Counting Timer0 and Timer1 overflows without any interrupt handler

Hi Gang,
Today, I played around and got a couple of programs running on a 16F877A. One program monitors internal Timer1 overflows and flips a port bit every 210 counts, to produce a 1 second clock pulse. And this without an interrupt handling routine.

(snippet)
'setup option register for TMR0
option_reg = %01010111 ' interrupt w/max

mainloop:
if TMR0 = 255 THEN
counting = counting + 1
if counting = 210 then
counting = 0
PORTD = PORTD ^ %00000001 'flip bit 0 for a clock
endif
endif
goto mainloop


The other program sets Timer1 up with pin-15 as its clock source, with prescaler=8. The main-loop starts Timer1, pauses for 1 second then turns off Timer1. Then displays the max count held within TMR1, which is offcourse <= 65535 with a max frequency input of 500khz. And this without an interrupt handling routine.

So here is what I'm wondering:
What I think I should be able to do is count in the main-loop, both Timer0 and Timer1 overflows. Timer0 clocking from internal source. Timer1 clocking from pin 15.
When timer0 overflow count = 210, then exit out of the loop, turn off Timer1 and process Timer1's overflow count.

The number of Timer1 overflows will act as a multiplier value.
An overflow count of 2, for example would represent an clock input frequency of a little over 1mhz.

Wondering how realistic you think my plan might be?
I hoping to code a frequency counter that will count up to 30mhz without having to use an external ripple counter.

Big thanks!!
dw

Hi,
Just a couple of random notes:

Doing IF TMR0=255 may not be the best. The reason for that is that your program may be off doing something else while TMR0 "transitions thru" 255, ie the program checks it and it's 254 next time it checks it's 0. TMR0 has overflowed but your program missed it. In THIS particular case your program is fairly "tight" so it likely runs faster than the increment rate of TMR0 but if/when you add stuff to the loop it may not work so well. Instead, check the TMR0IF, it'll get set when the timer overflows (even if you're not actually using interrupts) - just remember to clear it again.

I'm not sure TMR1 can cope with a frequency as high as 30MHz, check the electrical specifications in the datasheet.

/Henrik.

Thank you Henrik!
Much appreciate your knowledge of the devices!
I just read in MicroChips DS33023A section 8 on interrupts that the interrupt flag bit gets set regardless of the corresponding enable bit setting. So yes, I can see that monitoring the flag bit allows for processing that condition while the timer is still running towards its next overflow. Just "as you mentioned" zero the flag bit until the next one.
I'm hoping I can track both timer overflows without having to code the main loop in asm.
I'll try it :-]
Check Timer0 with TMR0IF status and Timer1 with PIR1.0 status.
Thanks!!!! :-]

The following PBP3 code is my first attempt at coding for a frequency counter that can read above 500khz. The goal was to see if it could accurately capture frequency below 30mhz without using a frequency-prescaler, or ripple-counter. It does work, but its not accurate enough to be considered anything but educational or a fun project.

Code:

---


'  Code for 16F877A to count frequency
'  Uses TMR0 and prescaler to gate 1 second counting time, internally clocked.
'  Uses Timer1, external clock, from PIN 15 
'  WDT, Analog, Comparitors OFF

clear
define loader_used 1
define osc 4

' Define LCD registers and bits
Define LCD_DREG PORTD
Define LCD_RSREG PORTE
Define LCD_RSBIT 0
Define LCD_EREG PORTE
Define LCD_EBIT 1
define LCD_RWREG PORTE
define LCD_RWBIT 2
define LCD_BITS 8
define LCD_LINES 2
define LCD_COMMANDUS 2500
define LCD_DATAUS 100

TRISC = %11110001    'using pin 15 as input for frequency
TRISD = 0            'PORTD all outputs
PORTD = 0
CMCON = 7            'disable comparators
ADCON1 = 7          'PORTA and E digital


'setup option register for TMR0
option_reg.7 = 0
option_reg.6 = 1    '1 for interrupts active
option_reg.5 = 0
option_reg.4 = 1    '1 for interrupts active
option_reg.3 = 0    'Assign prescaller to TMR0
option_reg.2 = 1    'prescaler 0-15
option_reg.1 = 1    'with this setting TMRO-Rate = 1:256
option_reg.0 = 1

