Real time clocks, out of the question?

[Dave K.]

Originally Posted by evan_wilson

Alright. I have another question about how the timer variables work in all of these. I've used storage classes/qualifiers occasionally but I don't understand the significance of using, say, extern and volatile to store the timer variables. Could anyone shed some light on this?

'extern' is much like a function prototype, meaning that it is allowing you declare the variable within the local scope such that the compiler will know what the variable type is and can generate the appropriate code. When the linker pulls all of the object files together, it will resolve the external dependancies of each of the object files, and if you haven't declared the variable itself anywhere within the program, will generate an error and abort.

'volatile' tells the compiler that the variable may change on its own and to generate code as appropriate. For optimization purposes, a compiler may read a variable from memory into a temporary register, and subsequent read/write operations to that variable _may_ only occur to the register without updating the memory location. Telling the compiler that the variable is volatile causes the compiler to re-read the variable's memory location each time it is used, and to write it back to memory each time it is updated.

'volatile' would be used for things like hardware periphial registers, as well as things like variables that might change inside of an interrupt routine.

Since the compiler can't know why you are telling it that the variable is volatile, it wouldn't generate additional instructions to disable interrupts during critical regions to ensure that a multi-byte variable read/write operation is performed without the potential for another interrupt routine corrupting the operation. Pre-emptive multi-tasking operating systems generally pose the same potential for corruption that an interrupt routine does.

The 'static' qualifier allows variables to be declared within a local, rather than global, scope and simply tells the compiler to declare the variable in a memory location that is static, rather than allocating it on the stack frame (or emulated equivilent thereof) of the local function.

Any variable declared outside of a function is normally considered a global variable, but unless other source files provide an 'extern' reference, the variable's use will remain limited to that source file, and is sometimes referred to as a 'local global'. The caveat here is that if you were to declare another 'local global' in another source file, the linker will (usually) have a problem with the duplicate declarations, and generate an error.



Recognizing that Microchips documentation already presumes a certain familiarity with the "C" language and doesn't really get into all of the specifics, for those that already have a general grasp on the "C" language, two books that I would recommend are:

The C Programming Language by Brian Kernighan and Dennis Ritchie, who are the origional authors of "C".
A book on C by Al Kelly and Ira Pohl.

K&R's book is very terse with minimal examples, but is the bible. The 2nd one is also a bit terse, and goes into depth more than K&R's book.

I've included links to Amazon's website as a convienent reference, but retailers such as Barnes & Noble also generally carry these and other good reference books where you can browse before you buy.

[evan_wilson]

Strangely enough, K&R is sitting right here on my bookshelf. Somehow I hadn't remembered "volatile". I'll go reread through the data types/variables stuff again.

Not to seem pestery... but I've once again confused myself in this code. I read through the PIC's datasheet (timer section) and studied up on the registers. On the good side, I now completely understand all the scalers, clocks, and flags, but I could not seem to understand how the high and low byte registers worked/were needed. I don't see how an integer could go past 65,535, so why does the high byte happen to be able to have a value over that?

I also could not find an equivalent for the PR2 register for Timer1. Is that simply not present and do I have to use conventional math to downscale further?

Thanks again for the help.

[Dave K.]

Originally Posted by evan_wilson

Strangely enough, K&R is sitting right here on my bookshelf. Somehow I hadn't remembered "volatile". I'll go reread through the data types/variables stuff again.

Not to seem pestery... but I've once again confused myself in this code. I read through the PIC's datasheet (timer section) and studied up on the registers. On the good side, I now completely understand all the scalers, clocks, and flags, but I could not seem to understand how the high and low byte registers worked/were needed. I don't see how an integer could go past 65,535, so why does the high byte happen to be able to have a value over that?

I also could not find an equivalent for the PR2 register for Timer1. Is that simply not present and do I have to use conventional math to downscale further?

Thanks again for the help.

