Maximum frequency output from a PIC

[NickMu]

Not too long ago there was a tread in this forum that wasdealing with maximum frequency output from a PIC for any given master clock. Unfortunatelymy searches did not return any good information.

I’m interested to get a small PIC device (10F or 12F type)to act as a high frequency clock generator using its internal clock and I wouldlike to know what to expect.
I have an application where the space is very tight and Ineed a modulated clock source with decent stability in the order of hundreds ofkHz but ideal will be 1MHz.

Using PBP I understand that something like:

main_loop:
Togglepin_x
GOTOmain_loop

should give maximum frequency output. Assuming that theToggle and GOTO commands will take two cycles each that will give me possibleoutput frequency of Main_clock/4/4 which will be 250 kHz for a 4MHz clock.
My question is if one uses Assembly (which is not my strongestfield) will it get any faster? Will itbe possible to make it run in the background?
Ideally I need a 1 MHz output that will be ON for 15 mS andOFF for 5 mS.

Thank you in advance for any input in this matter.

[Dave]

Nick, Here ia a program I wrote a few years ago to take a 10F222 and use it as a modulator for some serial data I was pushing to the surface about 200 feet below the water surface. It worked quite well. As you can see it is done in assembly wrapped inside PBP. I hope this will give you a starting point.

' SURFCOMM1.BAS
' WRITTEN FOR 10F222
' PROGRAM TO SEND:
'STATUS DATA READING,TEMPERATURE REDAING,SERVO POSITION READING and
'DEPTH SENSOR READING TO SURFACE @ 1 SECOND INTERVALS. TRANSMIT DATA
'@ 3000 BAUD MODULATED @ 125Khz.
'
' WRITTEN BY DAVID PUROLA 07/13/2006
'
' INCORPORATED INTO 216229 REV.D "NEW TUBSUB" (12/28/2006)
'
DEFINE OSC 8
'
' ************************************************** ******************
' Define Port Variables
' ************************************************** ******************
DATA_OUT VAR GPIO.0 '0-MODULATED DATA TO HEAD UNIT
TXENAB VAR GPIO.1 '1-TRANSMIT ENABLE INPUT
DATA_IN VAR GPIO.2 '1-INPUT DATA FROM SOURCE
SPARE3 VAR GPIO.3 '1-

' ************************************************** ******************
' Declare Variables
' ************************************************** ******************
' DO NOT CHANGE THESE LINES
' ************************************************** ******************
TRISIO = %11111110 'SET PORT DIRECTION REGISTER
OPTION_REG = %11000000 'DISABLE PULL-UPS,PSA to TMR0,PSA 1:2
ADCON0 = %00000000 ' LEFT JUSTIFIED,REF = VDD,Channel 0,A/D Off

'************************************************* ********************
LOOP: 'MAIN LOOP FOR PROGRAM
'************************************************* ********************
ASM
BCF GPIO,0 ;DISABLE TRANSMITTER OUTPUT INITIALLY
NXTPLS BTFSS GPIO,1 ;TEST DATA ENABLE INPUT PIN BIT STATE
GOTO NXTPLS ;DON'T WAIT FOR PERIOD TO EXPIRE BEFORE TESTING ENABLE BIT
BTFSC GPIO,2 ;TEST DATA INPUT PIN BIT STATE
GOTO NXTPLS ;DON'T WAIT FOR PERIOD TO EXPIRE BEFORE TESTING BIT
BSF GPIO,0 ;IF NOT SET THEN MODULATE OUTPUT (ENABLE OUTPUT PIN)
NOP ;BIT ON TIME
NOP
NOP
NOP
NOP
NOP
NOP
BCF GPIO,0 ;(DISABLE OUTPUT PIN)
NOP
GOTO NXTPLS
ENDASM
GOTO LOOP
STOP

[NickMu]

Hi Dave,

Thank you for your sample code.
I do expect some PIC10F222 any day now and I will start testing with.
Just to make things easier for me understanding your code what is the output of this piece of code?

Regards,

Nick

[amgen]

Dave, that was a cool project. Did you use a 'depth sounder transducer' to modulate ? They are generally used at 52 and 125KHZ. How far in water do you suppose the signal can travle at that freq ?
don

[NickMu]

While waiting for my 10F PICs I had a little time today and did some experiments with a PIC12F683.
Using the internal oscillator set for 4MHz and a close loop with Toggle GPIO.x I was getting in the neighborhood of 70 kHz output and 143 kHz for 8 MHz clock.
Interestingly enough with simple High / Low GPIO.x loop I got about 200 kHz.

Then I realized that one of the clock options is to use internal clock with clock/4 on clock out pin. This method got me the 1 MHz output for the 4 MHz internal clock and 2 MHz for the 8 MHz internal clock.

The oscilloscope shows a pretty steady output and my frequency counter gives about 0.15% error.

If this will prove to be my path I will use a one gate chip to buffer and modulate the output.

I still wish I will be able to solve it with only one PIC10F and I will try my best to find a solution.

Nick

[HenrikOlsson]

Hi,
The toggle command first makes the pin an output, then it reads the state of the pin, then it inverts the state, then it writes the new state to the pin - takes time.
High/Low first makes the pin an outout then sets the pins state - takes a little less time.
You'll get more speed of you write to the pin directly

TRISO.0 = 0  'Make pin an output
DoIt:
GPIO.0 = 1
GPIO.0 = 0
Goto DoIt

If you can handle some jitter / discontinuity in the output then something likw this will give you even higher average frequency with the drawback that every 6th pulse will be "missing" due to the GOTO.

DoIt:
GPIO.0 = 1
GPIO.0 = 0
GPIO.0 = 1
GPIO.0 = 0
GPIO.0 = 1
GPIO.0 = 0
GPIO.0 = 1
GPIO.0 = 0
GPIO.0 = 1
GPIO.0 = 0
Goto DoIt

Another aproach would be to use the CCP module, though I'm not sure if the 10F series have that built in.

Of course if all you need is stable 1MHz (or whatever that can be derived from the oscillator) signal then use clock-out option of the chips oscillator.

/Henrik.