'Prescaller settings for option_reg
'option_reg 01011000 = 1:1 prescaler TMR0 roles over every 1mSec
'option_reg 01010000 = 1:2 prescaler TMR0 roles over every 152 uSec
'option_reg 01010001 = 1:4 prescaler TMR0 roles over every 1mSec
'option_reg 01010010 = 1:8 prescaler TMR0 roles over every 2mSec
'option_reg 01010011 = 1:16 prescaler TMR0 roles over every 4mSec
'option_reg 01010100 = 1:32 prescaler TMR0 roles over every 8mSec
'option_reg 01010101 = 1:64 prescaler TMR0 roles over every 16mSec
'option_reg 01010110 = 1:128 prescaler TMR0 roles over every 32mSec
'option_reg 01010111 = 1:256 prescaler TMR0 roles over ever 64mSec

INTCON.7 = 0  'disable interrupt routine on overflow
INTCON.5 = 0  'disable interrupt routine on Timer0 overflow
INTCON.4 = 0  'disable RB0 external interrupt function
INTCON.3 = 0  'disable port change interrupt function
INTCON.1 = 0  'not really needed, but clear possible pin change flag

T1CON = %00110010 'Timer1 prescaler = 8 - osc off- sync on, external source

lcdout $fe,1 'clear the display
lcdout "Frequency Counter"
pause 1000

Tmr1flg var word
counting var byte
calc var byte
mhz var byte
khz var word
hz var word

PORTD = 0    'We will be setting PORTD.0 high through the 1 second
              'gate-period to monitor it with Oscope
INTCON.2 = 0  'Clear Timer0 interrupt flag
TMR1 = 0      'Reset Timer0 counting value

' Main program loop
mainloop:
Tmr1flg = 0  'Clear variable which counts Timer1 overflows
TMR1H = 0    'clear Timer1 high register
TMR1L = 0    'clear Timer1 low register
PIR1.0 = 0    'clear Timer1 overflow flag bit
INTCON.2 = 0  'Clear Timer0 interrupt flag
TMR1 = 0      'Reset Timer0 value
counting = 0

PORTD.0 = 1      'raise the 1 second timer flag
T1CON.0 = 1      'start Timer1

while counting < 15  'Timer0 has to overflow this many times for 1 second gate period
while INTCON.2 = 0    'watch for Timer1 overflow and count if it occurs.
if PIR1.0 = 1 then    'if Timer1 overflow flag is up then
Tmr1flg = Tmr1flg + 1  'increment overflow count by 1. Each count represents value of 524280
PIR1.0 = 0              'Now reset the Timer1 overflow flag bit
endif
wend                    'end of inner while loop
INTCON.2 = 0            'Now reset the Timer0 overflow flag bit
counting = counting + 1  'increment counting for gate time
wend                    'end of outer while loop

T1CON.0 = 0 'stop Timer1
PORTD.0 = 0 'lower the 1 second timer flag representing the gate period


' calculations to get the numbers captured to the display
' we have to feed the number of Timer1 overflows into Mhz, Khz, hz
' Then we feed the Timer1 remaining count * 8 in as well
' and parse it out to mhz khz hz
hz = 0
khz = 0
mhz = 0

' Add Timer1 overflow values into Mhz, Khz, hz
if Tmr1flg > 0 then
for calc = 0 to Tmr1flg
khz = khz + 524
hz = hz + 280

while hz > 999
khz = khz + 1
hz = hz -1000
wend
while khz > 999
mhz = mhz + 1
khz = khz - 1000
wend
next
endif

' Add remaining Timer1 count * 8 into Mhz, Khz, hz
for calc = 0 to 7
hz = hz + TMR1

while hz > 999
khz = khz + 1
hz = hz -1000
wend

while khz > 999
mhz = mhz + 1
khz = khz - 1000
wend
next

'Display results
if mhz > 0 then
lcdout $FE,1,#mhz,".",#khz,".",#hz
else
lcdout $FE,1,#khz,".",#hz
endif

pause 2000
goto mainloop  ' Go get another reading
End


---

Quote:

---

Originally Posted by dw_picbasic

The following PBP3 code is my first attempt at coding for a frequency counter that can read above 500khz...
It does work, but its not accurate enough to be considered anything but educational or a fun project.

---

Since it's not a problem for the PIC chip, or for PBP ...
Will there be a Second Attempt?

I'd hate to interfere with the learning process. :)

But I would suggest using the Timer0 input (T0CKI) for the frequency to be measured.
The Timer0 prescaler has a minimum period of 20nS, which means it can take up to 50Mhz.
Timer1 (T1CKI) can only take up to 1Mhz with a 4Mhz OSC, or 5Mhz with a 20Mhz OSC.

Any frequency measurement requires an accurate "Time Base".
And the easiest way to get that is to use a 32768 Hz crystal on the Timer1 oscillator.
This way, Timer1 generates the 1 sec window, and Timer0 (and it's overflows) counts the frequency.

It is a little tricky to read the Timer0 prescaler to get the lowest byte of the count though.
The prescaler should be 1:256 (the default).

Oh, and the PICs oscillator should be as fast as possible, so that the program can respond to the hardware flags, which have to be polled.
The fastest oscillator for the 16F877A is 20 Mhz.

A simulation works good up to 600Kz. Reading the exact Hz. (actual results will depend on the 32768 crystal tolerance)
A real chip should go much higher, but the SIM crashes with the higher frequency.

Attachment 7216

Sometimes you just need to know it can be done ... so you can figure it out on your own.

Darrel,
That's an ***AWESOME*** post!!
It hadn't occurred to me to flip the timers around that way.
Also, I need to go back to the data sheet and look at the electrical characteristics again for timer0 clock minimum pulse width.
Per your info about the 20nSec I need to read through that again.
I definitely want to try it!!
This will be the first time, for me learning to setup Timer1 with external crystal for time base.
Perhaps I would do well reading up on how that functionality is used for a standard 12hour clock.

Also, what simulator are you using?
It looks totally awesome!!
Many thanks
Duane :-]

The simulator is Proteus VSM from Labcenter in the UK.
It's a bit pricey, but does an amazing job. Couldn't do my job without it.

And I just want to point out how I have RA3 and RA4 tied together in the above schematic.
It's a very important part of making the freq. counter work.

Thanks Darrel,
I never could get my head around how to do that before, now I see tmr0 is a counter, tmr1 is the time base.

Quote:

---

And I just want to point out how I have RA3 and RA4 tied together in the above schematic.
It's a very important part of making the freq. counter work.

---

OK, I'll bite.... Are using RA3 to gate the signal for the T0CKI-pin, hence the seriesresistor on the input?

/Henrik.

The two pins tied together (and the resistor) serve two purposes.

1. You're right Henrik, since Timer0 on 16F's can't be stopped and started like all the other timers, it's used to gate the incoming signal.

2. You can't directly read the value in Timer0's prescaler. So after counting pulses for the 1 sec. window, you have to provide additional pulses to the input until the value in TMR0 changes. The number of pulses you add tells you how many counts were already in the prescaler. This gives the low Byte of the final 32-bit value.

The result is made up of the Timer0 overflows (HighWord), the TMR0 value (HighByte of the LowWord) and the prescaler value (LowByte of the LowWord)

Hmmmmm.
I was thinking I was going to have to use a mosfet solid state relay at the input from the frequency source, so that I could pump Timer0 through the remaining clock cycles necessary to raise its flag again.
I can see how pulling the Timer0 input pin high will disable the incoming clock from the frequency source.
But I can't see how it could be used to pump the remaining cycles to get Timer0 to its next overflow since there is no way to dis-engage the frequency source signal. The remaining clocking and the frequency source would conflict with one another.

Additionally, I was wondering if there is another way of pumping Timer0 to overflow, by using an internal software loop rather than doing it through an external clocking pin? I was wondering if a routine of adding 1 to TIMRL register and testing for a TMR1IF flag might work just as well.


Unless I'm wrong that the intent is to pump Timer0 from an output pin from the PIC?

Thanks :-]

Hi,
To enable the counter you TRISA.3=1 (pin is input). To gate the count signal OFF you set TRISA.3=0 (pin is output) and pull the pin low (or high for that matter. The resistor in series with the input will protect the DUT from the "shorted" input.

Pulse the RA3 output in a loop until TMR0 again overflows. The pulses will override the input signal, there will be no conflicts because of the series resistor. As long as the RA3 output is LOW the input signal isn't seen by TMR0, when RA3 is high the input signal can't pull it low due to the series resistor. It'll work.

/Henrik.

Oh...that is just tooooo coool!!! :-D
Love these toys!!!
Thanks Henrik!!

