Timer 1 , 2 ---> 18F452

TaiChipY · Joined: 11 Mar 2005 Posts: 21

I saw the timers topic but this has solved party my problem...
I need just some explanation on part where im using two timers...

First timer is used like countdown and its working on 16F877.
Since i added the second timer and since the speed / MCU has been changed,i have some problems with displaying the value ( time ) of the first counter. Counted time value is wrong and i have had small problems with second timer ( fixed ), becuse of the first counter...

Im using 8MHZ clock on 18F452 and both internal timers.
This is the code part:

Ttelmah · Guest

Unfortunately, the 'key thing', is to forget setting timer1 to a value!...
The problem is that timer1, is counting in instruction times. When your code arrives in the interrupt handler, there will typically have been at least 25 instruction times from the moment the interrupt occurred, so setting the timer to a value, will be in error by this interval. Worse, the interval will change. With two interrupts present (doesn't have to be a timer, could be a serial interrupt, or any other source), if the timer interrupt occurs once the code has already entered the other interrupt handler, the whole of this handler will have to execute, then the code to restore the registers, then the return from interrupt, followed by the next interrupt call, and it's overhead to save the registers. Even with a quick interrupt handler, the error will jump to perhaps 75 to 100 instruction times in the worst case...
Do a search on this forum, for posts on handling a clock interrupt, and particularly a RTC. There has been a version published in the past, which shows a very efficient way (using an integer addition), to generate an accurate timing for a system 'tick', without tweaking the timer counter value. Alternatively, change timers, and use the hardware ability to reset the timer, available from the CTC. Realise also, that the RTCC, with a /16 prescaler, will give the same rate as timer1, with the tweak you show.
You can reduce the error, by adding an offset to the timer value (rather than an absolute value), but this is best applied when dealing with a timer 'ticking' at a much slower rate, than the instruction clock speed.

Best Wishes

TaiChipY · Joined: 11 Mar 2005 Posts: 21

Unfortunately, the 'key thing', is to forget setting timer1 to a value!...

**
aaaaaaaaaaaaaaaaaaaaaaaaa, sorry my mistake :--)

Thanx anyway !

TaiChipY · Joined: 11 Mar 2005 Posts: 21

I have tested three timers 0,1,2.
and regarding to last post i made this changes.

Instead Timer1 am using ccp1.

Timer0 is used for 10 seconds timeout
Timer1 for countdown --> ccp1
Timer2 for button clicks interrupts

Q.
Can i use ccp1 for displaying the countdown (so i can work normaly with no conflicts with other timers) ? Any idea would be good....

Just an example:
This are settings for 16F917 chip:

TaiChipY · Joined: 11 Mar 2005 Posts: 21

Can someone take a look and tell my if this is possible to make this on the described way ( with ccp )..

Thank you in advance.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

I'm not really sure what you want to do. Do you want to use the CCP1 to generate an interrupt every second? If yes, then how do you want to feed the CCP1 counter? With an external clock pulse or from one of the internal ltimers? If you are thinking about using one of the internal timers like in your first example, then using the CCP1 is overkill and a waste of resources.

Like Ttelmah already told you, search this forum for the keyword 'RTC' and you will find some very nice examples (I know because I reposted a nice example from Neutone a few times and am too lazy to do it again now. Sorry).

Please post a complete program, several variable declarations are missing now, for example sekunde.

TaiChipY · Joined: 11 Mar 2005 Posts: 21

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Your timer2 looks like my handy work. There is really no need to have all the other timers since you have such a small "system tick". You could just count to 1000 in the timer2 to get seconds or even do it in the main loop. You can either add the Miliseconds to another variable and anytime it is over 1000 increment the seconds and subtract 1000 from the counter or just have a loop while (Miliseconds) and increment the count by one.

TaiChipY · Joined: 11 Mar 2005 Posts: 21

Hi Mark!
Im realy sorry for so much time taken for replay!
Im making some experiment with segmented LCD.

Your timer2 looks like my handy work
Yep this is your part of code.

Ok. I have done like you suggested and its OK accept timing part ).

