Re: Real time clock + external interrupt in one PIC possible?
Timer1 can create an Interrupt when it "rolls over" from $FFFF to $0000. This creates TMR1IF = 1. Timer1 is a 16-bit counter/timer, which means it can reach 65,535d (Hex $FFFF) before going back to zero and tripping an Interrupt Flag. Since it is a 16-bit counter, the value in the counter needs 2 bytes, which are TMR1H(IGHBYTE) and TMR1L(OWBYTE). If you are running your OSC at 4 MHz, then your Fosc/4 = (4,000,000 / 4) 1,000,000 Instruction Cycles per Second. This equates to (1 / 1,000,000) 0.000001 second (1 us) time frame per Instruction Cycle. Options for Prescaler in the Timer1 SFR goes as high as 1:8. At max 1:8 you can get 8 us (0.000008 second) clicks on your Timer1. Since Timer1 maxes out at 65,535, you can run Timer1 up to 0.52428 seconds. This isn't a very "friendly" time frame to work with. However, it is slightly over a half second. If we calculate 0.5 seconds divided by our 0.000 008 second TMR1 clicks, we get 62,500 TMR1 increments yield 1/2 second (0.5 seconds).
Back to computerese, a byte = max 255 while a word = max 65535. The architecture of the 8-bit Microchip MCUs means each address holds 8 bits, or 1 byte. In order to assign 16 bits to the Timer1 counter, it takes 2 addresses, or 2 bytes. Microchip has decided to name the 2 bytes required by Timer1 "TMR1H" for the upper byte value and "TMR1L" for the lower byte value. Sometimes it's easier to deal with our familiar decimal system. PBP lets us do that. Sometimes it's easier to deal with binary, as in setting Special Function Registers. PBP lets us do that as well. Sometimes you are forced to work with hexadecimal. Well, I find working with Hex easier with a calculator that came standard on both my MAC OS and Windows 10. It has Programmer function that easily converts any format (decimal, octal, hexadecimal, or binary) to any format (ditto).
Back to your original question, since Timer1 trips a flag when it overflows, if you want a 1 second increment, it can't happen. [You could work with some of the slower factory oscillator speeds available in ALL PIC MCUs and do the conversion math for PBP.] Best you can get at PBP's slowest OSC speed is 0.52428 seconds. If you could work with 0.5 seconds, then all we have to do is start Timer1 part way through it's counting sequence so when it flips, we got a 1/2 second.
Review, 4 MHz OSC + 1:8 Prescaler X 65,535 gives us 0.52428. If we want 0.50000 then all we have to do is take 0.5 and divide it by 0.000 008 and we get 62500. We don't want to START there, we want to END there. So we must subtract 62,500 from 65,535 and get 3035. That's bigger than 255, which means it is bigger than a byte. In order to contain such a large number, it takes 2 bytes (which max out at 65,535). I like working with decimal since that's what I grew up using. However, PIC requires that this decimal number be separated into it's upper byte and lower byte. You could convert it to binary and get %0000 1011 1101 1011, or convert it to hex and get $0B DB. If you wanted to load TMR1 with binary you would:
TMR1H = %00001011
TMR1L = %11011011
Hex is shorter and accomplishes the same thing:
TMR1H = $0B
TMR1L = $DB
Of course you could do it with decimal and:
TMR1H = 11
TMR1L = 219
Bottom line, Timer1 will start counting at 3035 through 65,535 and trip a flag at the 1/2 second mark. With that flag (TMR1IF) you can direct your code to an Interrupt Handler to do whatever you require. Though lengthy, I hope this helps.
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