Hi Darrel and Hendrick,
I'm still moving along dabbling with this project.
Since I have the 16F887A in a LABX-1 project board, and I had a 12F683 mounted in a bread-board, I decided for convenience to see if I can make another step in this project using the 12F683. I got the 32khz crystal up and running on Timer1. By loading a preset value into Timer1's hi-byte, I can initiate the the frequency counting gate and Timer1 will interrupt right at 1Sec. (close enough on my Oscope).
So then I moved to add using Timer0 as the counter for the external frequency.
I'm only playing with a max input frequency of 300Khz right now, so I can utilize the PICs internal oscillator.

The following code is my progress.
It doesn't yet have the step of capturing the remainder value within Timer0.
Just got the point of attempting to counting Timer0 overflows.
But since I added the Timer0 register settings, it appears that I can't shut off the Timer1 overflow no matter what I do. Its supposed to display the timer0 overflow count and then go into a forever loop. But it keeps popping back up to "Display_Results". So I suspect timer1 is still initiating the interrupt routine.

Code:

---

' Name        : Frequency Counter Clock in Timer0 - Gate with Timer1
' Compiler    : PICBASIC PRO Compiler 2.6
' Assembler  : PM or MPASM
' Target PIC  : PIC12F683
' Hardware    : 32.768khz crystal on OSC1-ISC2 for 1sec gate
' Oscillator  : 4MHz internal osc for PIC
' Keywords    :
' Language    : PICBASIC PRO
' 
'                    VDD --- 1|----|8--- vSS
'    GP5/TICKI/OSC1/CLKIN --- 2|----|7--- GP0/AN0/CIN+/ICSPDAT/ULPWU
' GP4/AN3/T1G/OSC2/CLKOUT --- 3|----|6--- GP1/AN1/CIN-/VREF/ICSPCLK
'            GP3/MCLR/VPP --- 4|----|5--- GP2/AN2/T0CKI/INT/COUT/CCP1
'------------------------------------------------------------------------
 
Include "modedefs.bas"  ' Include serial modes for LCD display
define osc 4
OSCCON = %01100100    ' internal oscillator at 4 MHZ runs the pic
CMCON0 = 7            ' Comparators off
ANSEL = 0              ' Set all digital
WPU    = 0            ' Internal pull-ups = off
GPIO  = %00000000    ' clear GPIO port
TRISIO = %00000100    ' All outputs except gpio.2 (input to frequency)
CCP1CON=0              ' ccpm module off

LCD var GPIO.1                        'use GPIO1 pin 6 to LCD display
GATE var byte            'variable for 1 second gate flag
TMR_CNT var word        'variable for counting timer0 overflows

on interrupt goto TMR1_INT

'setup Timer0 option_register
OPTION_REG = %10100111  'bit 7 pullups disabled
                        'bit 6 don't care
                        'bit 5 Transition on TOCKI pin
                        'bit 4 Increment on low to high TOCKI pin
                        'bit 3 Prescaller assigned to TIMER0
                        'bit 2-0 Prescaller setting 1:256
INTCON.2 = 0                    'set Timer0 ovrflow flag zero
TMR0 = 0                'clear the Timer0 counter register

'clear Timer1 interrupt registers
T1CON.0 = 0    'TMR1ON disable interrupts for now
TMR1L = 0      'clear lower 8 bits of Timer1 counter
INTCON = 0    'clear the system interrupt control register
PIR1 = 0      'clear the system interrup flag bits register
PIE1 = 0      'clear perephrial interrupt enable register

pause 1000

'configure Timer1 interrupt settings
T1CON.3 = 1    'T1OSC of the T1CON register enabled
PIE1.0 = 1                'Timer1 overflow enable bit enabled
INTCON.6 = 1        'Perephrial enable bit set to enable
INTCON.7 = 1    'GIE Global interrupt enable bit enabled
T1CON = 14                'T1OSCEN enable
                'T1Sync disabled
                'TMR1CS external source for Timer1
TMR1H = 207    'preload hi-byte of Timer1 counter so
                'when interrupt occurs with 32khz crystal
                'on OSC1 + OSC2, it will be 1 second
T1CON.0 = 1    'T1CON.TMR1ON Timer1 enabled *NOW*

GATE = 1

main:
while GATE > 0      'After 1 Second Gate interrupt this ends
@ BTFSC INTCON,2    ;Test Timer0 overflow flag bit
@ incf _TMR_CNT,f  ;If flag set then increment TMR_CNT by 1
@ BTFSC INTCON,2    ;Test Timer0 overflow flag bit again
@ BCF INTCON,2      ;If flag set then clear it
wend

Display_Results:
serout LCD,T9600, [#TMR_CNT]
pause 2000
loop_here:
goto loop_here

' ************ TIMER1 GATE INTERRUPT ROUTINE AFTER 1 SEC **************
disable
TMR1_INT:
GATE = 0
T1CON.0 = 0    'TMR1ON disable interrupts for now
INTCON = 0    'clear the system interrupt control register
PIR1 = 0      'clear the system interrup flag bits register
PIE1 = 0      'clear perephrial interrupt enable register
INTCON.2 = 0  'set Timer0 ovrflow flag zero
INTCON.7 = 0  'GIE OFF - Turn off all Interrupts
resume        'goto main

       
End

---

For this project ... you should not be using interrupts.
Especially not ON INTERRUPT. There's no guaranteed timing with ON INTERRUPT.

207 isn't a good value to load in the Timer.
Since the Timer will normally overflow in 65536 counts, you just need to set bit-7 of the high byte to make it overflow in 32768 counts.
With a 32768 Hz crystal, it will overflow in exactly 1 second.

Timer1 doesn't free-run. You need to stop and start it so that you can synchronize the Timer0 Gate output with Timer1's 1 second window.

Since the PIC's changed now, the oscillator's changed and interrupts are introduced ...
I think I'll just post my code for the 877A simulation in post #5.
Hopefully you can convert it for your needs with a different PIC.

Code:

---

; DT_FREQy.pbp
;
; Created    : Mon Feb 3 2014
; Processor  : PIC16F877A
; Compiler    : PicBasic Pro 3.0

;----[Device Configuration]--(See manual section 4.9)---------------------------
#CONFIG
  __config  _HS_OSC & _WDT_OFF & _PWRTE_OFF & _BODEN_OFF & _LVP_OFF & _CPD_OFF & _WRT_OFF & _DEBUG_OFF & _CP_OFF
#ENDCONFIG

;----[DEFINEs]------------------------------------------------------------------
DEFINE OSC 20

DEFINE LCD_DREG PORTD        ; LCD data port
DEFINE LCD_DBIT 4            ; LCD data starting bit 0 or 4
DEFINE LCD_RSREG PORTD        ; LCD register select port
DEFINE LCD_RSBIT 2            ; LCD register select bit
DEFINE LCD_EREG PORTD        ; LCD enable port
DEFINE LCD_EBIT 3            ; LCD enable bit
DEFINE LCD_BITS 4            ; LCD bus size 4 or 8
DEFINE LCD_LINES 2            ; LCD Number lines on LCD
DEFINE LCD_COMMANDUS 2000    ; LCD Command delay time in us
DEFINE LCD_DATAUS 50          ; LCD Data delay time in us

;----[Aliases]------------------------------------------------------------------
Timer1    VAR WORD EXT : @Timer1 = TMR1L
TMR0IF    VAR INTCON.2
TMR0CS    VAR OPTION_REG.5
TMR1IF    VAR PIR1.0
TMR1CS    VAR T1CON.1        ; Timer1 Clock Source Select bit
T1OSCEN  VAR T1CON.3        ; Timer1 Oscillator Enable Control bit
TMR1ON    VAR T1CON.0        ; Timer1 On bit
T1OSI    VAR PORTC.1        ; 32768 Hz input pin
PSCLKOUT  VAR PORTA.3        ; pin to clock out the remaining prescaler
PSA      VAR OPTION_REG.3    ; Prescaler Assignment bit

;----[Variables]----------------------------------------------------------------
Result    VAR WORD[2]        ; Final 32-bit Frequency measurement
T0overflow VAR Result[1]      ; Counts Timer0 overflows (HighWord of the result)
T0value    VAR Result.BYTE1  ; Final Timer0 value
PScount    VAR Result.BYTE0  ; Final Timer0 Prescaler value

FreqHigh  VAR WORD          ; Top five digits of the decimal result
FreqLow    VAR WORD          ; Bottom 3 digits of the decimal result
MeasCount  VAR WORD          ; Counts number of measurements

;----[Initialize]---------------------------------------------------------------
ADCON1 = 7                    ; All Digital
CMCON  = 7
T1OSCEN = 1                  ; Start Timer1 oscillator
PAUSE 250                    ; LCD Power-Up delay
LCDOUT $FE,1,"DT's FREQy"
MeasCount = 0                ; clear the measurement count

;----[Main Program Loop]--------------------------------------------------------
Main:
    GOSUB MeasureFreq        ; Get a Frequency Measurement
    GOSUB DisplayFreq        ; Display the Frequency

    MeasCount = MeasCount + 1 ; increment and display measurement count
    LCDOUT $FE,$94,"Meas = ", DEC MeasCount
GOTO Main

;----[Measure frequency on T0CKI]-----------------------------------------------
MeasureFreq:
    LOW PSCLKOUT          ; prevent signal from getting to Timer0
    PSA = 0              ; Prescaler is assigned to the Timer0 module
    TMR0CS = 1            ; Transition on T0CKI pin
    TMR1CS = 1            ; Timer1 Clock source = T1CKI (T1OSC)
    TMR1ON = 0            ; Stop Timer1
    TMR0IF = 0            ; Clear Timer0 overflow flag
    T0overflow = 0        ; Clear Timer0 overflow count
    TMR0 = 0              ; Clear Timer0 value

    Timer1 = $FFFF        ; Timer1 overflows on next cycle of 32768 Hz clock
    TMR1IF = 0            ; Clear Timer1 overflow flag
    TMR1ON = 1            ; Start Timer1 1 sec. period
    WHILE !TMR1IF : WEND  ; Synchronize to 32768 clock
    INPUT PSCLKOUT        ; Start counting freq input
    Timer1.15 = 1        ; Set Timer1 to 32768 for 1 Sec. period
    TMR1IF = 0            ; Clear Timer1 overflow flag

    WHILE !TMR1IF        ; while 1 sec. window is open  ---|
        IF TMR0IF THEN    ; count TMR0 overflows            | This section
            T0overflow = T0overflow + 1  ;                  | does the actual
            TMR0IF = 0                  ;                  | 1-Sec measurement
        ENDIF                            ;                  | Window
    WEND                                ;              ---|
    LOW PSCLKOUT          ; Stop further counting
    T0value = TMR0        ; get Timer0 value

    PScount = 0            ; Clock out remaining prescaler
    WHILE TMR0 = T0value  ; until TMR0 value changes
        HIGH PSCLKOUT
        PAUSEUS 3
        LOW PSCLKOUT
        PAUSEUS 3
        PScount = PScount + 1
    WEND
    PScount = -PScount    ; negate to get prescaler count
RETURN

;----[Display frequency on LCD]-------------------------------------------------
DisplayFreq
    R2 = Result            ; Load Result into PBP's system vars for DIV32
    R0 = Result[1]
    FreqHigh = DIV32 1000  ; get left most 5 digits
    FreqLow = R2          ; remainder (R2) is right 3 digits

    LCDOUT $FE,$C0,"Freq = "
    IF FreqHigh > 0 THEN                        ; if any left digits
        LCDOUT DEC FreqHigh, DEC3 FreqLow,"  " ; display both left and right
    ELSE                                        ;  otherwise
        LCDOUT DEC FreqLow,"    "              ; display only right digits
    ENDIF
RETURN

END

---

Thanks Darrel!!
I think I'll try to figure out how to mount the 32khz crystal onto the Labx-1 board.
I may be able to create a small pcb with headers that I can plug into the female header pins on the side of the 887a.

I should be able to see the 32khz oscillator with an Oscope to verify its running.
Interesting, I did play with loading different values into the high byte of Timer1s counting register and had it pull a pin high for the duration until timer1 rolled over and into the interrupt routine. 207 was what gave me a 1 sec pulse.
So I assumed it was the right number to load.

Thanks again for providing the code.
It will be super fun to try it out :-]
Duane

I had a question...but connected the dots when I posted it.

Hi Darrel,
I'm curious about something in the code
I see that you capture the Timer1 overflow count and load it into RESULTS.[bit16-31]
Then load the remaining count in TMR0 into RESULTS.[bit8-15]
But I thought that TMR0 is a 16 bit register....how is it we're loading its value into an 8 bit location in RESULTS?

Thanks!!
Duane :-]

Hi Duane,
No, the TMR0 register is only 8 bits wide, that's why you need to use the trick with getting the remaining count out of the prescaler.
So, RESULT is a total of 4 bytes where the two most significant bytes are the number of TMR0 overflows, the next byte is the TMR0 count (where each count equals 256 transitions of the inputsignal due the 1:256 presacaler ratio) and the least significant byte is the number of transitions "stored" in the prescaler.