Q1. Timer1:
clock = 8MHZ
8MHZ / 4 = 0.5 uS
1 seconds = 2000000
2000000 / 256 = 7812,5
If my counting is correct this should count (correctly) seconds and substract hours. But :--) how to define this in the timer1 ( becuse of the precision ).

TaiChipY · Joined: 11 Mar 2005 Posts: 21

... i forgot to ask just one more thing.
Becuse the switch part is made thru timer2, and wenn chip goes to sleep,
timer 2 is inactive, can i wakeup the chip thru button click ??.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Do you really need a millisecond counter? Counting milliseconds when you are only using timeouts in multiples of whole seconds gives a lot of overhead.

Timer1 is one of the few devices still operating in sleep mode, but ONLY when the clock pulse is provided by an external clock or crystal connected to timer1. Is this the way you have setup your hardware? Your call to setup_timer_1 is using the internal timer.............

I have the feeling that you are creating a way more difficult design than nescessary. I have no clue as to what your system is supposed to do , so I might be wrong. It might help if you tell us what your application is supposed to do.

When your application requires an accurate clock but also low power consumption then you have the following options:
1) Your current 18F452 with a 32768 Hz crystal connected to timer1
2) Using the watchdog for timekeeping during sleep (less accurate).
3) Use an external RTC like the DS1305 (SPI bus)
4) Choose another processor from the Microchip nanowatt line with inbuilt RTC.

Have you realized the 18F452 is not recommended for new designs and more expensive than most newer chips with more capabilities?

TaiChipY · Joined: 11 Mar 2005 Posts: 21

Im using 16F917 and i want to make low power application.
This are the infos from 16F917 Timer1.

• 16-bit timer/counter (TMR1H:TMR1L)
• Readable and writable
• Internal or external clock selection
• Synchronous or asynchronous operation
• Interrupt-on-overflow from FFFFh to 0000h
• Wake-up upon overflow (Asynchronous mode)

In Timer mode, Timer1 is incremented on every
instruction cycle. In Counter mode, Timer1 is incremented
on the rising edge of the external clock input T1CKI.
Timer1 can use the LP oscillator as a clock source.
-----------------------------------------------
To minimize the multiplexing of peripherals on the I/O
ports, the dedicated TMR1 oscillator, which is normally
used for TMR1 real time clock applications, is eliminated.
Instead, the TMR1 module can enable the LP oscillator.
-----------------------------------------------
Sleep mode does not shut off the LP oscillator
operation (i.e., if the INTOSC oscillator runs the
microcontroller, and T1OSCEN = 1 (TMR1 is
running from the LP oscillator), then the LP
oscillator will continue to run during Sleep mode.
------------------------------------------------
When INTOSC without CLKO oscillator is
selected and T1OSCEN = 1, the LP
oscillator will run continuously independent
of the TMR1ON bit.
-------------------------------------------------------------
I have no external clock becuse i thought that i can access to internal clock as it was described before…

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

It looks like a nice processor with Nanowatt tecnologie and LCD-driver logic built in. The Timer1 clock driver in this chip is different from what I've seen in other chips, and yes it seems perfect for running in low-power applications. Use a 32768 external crystal in LP-mode during sleep and for high performance switch to the internal clock oscillator.

I guess you can use this processor but I only have hands on experience with the PIC18F458 and this processor is very different, so don't take my word for it.

Just two final remarks:
1) In the Microchip product selector list this particular chip is listed as a 'future product'. PIC16F913 and PIC16F914 are in production.
2) 14336 bytes of program memory equals 8192 instructions. This is not much and currently no higher capacity models are available, be aware of this risk.

TaiChipY · Joined: 11 Mar 2005 Posts: 21

Well i thought that i solved this one out, but :--).

My timer1 is too fast (cca 1 min. == 122 sec.)
and my Timer2 can't get the right count (not near).

MCU is running on 8MHz and this is what i have tryed:

I have defined new prescaler (in devices dir )... no effect.
I have set the calculated value in the set_timer1... no good (infact, no changes)

I thought to access directly thru registers -- didn't try

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands