Maximum frequency output from a PIC

[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.

[Dave]

Amgen, The project was a cannon ball replacement for COHO fishing. The sub would track the temperature gradients and depth then control the lure depth as it was connected to the rear of the sub. The data link was the stainless tow wire going up to the boat. The remote readout would display the depth, temperature as well as plot the depth during the tow on an VFD dot matrix display.

[NickMu]

Thank you Henrik for your suggestions.
I will try the tonight and report the findings.
If I must I will stay with 12F series which might give me the corect output.
The goal is to get 1MHz with output with a 15 mS ON and 5 mS OFF from one single chip.

Regards,

Nick

[NickMu]

One other choice will be to have the PIC12f output (the 1MHz) controlled ON and OFF by one of its input pins kind like Dave did in his application. This will give me more flexibility with the rest of the concept which is not completely defined yet.
Once I have a clearer picture I will present it to the forum.

Nick

[NickMu]

If anyone is interested here are the results of the speed test using the two methods suggested by Henrik:
• Writing directly to the output pin at 8MHz internal clock I got a little over 500 kHz but the duty cycle is 25% or 75% depending on the order you write to the pin 0 or 1.
• Using x consecutive writes to the pin (but with a gap at the end of the sequence) increases the burst frequency to about 1 MHz with a duty cycle of 50%. BTW I used 10 cycles.

So far I learned a lot in the process.

Henrik, based on your prior experience with CCP module I have few questions that you might be able to answer:
1. What will be the maximum frequency out that can be achieved from a given master clock?
2. Can the output be turned ON and OFF at some predetermined intervals (let's say 15 mS ON, 5 mS OFF)?
3. What other requirements are there for the PIC (other the CCP module) to be able to get that?
4. Will PIC12F683 do the job?

I will start searching the forum for this kind of applications and I’m sure I might be able to find something to get me started.

Thank you to all the members of this forum that are making PIC programming accessible to more and more hobbyists.

Who said you can’t teach old dogs new tricks?:

Regards,

Nick

[HenrikOlsson]

Hi Nick,

Henrik, based on your prior experience with CCP module I have few questions that you might be able to answer:
1. What will be the maximum frequency out that can be achieved from a given master clock?

As per the datasheet, PWMPeriod = (PR2+1) * 4 * Tosc so with an 8MHz oscillator and PR2=0 the maximum PWM frequency would be 2MHz (1/(1*4*0.000000125)

2. Can the output be turned ON and OFF at some predetermined intervals (let's say 15 mS ON, 5 mS OFF)?

Yes, that's what the PWM module does, however the number of available dutycycle ratios depends on the selected output frequency. At the maximum output frequency you have 2bits of resoultion, ie the dutycycle can be 0%, 33%, 66% or 100%.

You keep saying that you want 1MHz output frequency with 15ms on-time and 5ms off-time. That's a period of 20ms which equals a frequency of 50Hz - not 1MHz. If you mean 15us on-time and 5us off-time it'll be 50kHz, still not 1MHz.

3. What other requirements are there for the PIC (other the CCP module) to be able to get that?

No, not really.

4. Will PIC12F683 do the job?

I think it might, I'll have to look into it a bit further though.

/Henrik.

[Dave]

Henrick, I think what NickMu means is, you have a carrier frequency of 1 Mhz. being generated by an oscillator and you modulate it with a gated signal of 15 Ms. on and 5Ms. off. What looks like OOK modulation. NickMu, am I right? Is this what you are looking for?

[Dave]

NickMu, If this is what you are looking for then maybe you should look at using an 12F1840. It has a DATA SIGNAL MODULATOR module that is quite usefull for generating waveform like this.