/Henrik.

Ah!
Thats right!!...sorry... I forgot the Timer0 is an 8 bit register.
I slipped on that one :-/

Thanks!

Can I pose another question:
Lets say that the frequency that is being measured within the 1 second gate period is 301234hz
Is it the case that the value that will be captured within T0overflow, RESULTS.[bit16-31] = (301234/256)?
Which (in integer form) would resolve to 1176 overflows of the Timer0 8 bit register?
And then the remainder value of 178, which remains within TMR0 will be loaded into T0value RESULTS.[bit8-15]?
Thanks!!
Duane

The prescaler divides by 256, and TMR0 divides by 256.
So the overflows only happen every 65536 counts.

With a frequency of 301234, there will only be 4 overflows, with 39090 remaining.
That's 152 in TMR0, and 178 in the prescaler.

Another way to look at it is to convert the Frequency to a HEX number
#301234 = $4:98B2

Now it's easy to see that there will be 4 overflows, TMR0 will have $98 = 152, and the prescaler has $B2 = 178.

HTH

Thanks Darrel,
I didn't see the prescaler settings in setup code.
I see "PSA [OPTION_REG.3] set to 0 which sets the prescaller from the WDT to Timer0
But I don't see any settings for OPTION_REG.0, 1 and 2.
Are they there in the code and I missed them?
Thanks!!
Duane

On power-up, the default value of OPTION_REG is %11111111.
Which means the prescaler is already set to 1:256.
There is no need to set it again in the code.

Attachment 7254

Hi Duane,
If you look up OPTION_REG in the datasheet, you'll find that it defaults to "all ones", $FF, 255 whichever way you want to look at it.
This means that the prescaler ratio, when assigned to TMR0 defaults to 1:256 - so no need to change it.

/Henrik.

EDIT: Damn, you're fast today Darrel!

Sweet!!
Thanks!
I had a suspicion that was the case, but I have an old habit of wanting to make sure in the code.
This gives the old dog a new trick!! :-]

Quote:

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Originally Posted by Darrel Taylor

The prescaler divides by 256, and TMR0 divides by 256.
So the overflows only happen every 65536 counts.

With a frequency of 301234, there will only be 4 overflows, with 39090 remaining.
That's 152 in TMR0, and 178 in the prescaler.

Another way to look at it is to convert the Frequency to a HEX number
#301234 = $4:98B2

Now it's easy to see that there will be 4 overflows, TMR0 will have $98 = 152, and the prescaler has $B2 = 178.

HTH

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Hello Darrel,

found your excellent example and i would like to build the same counter based on PIC16F870. From your above post i understand, that with prescaler 256 minimum measured frequency is 65536Hz, otherwise TMR0 will not overflow, or am i wrong? To measure lower frequencies i would have to change the OPTION_REG bits 2-0 , which will set prescaler to 1:2?

P.S. My target measurement range is 1 HZ - 1 MHz.

Hi Rufinus,
Sadly Darrel has passed away and is no longer with us.

I'll try to answer your qeustion.
It's correct that with a prescaler of 1:256 the timer will only overflow every 65536 "tick" but that doesn't prevent Darrels code from measuring frequencies lower than 65536Hz. As I wrote in a previous post the two most significant bytes of the 4 byte result will contains the number of overflows (ie "units" of 65536Hz), the next byte is the TMR0 value which is in units of 256Hz and the least significant byte is what's stored in the prescaler, in units of 1Hz. All provided your "gate time" is precisely 1Hz of course.

So, if your input frequency is 125Hz then after the one second gate time there will be 0 overflows, 0 counts in TMR0 and 125 counts in the prescaler. The prescaler isn't readable so the code then "manually" pulses the counter input to find out how many pulses is needed before the prescaler overflows and TMR0 increases. In this example it would take 131 (125+131=256) which, when stored in a byte and negated "becomes" the value 125.

Changing the prescaler won't help you measure the low frequencies. The only way to gain resolution at the lower end is to increase the gate time, count for 2, 5, 10 or whatever seconds instead of 1 and adjust the calculations accordingly.

/Henrik.

Holy Christ, i had no idea! So sorry about that.. R.I.P Darrel..

