PIC timing

R

randy.day

Guest
Okay, I was curious about the actual speed my controller
is running at, so I ran the following loop on it. If I'm not mistaken,
I've set OSCCON to 8MHz, so at 4 clock/instruction I should see
0.5 uS pulses. My scope on RB4 shows pulses 5uS wide!

I'm out by a factor of 10. What have I not taken into account?


#include <p16F690.inc>
__config (_INTRC_OSC_NOCLKOUT & _WDT_OFF & _PWRTE_OFF & _MCLRE_OFF &
_CP_OFF & _BOR_OFF & _IESO_OFF & _FCMEN_OFF)

Org 0
goto Start
Org 4
goto ISR
ISR
retfie

Start movlw 0x71 ;b'01110001'
BANKSEL OSCCON
movwf OSCCON

; initialize any register that might affect I/O
BANKSEL IOCB
clrf IOCB
clrf WPUB
clrf ANSEL
clrf ANSELH

BANKSEL TRISA
clrf TRISA ; PORTA all output
clrf TRISB ; PORTB all output
clrf TRISC ; PORTC all output

BANKSEL SSPCON
bcf SSPCON, 5

;=== run timing loop ===
BSF PORTB,4
BCF PORTB,4
goto $-2
end
 
"randy.day" <randy.day@shaw.cax> wrote in message
news:nTedj.29074$DP1.13331@pd7urf2no...
Okay, I was curious about the actual speed my controller
is running at, so I ran the following loop on it. If I'm not mistaken,
I've set OSCCON to 8MHz, so at 4 clock/instruction I should see
0.5 uS pulses. My scope on RB4 shows pulses 5uS wide!

I'm out by a factor of 10. What have I not taken into account?


#include <p16F690.inc
__config (_INTRC_OSC_NOCLKOUT & _WDT_OFF & _PWRTE_OFF & _MCLRE_OFF &
_CP_OFF & _BOR_OFF & _IESO_OFF & _FCMEN_OFF)

Org 0
goto Start
Org 4
goto ISR
ISR
retfie

Start movlw 0x71 ;b'01110001'
BANKSEL OSCCON
movwf OSCCON

; initialize any register that might affect I/O
BANKSEL IOCB
clrf IOCB
clrf WPUB
clrf ANSEL
clrf ANSELH

BANKSEL TRISA
clrf TRISA ; PORTA all output
clrf TRISB ; PORTB all output
clrf TRISC ; PORTC all output

BANKSEL SSPCON
bcf SSPCON, 5

;=== run timing loop ===
BSF PORTB,4
BCF PORTB,4
goto $-2
end
Click to expand...
The "goto $-2" takes two instruction times.
I make it as, (in 0.5uS steps and 0V,5V) ...
5-0-0-0-5-0-0-0-5-0-0-0-5-0-0-0-5 ...
I.e. 2uS repetition (500kHz), at a 1:3 duty cycle with 8MHz clock=2 mips.
 
"randy.day" <randy.day@shaw.cax> wrote in message
news:nTedj.29074$DP1.13331@pd7urf2no...
Okay, I was curious about the actual speed my controller
is running at, so I ran the following loop on it. If I'm not mistaken,
I've set OSCCON to 8MHz, so at 4 clock/instruction I should see
0.5 uS pulses. My scope on RB4 shows pulses 5uS wide!

I'm out by a factor of 10. What have I not taken into account?


#include <p16F690.inc
__config (_INTRC_OSC_NOCLKOUT & _WDT_OFF & _PWRTE_OFF & _MCLRE_OFF &
_CP_OFF & _BOR_OFF & _IESO_OFF & _FCMEN_OFF)

Org 0
goto Start
Org 4
goto ISR
ISR
retfie

Start movlw 0x71 ;b'01110001'
BANKSEL OSCCON
movwf OSCCON

; initialize any register that might affect I/O
BANKSEL IOCB
clrf IOCB
clrf WPUB
clrf ANSEL
clrf ANSELH

BANKSEL TRISA
clrf TRISA ; PORTA all output
clrf TRISB ; PORTB all output
clrf TRISC ; PORTC all output

BANKSEL SSPCON
bcf SSPCON, 5

;=== run timing loop ===
BSF PORTB,4
BCF PORTB,4
goto $-2
end

If you are only measuring the part of the pulse that is high, it appears it
Click to expand...
should be 0.5 us. Check if your scope has a X10 horizontal magnification
switch (my old Leader has a knob on the inside of the Variable Possition
control (horizontal position) that is labeled, "PULL X10 MAG". Inadvertantly
activating that control can throw you off.

By the way, goto takes 2 instruction cycles, so you may be getting a 0.5 us
high pulse, followed by a 1 us low pulse.

Richard
 
Richard Seriani wrote:

[snip]

If you are only measuring the part of the pulse that is high, it appears it
should be 0.5 us. Check if your scope has a X10 horizontal magnification
switch (my old Leader has a knob on the inside of the Variable Possition
control (horizontal position) that is labeled, "PULL X10 MAG". Inadvertantly
activating that control can throw you off.
Click to expand...
ROFL! That's EXACTLY what it was! :D

I just got the scope last week as my early xmas present.
I'm going to be giggling over this for the rest of the night...

By the way, goto takes 2 instruction cycles, so you may be getting a 0.5 us
high pulse, followed by a 1 us low pulse.
Click to expand...
Yep. The on/off ratio was correct, but it was all
*stretched* by a factor of 10, which seemed
suspicious on a base-8 micro, but I *never* would
have spotted that. :)

Cheers mate! You made my day!
 
"Randy Day" <randy.day@shaw.cax> wrote in message
news:4775A903.97662200@shaw.cax...
Richard Seriani wrote:

[snip]

If you are only measuring the part of the pulse that is high, it appears
it
should be 0.5 us. Check if your scope has a X10 horizontal magnification
switch (my old Leader has a knob on the inside of the Variable Possition
control (horizontal position) that is labeled, "PULL X10 MAG".
Inadvertantly
activating that control can throw you off.

ROFL! That's EXACTLY what it was! :D

I just got the scope last week as my early xmas present.
I'm going to be giggling over this for the rest of the night...

By the way, goto takes 2 instruction cycles, so you may be getting a 0.5
us
high pulse, followed by a 1 us low pulse.

Yep. The on/off ratio was correct, but it was all
*stretched* by a factor of 10, which seemed
suspicious on a base-8 micro, but I *never* would
have spotted that. :)

Cheers mate! You made my day!
Click to expand...
Randy,

Glad to be able to help. Thanks for letting me know.

Happy New Year.

Richard
 
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