Timer's 2 & 4 are 8 bit wide timers and have the "PR" registers, the other timers are 16 bit.

Timer 1 is a bit special and can be used for a low power, real time clock function.

Without a specific reference, I'm not sure what you mean about the high byte having a value greater than 65535. My only guess would be that by setting a value to zero, you'll get an effective divide rate of 65536, but I don't immediately see that in the documentation.

The reason that the 16 bit counter is split into two 8 bit registers is that the CPU only operates on data 8 bits at a time. So a 16 bit read/write operation requires two data bus operations to manipulate the 16 bit value.

Does that help or did I miss one or more of the questions?

[evan_wilson]

Ahhh, that explains things a bit. I should have thought about that. RD16 two bytes wide, duh.

Thanks for that tidbit.

And the high and low bytes. In the wallclock code (http://www.chiefdelphi.com/forums/sh...t=wallclock.c), the high byte value is.... NEVER MIND

Someone can't count their zeros.

I still don't exactly understand what the purpose of the high and low bytes are. Is it like an loop condition? Above the max value for a 16bit number counts as an overflow and it resets, setting the flag with it?

THANK YOU

[Dave K.]

Originally Posted by evan_wilson

Ahhh, that explains things a bit. I should have thought about that. RD16 two bytes wide, duh.

Thanks for that tidbit.

And the high and low bytes. In the wallclock code (http://www.chiefdelphi.com/forums/sh...t=wallclock.c), the high byte value is.... NEVER MIND

Someone can't count their zeros.

I still don't exactly understand what the purpose of the high and low bytes are. Is it like an loop condition? Above the max value for a 16bit number counts as an overflow and it resets, setting the flag with it?

THANK YOU

One of the interrupt options is to interrupt on timer overflow, and that is how some of the other examples posted work.

If you:

- stop the timer
- load a 16 bit value into the timer register
- reset the interrupt flag
- start the timer

You will get an overflow interrupt in however many timer ticks it takes for the pre-load value to reach the point of overflow (0xffff -> 0x0000).


Having the timer value available could also be useful for calculating the period between two events, such as a transition on an input pin that triggers an interrupt. The input interrupt could read the current timer value, and subtract the timer value of the last input transition to calculate the amount of time elapsed since the last input interrupt.

Ideally, one would use something like the CCP module to time stamp an input transition, but in the case of IFI's RC, those pins aren't available as for user Digital I/O pins.

As an example, for something like motor velocity control, at slow speeds, if a rotational speed slower than 1 pulse per sample period was desired, you could use period measurement to gain some additional resolution, and above a certain rotational speed, you could switch to counting the number of pulses per period method.

Each microcontroller manufactuer has their own ideas on the degree's of flexibility that they make available. Microchip's 8 bit PIC family is pretty average. Some parts have less flexibility, others have quite a bit more. In this case, you have to do the best you can with what they give you...

[Nathan]

I thought the WPI library had built in timers? What's the difference between those and "real time" timers?

Thanks,
Nathan

[Astronouth7303]

Setting up just a realtime clock isn't that hard. I was thinking of doing it myself. I just have to figure out how to configure which timer to achieve something reasonable.

[Mike Bortfeldt]

Here is the code we used for a "clock". It uses Timer 2 and has a number of options for flexibility. It is not a "real time clock" in the sense that it doesn't compute hour/minute/second but works off clock "ticks", which for a 2 minute match, or a 15 second autonomous, works fine for us. By default, it's configured to count milliseconds (1 tick = 1 millisecond), but has 2 other predefined options for increased accuracy.

Mike

[Mark McLeod]

In teaching timers I use a demonstration program to calculate all the possible timer prescaler/postscaler combinations for a given Hertz or time period. It's an easy beginner exercise to write such a program for the PIC (or VB), and also can be used to teach them about maintaining the Master processor communication leading to an easy elimination of the Master handshake.

Here's a sample output calculating 2400 Hz used with an LCD display off a digital output.