Thanks for fast answer Henrik, but please let me pull my thought together before i can understand your explanation..

Quote:

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Originally Posted by HenrikOlsson

Hi Rufinus,
Sadly Darrel has passed away and is no longer with us.

I'll try to answer your qeustion.
It's correct that with a prescaler of 1:256 the timer will only overflow every 65536 "tick" but that doesn't prevent Darrels code from measuring frequencies lower than 65536Hz. As I wrote in a previous post the two most significant bytes of the 4 byte result will contains the number of overflows (ie "units" of 65536Hz), the next byte is the TMR0 value which is in units of 256Hz and the least significant byte is what's stored in the prescaler, in units of 1Hz. All provided your "gate time" is precisely 1Hz of course.

So, if your input frequency is 125Hz then after the one second gate time there will be 0 overflows, 0 counts in TMR0 and 125 counts in the prescaler. The prescaler isn't readable so the code then "manually" pulses the counter input to find out how many pulses is needed before the prescaler overflows and TMR0 increases. In this example it would take 131 (125+131=256) which, when stored in a byte and negated "becomes" the value 125.

Changing the prescaler won't help you measure the low frequencies. The only way to gain resolution at the lower end is to increase the gate time, count for 2, 5, 10 or whatever seconds instead of 1 and adjust the calculations accordingly.

/Henrik.

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Ok, now i think i got it. It was the last step (remainder in prescaler), which I could not understand.
But it would mean then, that the smallest frequency, which I could measure in one second is 256 Hz.
For lower frequencies i would have to increase sampling time.

Lets say, it is very rare, that one needs to measure frequency below 20 Hz in normal application.
That would mean, that in 1 second sampling time there will be 20 counts in prescaler, which in turn means we would have to provide 256-20=236 counts more to overflow it. So, 236 remaining counts divided by 20 gives us 11.8 seconds! We will need to increase sampling time to 11.8 seconds, in order to measure 20 Hz frequency, or ?!

I think I will have to think about some other algorithm to measure frequencies below 256 Hz, maybe with COUNT command or so. Precision might suffer, but that would be a reasonable compromise. Target would be to keep the one second sampling rate over whole range.

Thanks again Henrik, for explaining.

Or "manual counting" in your example is the part of the code, which fills out the precaler by toggling PSCLKOUT for 3uS instead of using actual measured frequency to increase the count? Than it schould be significantly faster, than 11.8 seconds. In fact, it should be just very sligtly over 1 second for 20 Hz, or?

Hi,

Quote:

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But it would mean then, that the smallest frequency, which I could measure in one second is 256 Hz.
For lower frequencies i would have to increase sampling time.

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Not really but it depends on the resolution you need. If the gate time is one second and the input frequency is 2Hz then the prescaler will count to 2 during each gate period - therefor you'll get a result of 2Hz. But if the frequency is 2.5Hz then the displayed result will "flicker" between 2 and 3 Hz. If the gate time is increased to 10 seconds the prescaler will accumulate 25 counts which, when divided by 10 is 2.5.

[quote]Lets say, it is very rare, that one needs to measure frequency below 20 Hz in normal application.
That would mean, that in 1 second sampling time there will be 20 counts in prescaler, which in turn means we would have to provide 256-20=236 counts more to overflow it.[quote]
That's correct!

Quote:

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So, 236 remaining counts divided by 20 gives us 11.8 seconds! We will need to increase sampling time to 11.8 seconds, in order to measure 20 Hz frequency, or ?!

---

No, not at all.
After the one second gate time the prescaler will hold 20 (which is the correct input frequency since the gate time was 1 second). By counting the number of additional pulses it takes for the prescaler to overflow you know what value was in the prescaler. It'll require 236 pulses which when negated is 20.

Quote:

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Or "manual counting" in your example is the part of the code, which fills out the precaler by toggling PSCLKOUT for 3uS instead of using actual measured frequency to increase the count? Than it schould be significantly faster, than 11.8 seconds. In fact, it should be just very sligtly over 1 second for 20 Hz, or?

---

Yes, the "gate time" needs to be exactly 1 second but the total time for the measurment, the "prescaler value extraction", calculation and display will of course be a bit more than the gate time alone.

/Henrik.

Great, thanks, now i do have a complete picture. Surely resolution below 1 Hz in my case is overkill. Crystals will come tomorrow and i can start testing.

Also started reading about reciprocal counters ;)