PBP projects for R/C models

[Kenjones1935]

Can anyone point me to a definition of "prescaler".
What I thought it meant does not seem to jive with what I am seeing.

Ken

[Kenjones1935]

"Running Small Motors with PIC Microcontrollers" says, on page108

"A pre-scaler is applied to the system clock and affects the timer by slowing down the system clock as it applies to the timer. Normally the timer is fed by a fourth of the basic clock frequency, which is called Fosc/4. In a system running a 4 MHz the timer sees a clock running at 1 MHz. If the pre-scaler is set for 1:8, the clock will be slowed down by another eight times, and the timer will see a clock at 125kHz."

"The post-scaler setting determines how may overflows will go by before an interrupt is triggered."

If I pre-scale TMRO 1:256 I will get overflows at about 4K per second. If I add a post-scaler of 1:64 I will end up with about 62 interrupts per second.

Close enough for government work?

Ken

[Kenjones1935]

I just found in DS33023 section 11.6.

"There is only one pre-scaler available which is mutually exclusively shared between Timer 0 and Watchdog Timer."

So I have to count the 4000 Interrupts per second using a

FOR CNT To 80
NEXT

loop. That will give me much closer to 50 pulses per second.

Correct?

[rmteo]

See my post #84 above. Which part is unclear to you?

[mackrackit]

Hi Ken,

I always get confused with this part of things too, so I will just point you to a couple of threads that I refer to when the need arises.

I think maybe the parts about
TMR1L
TMR1H
are the things you need to look at???

http://www.picbasic.co.uk/forum/showthread.php?t=961
http://list.picbasic.com/forum/messa...tml?1049090059

[Bruce]

You could simplify this a great deal if your PIC has Timer1 available, and Compare,
Capture, PWM .. by just using the Compare peripheral.

Here's an example:

    DEFINE OSC 4

    Match VAR WORD
    
    Match   = 20000         ' 20,000 * 1uS = 20mS until compare match
    CCPR1H  = Match.HighByte' Load compare register high
    CCPR1L  = Match.LowByte ' Load compare register low
    CCP1CON = %00001011     ' Compare mode, auto reset Timer1
    
    '/ TRIS & Port Setup
    PORTB.0=1
    TRISB = $FE  ' PORTB.0 output, rest inputs
    
    '/ disable interrupts
    INTCON.7 = 0 ' Disable global ints
    INTCON.6 = 0 ' Disable peripheral ints
    PIE1.0   = 0 ' Disable Timer1 int
    PIE1.2   = 0 ' Disable CCP1 int

    T1CON = %00000000 ' Timer1 off, internal clock, 1:1 prescale
    TMR1H = 0         ' Clear Timer1 high
    TMR1L = 0         ' Clear Timer1 low
    
    T1CON.0 = 1  ' Turn on Timer1

LOOPS: ' NOTE: PIR1.2 is the CCP1IF compare interrupt flag bit.
       ' this bit is set when Timer1 reaches the value in
       ' CCPR1L & CCPR1H
    IF PIR1.2 = 1 THEN   ' If CCP1 to TMR1 compare match occured
       PORTB = PORTB ^ 1 ' Toggle PORTB.0
       PIR1.2 = 0        ' Clear compare interrupt flag bit
    ENDIF
    GOTO LOOPS

    END

Just 1 single interrupt could take care of your 20mS timing VS a bunch to accumulate the
total 20mS time period.

A single interrupt every 20mS & no reloading anything. Timer1 is automatically cleared on
match, and the CCPR1L/CCPR1H registers contain the value loaded into them until you
change it.

[rmteo]

For 20mS timing, you can do it with a single interrupt with any of the 3 timers on the PIC16F887.

Timer0 - Set prescaler to 1:256
Timer1 - As shown above by Bruce
Timer2 - Set prescaler to 1:16, set postscaler to 1:16