Program a timer for a fixed interrupt period of 10 msec. This is same as 100 Hz frequency.

OK note that in 5 minutes there are 5*60 = 300 seconds. In 300 seconds there will the 300 * 100 = 30000 of the timer interrupts.

The number 30000 fits nicely in to a two byte sized variable.

3 minutes and 1 minute are counts of 3*60*100 and 1*60*100 or 18000 and 6000 respectively.

So have your program setup the timer interrupt to decrement a two byte counter starting from 30000 and going down to 0. At each time the count goes to zero reset it back to 30000 so you can repeat the cycle. At each interrupt time check the decremented count value for equality to 18000 and 6000 and at these times trigger your desired port bit.

Michael Karas