SOLVED - How to calculate Timer preload

[Demon]

From Art's Timer thread here:

https://www.picbasic.co.uk/forum/sho...978#post131978

Hi Guys,

...For the 20MHz example where the constant is 03B7h, it's the reverse of this:

FFFFh - 03B7h = C348h (49992 decimal),
49992 + 8 = 50000 (it must take eight instruction cycles to reload the timer)
50000 / 10000 = 5
5 x 4 = 20MHz

...Cheers, Art.

If I reverse these steps for a 16F1936 at 32MHz, I get this:

32 / 4 = 8
8 x 10000 = 80000
80000 - 8 = 79992

And that's where I block; 79992 is > 65535

What am I doing wrong?

I assume this is where the Prescaler comes into play, but I don't know how.


EDIT: So far, I have this for T1CON:

T1CON    = %01010100     ; Prescaler=1:1, TMR3ON.
'           bit 7-6 TMR1CS<1:0>: Timer1 Clock Source Select bits
'                   01 =Timer1 clock source is system clock (FOSC)
'           bit 5-4 T1CKPS<1:0>: Timer1 Input Clock Prescale Select bits
'                   01 = 1:2 Prescale value                                        
'           bit 3   T1OSCEN: LP Oscillator Enable Control bit
'                   0 = Dedicated Timer1 oscillator circuit disabled
'           bit 2   T1SYNC: Timer1 External Clock Input Synchronization Control bit
'                   1 = Do not synchronize external clock input
'           Unimplemented: Read as ‘0’
'           bit 0 TMR1ON: Timer1 On bit
'                   1 = Enables Timer1

I'm still looking how to use Prescaler in Art's formula.

(moved my post out of Art's thread)

[Demon]

SOLVED

16F1936 at 32MHz:

1) Elapsed_INT.bas, used Preload from 16 MHz,

  If OSC == 32
TimerConst = 063C7h
  EndIF

2) Set TMR1CS to Fosc/4

3) Set T1CKPS to 1:2 Prescaler

It runs at same speed as timer on my cell phone.

Compete code:

@ ERRORLEVEL -301   ; turn off ADC clock ignored message
@ ERRORLEVEL -306   ; turn off crossing page boundary message

#CONFIG
 __CONFIG _CONFIG1, _FOSC_INTOSC & _WDTE_OFF & _PWRTE_ON & _MCLRE_ON & _CP_OFF & _CPD_OFF & _BOREN_OFF & _CLKOUTEN_OFF & _IESO_OFF & _FCMEN_OFF 
 __CONFIG _CONFIG2, _WRT_OFF & _VCAPEN_OFF & _PLLEN_OFF & _STVREN_OFF & _BORV_LO & _LVP_OFF
#ENDCONFIG

DEFINE OSC 32

SPLLEN          CON %1                  ' PLL enable
IRCF            CON %1110               ' to enable 8 MHz
SCS             CON %00                 ' system clock determined by FOSC
OSCCON = (SPLLEN << 7) | (IRCF << 3) | SCS

INCLUDE "DT_INTS-14.bas"
INCLUDE "ReEnterPBP.bas"
Include "Elapsed_INT.bas"

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   TMR1_INT,  _ClockCount,   PBP,  yes
    endm
    INT_CREATE            ; Creates the interrupt processor
ENDASM

T1CON    = %00010101     ; Prescaler=1:1, TMR3ON.
'           bit 7-6 TMR1CS<1:0>: Timer1 Clock Source Select bits
'                   00 =Timer1 clock source is instruction clock (FOSC/4)
'           bit 5-4 T1CKPS<1:0>: Timer1 Input Clock Prescale Select bits
'                   01 = 1:2 Prescale value                                        
'           bit 3   T1OSCEN: LP Oscillator Enable Control bit
'                   0 = Dedicated Timer1 oscillator circuit disabled
'           bit 2   T1SYNC: Timer1 External Clock Input Synchronization Control bit
'                   1 = Do not synchronize external clock input
'           Unimplemented: Read as ‘0’
'           bit 0 TMR1ON: Timer1 On bit
'                   1 = Enables Timer1

DEFINE  LCD_DREG      PORTB             ' Set LCD data port
DEFINE  LCD_DBIT      0                 ' Set starting data bit
DEFINE  LCD_RSREG     PORTC             ' Set LCD register select port
DEFINE  LCD_RSBIT     5                 ' Set LCD register select bit
DEFINE  LCD_EREG      PORTC             ' Set LCD enable port
DEFINE  LCD_EBIT      4                 ' Set LCD enable bit
DEFINE  LCD_BITS      4                 ' Set LCD bus size
DEFINE  LCD_LINES     4                 ' Set number of lines on LCD
DEFINE  LCD_COMMANDUS 1000              ' Set command delay time in microseconds
DEFINE  LCD_DATAUS    50                ' Set data delay time in microseconds

define  CCP4_REG     PORTC              ' PWM Pulse out to LCD contrast
DEFINE  CCP4_BIT     1                  '   2N2907 PNP with 1K on base
define  CCP5_REG     PORTC              ' PWM Pulse out to LCD backlight
DEFINE  CCP5_BIT     2                  '   2N2222A NPN with 1K on base

ANSELA = %00000000
ANSELB = %00000000

TRISA = %00000111
TRISB = %00110000
TRISC = %10000000
TRISE = %00000000

    HPWM 2,100,1953
    HPWM 1,180,1953
    
@ INT_ENABLE TMR1_INT ; Enable Timer 1 Interrupts
    Gosub ResetTime                ' Reset Time to  0d-00:00:00.00

    Pause 500                           ' Let PIC and LCD stabilize

    LCDOUT $FE, 1 : Pauseus 1

    GOSUB StartTimer          ' Start the Elapsed Timer

Mainloop:

    IF SecondsChanged = 1 THEN  
        SecondsChanged = 0
        LCDOUT $FE,2, DEC Days,"d-",DEC2 Hours,":",DEC2 Minutes,":",DEC2 Seconds
    ENDIF
    
    goto mainloop
end

[Demon]

Art's formula with Prescaler:

65535 - (((( MHz / Prescaler ) / 4 ) x 10000 ) - 8 )

[Ioannis]

What are the 10000 and 8 stand for in the equation?

Ioannis

[Demon]

I couldn't tell you. See Art's thread above.

[tumbleweed]

What are the 10000 and 8 stand for in the equation?

The 10000 comes from the fact that the intr code is assuming a 100Hz interrupt (MHz -> 100Hz = 10000),
and the 8 is an attempt to compensate for the number of instruction cycles it takes to reload the timer.

Demon -
How long of a time period are you looking to measure?
If you can live with 65536 counts (the 16-bit timer count) then you can just drop all the interrupt code, let the timer free-run, and just subtract two unsigned timer values... something like this:

start_time = readtimer()
'code you wish to time is here
elapsed_time = readtimer() - start_time

[tumbleweed]

If you let the timer free-run, then at 32MHz using FOSC/4 as the timer clock and 1:1 prescaler you can count up to 8192us (8ms) with 125ns/count.
Setting the prescaler to 1:8 extends this to 8x8192 = 65536us (roughly 65ms), but your resolution drops to 8x125ns = 1us