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Advice needed...LED multiplexing...timer0

 
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KamPutty
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Advice needed...LED multiplexing...timer0
PostPosted: Tue Jan 10, 2006 10:22 am     Reply with quote
hi all,

Hope all is well with you.

I need some advice (what?! again?!)

- My project contains 8 7-segment LED's
- I am using a CC LED's
- I am using a picf452@4mhz
- My code can update the LEDs

<question>

Now the issue is this, I need to update the LED's "n" times a second and I was going to do this via the timer0. Running at 4mhz, that means something like only 15 times a second that the timer0 is hit. I am now thinking that I need to speed up my processor (lets say 20mhz) so I can get approx 75 times the timer0 hits per second.

Now am I going this the wrong way? (BTW, I have chosen NOT to use the 7219, for now(?)...)

All the interrupt needs to do is display/update the LED's, it does NO calculations, processing etc other then toggling the LED's...

Any advice would be greatly appriciated!

Thanks all,

~Kam (^8*
MikeValencia



Joined: 04 Aug 2004
Posts: 238
Location: Chicago

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PostPosted: Tue Jan 10, 2006 10:41 am     Reply with quote
I am doing something similiar:
PIC16F747, internal osc. x 4 (PLL x 4) (thus 32 mhz)
4-segment LED common anode display

I use the following to setup my my timer0 isr:

Code:

    int_count = INTS_PER_SECOND;
    set_timer0(0);
    setup_counters( RTCC_INTERNAL, RTCC_DIV_128 | RTCC_8_BIT);
    enable_interrupts(INT_RTCC);
    enable_interrupts(GLOBAL);


This amounts to
(4 * 128 * 256) / 32000000 = .004096 sec per interrupt

For a 4-digit led display, this 4ms refreshing seems ideal. Maybe it's okay for your 8-digit display, or maybe you'll need to do an RTCC_DIV_64.
Notice that i'm using the one of the cheapest possible PICs and not using a crystal.

FYI, my isr looks like this: ('x' goes from 0 to 3, and it marks which digit i am currently refreshing)

Code:

#INT_RTCC
void timer0_isr(void)
{
    static int x = 0;

    if (!GLOBAL_inhibit_all_leds)
    {
        port_a = ca_io[x];
        port_d = GLOBAL_character[x];
    }
    else
    {
        port_a = 0xFF;  // all leds off
    }
 
    x++;
    if (x == 5)
    {
        x = 0;
    }

    if (--int_count == 0)
    {
        ++seconds;
        int_count = INTS_PER_SECOND;
    }
    GLOBAL_led_counter++;
}
MikeValencia



Joined: 04 Aug 2004
Posts: 238
Location: Chicago

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PostPosted: Tue Jan 10, 2006 10:51 am     Reply with quote
I read your post carefully again...
Given that you are using a 4MHz crystal on the PIC18F452, and let's say you will need 2ms to strobe thru each display:

Code:

    set_timer0(0);
    setup_counters( RTCC_INTERNAL, RTCC_DIV_8 | RTCC_8_BIT);
    enable_interrupts(INT_RTCC);
    enable_interrupts(GLOBAL);


This amounts to
(4 * 8 * 256) / 4000000 = .002048 sec per interrupt
KamPutty
Guest
PostPosted: Tue Jan 10, 2006 11:00 am     Reply with quote
Mike,

Thanks for the replay,

This is what I currently have in setup:

Code:

    set_timer0(0);
    setup_counters(RTCC_INTERNAL, RTCC_DIV_256 | RTCC_8_BIT);
    enable_interrupts(INT_RTCC);
    enable_interrupts(GLOBAL);


Do you think you can explain the RTCC_DIV_??? arg. It just has not "clicked" in my brain.

~Kam (^8*
KamPutty
Guest
Success!
PostPosted: Tue Jan 10, 2006 11:35 am     Reply with quote
Mike, et al.,

100% success! I just changed to rtcc_div_8 and viola!

Here is my isr code...

Code:

#int_timer0
/**
 * This method is the timer0 isr
 */
void timer_0_isr(void)
{
    // lets turn off the current LED
    pinIO(PORT_C+currentActiveLED, OFF);

    // go to next LED and display
    currentActiveLED++;
    if(currentActiveLED>=LED_COUNT)
    {
        currentActiveLED=0;
    }

    portIO(PORT_B, LEDValues[currentActiveLED]);    // Set the segments
    pinIO(PORT_C+currentActiveLED, ON);  // turn on digit (using Common Cathode LED's)
}


Thanks for the suggestions

~Kam (^8*
newguy



Joined: 24 Jun 2004
Posts: 1908

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PostPosted: Tue Jan 10, 2006 12:34 pm     Reply with quote
KamPutty wrote:
Do you think you can explain the RTCC_DIV_??? arg. It just has not "clicked" in my brain.


http://www.ccsinfo.com/forum/viewtopic.php?t=22467

That should help explain it.
kam



Joined: 15 Jul 2005
Posts: 59

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KamPutty
PostPosted: Tue Jan 10, 2006 12:59 pm     Reply with quote
Thanks!
MikeValencia



Joined: 04 Aug 2004
Posts: 238
Location: Chicago

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PostPosted: Tue Jan 10, 2006 4:07 pm     Reply with quote
KamPutty wrote:
Mike,

Thanks for the replay,

This is what I currently have in setup:

Code:

    set_timer0(0);
    setup_counters(RTCC_INTERNAL, RTCC_DIV_256 | RTCC_8_BIT);
    enable_interrupts(INT_RTCC);
    enable_interrupts(GLOBAL);


Do you think you can explain the RTCC_DIV_??? arg. It just has not "clicked" in my brain.

~Kam (^8*


Think of it this way:

(4 * X * 256) / Fosc = time between tmr overflow interrupts [seconds]
where X is the post-divider value (e.g. RTCC_DIV_256); the 256 in the 3rd term is if you're using RTCC_8_BIT,

Otherwise it should be
(4 * X * 65536) / Fosc if it's RTCC_16_BIT
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