Stopping and re-starting a timer

kgng97ccs · Joined: 02 Apr 2022 Posts: 46

The PIC18LF46K22 datasheet has this paragraph in Section 12.5.1 (READING AND WRITING TIMER1/3/5 IN ASYNCHRONOUS COUNTER MODE):
"Reading TMRxH or TMRxL while the timer is running from an external asynchronous clock will ensure a valid read (taken care of in hardware). However, the user should keep in mind that reading the 16-bit timer in two 8-bit values itself, poses certain problems, since the timer may overflow between the reads. For writes, it is recommended that the user simply stop the timer and write the desired values. A write contention may occur by writing to the timer registers, while the register is incrementing. This may produce an unpredictable value in the TMRxH:TMRxL register pair."

Questions:
1. How does one stop and re-start Timer1, or for that matter, any other timer, without accessing the register directly and without affecting the other settings of the timer?
2. Is it correct that if we use the get_timer1() function, which returns a 16-bit integer, the above read problem will never happen?
3. Similarly, is it correct that if we use the set_timer1() function to write a value to Timer1, the above write problem will never happen?

PCM programmer · Joined: 06 Sep 2003 Posts: 21644

For question #1, see the #int_timer1 routine in this post:
http://www.ccsinfo.com/forum/viewtopic.php?t=45458&start=5

PCM programmer · Joined: 06 Sep 2003 Posts: 21644

For question #2:

Ttelmah · Joined: 11 Mar 2010 Posts: 17981

And, on '1', this depends on the timer, and what you actually 'mean'.
For example if you look at Timer3, there is an 'on' bit, but it makes the
comment (for off), "Clears Timer1/3/5 Gate flip-flop". So turning this
'off' (which will stop the timer), also clears this flip-flop. Means you cannot
stop it without this changing.
Timer0, does not have this problem, and this has a T0_OFF setting.
Generally, you don't stop timers. Just reset them, read and write them.
If you want to stop their 'effect', just disable the interrupt from them.

kgng97ccs · Joined: 02 Apr 2022 Posts: 46

Thank you, PCM Programmer, for the link and for directing me to the assembly code. I am not familiar with assembly language but will make an effort to understand the code.

Thank you, Ttelmah, for your advice regarding stopping a timer and for highlighting its effect on the Timer1/3/5 gate flip-flop. I have taken note of that.

If you know of any good resources for learning assembly language relevant to PIC MCU programming, please do let me know. Thank you.