 0x8C
SPI.transfer(0x80); //and to 0x8D
digitalWrite(RTC_CS, HIGH); //end the first write operation
//need to wait at least 400ns in between read/write operations, so we'll wait 1 microsecond, which is 1000ns.
delayMicroseconds(1);
//Enable Alarm Interrupt 2
digitalWrite(RTC_CS, LOW); //start the second write operation
SPI.transfer(0x8E); //this is the register address
SPI.transfer(0x1E); //write 0x1E to the control register (0x8E) to enable Alarm 2 interrupts
digitalWrite(RTC_CS, HIGH); //end the second write operation
}
Hope that helps. Again, I'd forget the interrupts for now, and just try to get it to work playing with the time registers. Once you can read and write them, the others are just the same except for different addresses.
