Using a Crystal just for Timer1

SkeeterHawk · Joined: 16 Oct 2010 Posts: 64 Location: Florissant, CO

Hello Friends,

I want to make sure that I am on the right track, so I would appreciate your input.

What I want to do is simple. I have a PIC18F57Q84 that I would like to have an accurate timer on so that I can count every 10mS with only a small amount of deviation in about 15 minutes or so. After doing some research, it appears that Timer1 is the one to use. To use the internal oscillator for the system clock, and only use the crystal on Timer1, is this how I would configure it?

Drop crystal across RA6 & RA7

temtronic · Posted: Thu Jul 16, 2026 4:47 pm

I had a very quik look at the data sheet....
don't see how you can 'just add a crystal as the ext clk to timer 1
1st it's a PPS PIC....
2nd no 'clk osc' section ahead of the timer1 input.

I suggest using a real crystal+2 caps for the main PIC clock, then using it as the input to timer1

This gives you a very stable 'main clock' so ALL PIC timings will be accurate, doesn't cost any 'real estate' either !

Others that use that PIC , or family will know more than my 'gut'.

SkeeterHawk · Joined: 16 Oct 2010 Posts: 64 Location: Florissant, CO

Thank you very much for interjecting, Temtronic!

I must admit that I "cheated" at the onset and asked AI this question. Here are my thoughts, and is why I am asking you more experienced developers on here to make sure that I am on the right track.

In my datasheet for the PIC18F57Q84 (DS40002213F) it has Timer 1 as chapter 25. In the initial bullet points (P.405), the second bullet says

Ttelmah · Joined: 11 Mar 2010 Posts: 20110

You can do this.
First thing though is that the XT fuse is not wanted.
What you do is use the secondary oscillator. This chip offers routing this
to other peripherals (on most chips it'll only go to Timer1 by default).
You need the fuse RSTOSC_SOSC which enables the secondary oscillator.
This is then routed to Timer1 using T1_SOSC in the timer1 setup.
This oscillator uses the SOSCI and SOSCO pins that are not remappable.
Pins RC0 and RC1. You cannot put these on RA6/7,
The SOSC pins are special. Offering very low power operation and a low drive
for a 32KHz crystal.

You could do it using an external oscillator, then your EXTOSC setting would
be right, but you would need an oscillator module not just a crystal.

SkeeterHawk · Joined: 16 Oct 2010 Posts: 64 Location: Florissant, CO

Thank you very much for your input, as always, Ttelmah!

I was planning to use a 4MHz crystal (Sorry I didn't mention that detail in my original post), and I was under the impression that the SOSC could only power a 32.768kHz crystal and I wouldn't be able to get an accurate 10mS tick from that frequency.

I can use an oscillator module if that is my option. I would rather keep this counter internal to the chip than trying to find a real time clock that will give me 10mS ticks since this is a pretty straight forward application. I'd prefer not to add a bunch of bus traffic to my timing because the timing is critical on this application.

Is it true that I can't run a 4MHz crystal on either the SOSC or the EXTOSC and route it to Timer1, would I be running an external oscillator to SOSCIN (RC1) or to CLKIN (RA7)? I am assuming that it is RA7 since you said to keep the EXTOSC. So, if I am required to use an external oscillator, I am thinking that CB3-3C-4M000000 from CTS (Digi-Key P/N 110-CB3-3C-4M000000CT-ND) would be a good choice.

The goal here is to have a really accurate timer that counts 100 "ticks" a second and is accurate to at least 15 minutes, but preferably to 30 minutes. I want to have this as an internal timer so I can clear the counter and start the timer counting at exactly the same time as I toggle an I/O pin. If it is better to use another timer and possibly an external trigger source, that would be good too (Like TOCKI/T0CKIPPS on Timer0). I don't really care how it gets done as long as it is accurate.

Thanks so much for your input to make sure that this application works out of the box!!
_________________
Life is too short to only write code in assembly...

Ttelmah · Joined: 11 Mar 2010 Posts: 20110

Yes, the secondary oscillator can only power a 32KHz crystal.
Why would you want to use 4MHz????
Uses more power, and generally will not be as accurate. The 'point' about
the 32KHz crystals is that they are a different cut from standard crystals
that offers better temperature stability and accuracy, without having to
go to things like TCXO's. Hence for 'timers' they are the preferred crystal.

gaugeguy · Joined: 05 Apr 2011 Posts: 365

If you need a 10ms interrupt then a 32kHz crystal will not work well.
You have not stated what your definition of accurate is. Crystal accuracy is usually given in parts per million (ppm) and temperature deviation in ppm/'C
Accurate over 15 minutes does not really give a definition of what the level of error tolerated would be.

gaugeguy · Joined: 05 Apr 2011 Posts: 365

Also instead of loading timer1 in the interrupt it will be more accurate to use the compare function and update the compare value in the interrupt (add the number of counts you want to the current compare value). That way the timer is free running. When writing the timer in the interrupt you need to figure in the interrupt latency and time to execute the write as well.

temtronic · Posted: Fri Jul 17, 2026 1:49 pm

OK, I/m curious..

Why not use the 4MHz xtal as your primary clock, then connect Timer1 to it 'somehow' ?

This will give you very accurate timing for the entire program.

SkeeterHawk · Joined: 16 Oct 2010 Posts: 64 Location: Florissant, CO

temtronic · Posted: Sat Jul 18, 2026 5:17 am

hmm, guess I wrote it wrong.... I meant to say use the 4MHz xtal ,but configure the 'primary clock section' to run at 64MHz. Some combination of PLL may allow that ? Might nee a faster xtal...have to ,ugh, read, the databook to see if it's possible.
My real point is to use a xtal as the 'main' clock' ,as it'll provide better timing for everything.

edit.... looked at the 'clock' internals.....
if you use a 16 MHz xtal and X4 PLL, you'll get a solid 64 MHz clock....