How to set TMRO & TMR3 as Counter?.

[NatureTech]

Hi all,

I am a bit confused on how to set TMR0 and TMR3 as a Counter in PIC18F4550?.I plan to use these 2 Counter as a pulse counter from my robots encoder.Thanks for your guidance guys.I am kinda new in this.

[HenrikOlsson]

Hi,
TMR3 seems to share it's clock pin with TMR1 and I haven't really looked into what effects that has but here's what I'd try:

TRISA.4 = 1		'TMR0 CKI Pin as input
TRISC.0 = 1		'TMR3 (and TMR1) CKI Pin as input

'From Datasheet Section 11
T0CON.7 = 1		'TMR0 is ON
T0CON.6 = 0		'TMR0 is 16 bits
T0CON.5 = 1		'TMR0 clock source is EXTERNAL...
T0CON.4 = 0		'...and counts on rising edge
T0CON.3 = 1		'NO prescaler is assigned
T0CON.2 = 0		'Prescaler 1:1
T0CON.1 = 0		'Prescaler 1:1
T0CON.0 = 0		'Prescaler 1:1


'From Datasheet Section 14
T3CON.7 = 0		'Disables 16bit read/write
T3CON.6 = 0		'TMR1 is clock for both CCP's (together with BIT 3)
T3CON.5 = 0		'Prescaler 1:1
T3CON.4 = 0		'Prescaler 1:1
T3CON.3 = 0		'TMR1 is clock for both CCP's (together with BIT 5)
T3CON.2 = 1		'Don't sync with internal clock.
T3CON.1 = 1		'TMR3 clock source is EXTERNAL
T3CON.0 = 1		'TMR3 is ON

I've set the bits one by one here so that it's easier to see what each one does. Note that I haven't tested this so it's perfectly normal if it doesn't work...

/Henrik Olsson.

[NatureTech]

Thats a great guide HenrikOlsson,

Well,the truth is .. this is my first time in my life setting up Counter in PIC.Although i know whats it function would work like,i am still confused ( Well,not the pre-scaler part,rising edge or falling edge and so on ..) but how to set it,accumulate it and clear it?.That's what something new to me.I am able to read the datasheet but understand 50/50...

[HenrikOlsson]

Hi,
Ahh, well the TMR registers are like any other register in the PIC so you can read/write to/from them as you would with any other register. So to set the TMR0 registers to 0 (assuming a 16 bit TMR0 here):

MyCount var WORD		'This is my counter...

TMR0L = 0			'Reset lower byte
TMR0H = 0			'Reset hogher byte

Start:
  MyCount.HighByte = TMR0H	'Get high byte of counter
  MyCount.LowByte = TMR0L	'Get low byte of counter
  LCDOUT $FE,1,#MyCount		'Display it
  Pause 100
Goto Start

To PRESET it you write whatever value you wish to the two registers:

TMR0H = 3        'Preset TMR0 to 878....
TMR0L = 110     ' ( 3 * 256 + 110 )

That's really all there is to it. Well, one more thing. When reading the two TMR registers in the way shown above there is a slight chance the lower byte rolls over in the time between reading TMR0H and TMR0L. To come around that problem you can either enable 16bits read/writes (on PICs that supports it) or read the high byte two times, like:

MyCount VAR WORD
Temp var byte


MyCount.HighByte = TMR0H	'Get high byte of counter
MyCount.LowByte = TMR0L		'Get low byte of counter
Temp = TMR0H			'Get high byte again

IF Temp <> MyCount.HighByte then  'Do a fresh read
  MyCount.HighByte = TMR0H	'Get high byte of counter
  MyCount.LowByte = TMR0L	'Get low byte of counter
EndIf

HTH
/Henrik Olsson.

[NatureTech]

Hi HenrikOlsson,

Ok.I understand now.I gonna give a try and let you know the result.Thanks a bunch my "Sail on the same ship" friend.

[NatureTech]

To HenrikOlsson,

Hope you remember me.

Is the TMR0 and TMR3 were shared?.So that means i could only use 1 counter at a time right?.