Maximum frequency output from a PIC

[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.

[NickMu]

Hi Henrik,

Thank you for your help and my apologies for not making myself clear.
I am in need of a modulated high frequency source. The main frequency (carrier) should be between 250 kHz and 4 MHz pending some results of testing in progress. I will start with 1 MHz and move up or down on carrier frequency.
I need to be able to turn this carrier ON and OFF in bursts of 15 mS ON and 5 mS OFF.

My project is still in its infant stages, so right now I’m at the point of poking around to see what can be done with a minimum amount of components.

The larger project this is part of is a non-contact power and data transfer. This week and next week I will be doing a lot of testing on the power transfer part to find an optimal frequency that will make it easier to do both with as little loss as possible.

My first power transfer test is encouraging (I can transfer about 1.5 W which is more than I need) with about 66% efficiency. For this I’m using a Mosfet driven by a variable gated oscillator. It serves the purpose for testing but in the final design I would like to have something much simpler especially at the slave unit side where I have a lot of limitations space being one of them.

My research also shows that for power transfer lower frequencies (100 – 400 kHz) are better. For data transfer I would prefer to use higher frequencies since I already tested a wired 4 MHz power and data transfer. Also I understand that higher carrier frequency will allow higher data transfer rate. I must have 9600 but 19200 will be better.

This week I will do some research on employing the CCP module to do most of this part.
I will also study Dave’s example which seems to be doing almost what I need only at lower frequencies.

Dave, you are right and thanks for the suggestion, I will look into it. I am familiar with PIC12F683 but if I must learn new tricks I will.

So many things, so little time.

Thanks again for your help.

Regards,

Nick

[HenrikOlsson]

Hi Nick,
No problem, I didn't have a 12F683 at home but I did have a 12F1840. Now, the 1840 has an ECCP module while the 683 has a standard CCP module so the following code MAY need to be changed slightly (the CCP1CON register) in order to work properly on the 683 (I'm not using the data modulator here). It sets the CCP module up to output a 1MHz, 50% dutycycle PWM on PortA.2. The main program then simply toggles the timebase (TMR2) for the CCP module on and off to modulate the output.

' ***************************************************************
' Pin Connections
' ***************************************************************

' RA0                       -> Not used
' RA1                       -> Not used
' RA2                       -> CCP1 output
' RA3                       -> Not used
' RA4                       -> Not used
' RA5                       -> Not used

' ***************************************************************
' Device Fuses
' ***************************************************************

#CONFIG
   __config _CONFIG1, _FOSC_INTOSC & _WDTE_ON & _PWRTE_ON & _MCLRE_OFF & _CP_OFF & _CPD_OFF
   __config _CONFIG2, _PLLEN_OFF & _STVREN_ON & _BORV_LO & _LVP_OFF
#ENDCONFIG

' ***************************************************************
' Compiler directives
' ***************************************************************
DEFINE OSC 8               ; We're running at 16Mhz

' ***************************************************************
' Initialization
' ***************************************************************

OSCCON   = %01110000        ' 8MHz internal osc
ANSELA   = 0                ' Digital only for all PortA pins
TRISA    = %00000000        ' Make PORTA outputs

CCP1CON = %00001100     ' Normal PWM 
PR2 = 1                 ' 1MHz output @8MHz system clock
CCPR1L = 0              ' 50% Dutycycle
CCP1CON.5 = 1
CCP1CON.4 = 1

' ***************************************************************
' Actual program
' ***************************************************************
Main:
    T2CON.2 = 1             ' Timebase ON
    PAUSE 15
    T2CON.2 = 0             ' Timebase OFF
    PAUSE 5
Goto Main

I have a scope screen grab of the output but for some reason the file upload manager on the forum refuses to work, the upload just hangs...aarghh....

Now, if you intend to do other things with the PIC as well then this simple aproach obviously won't work. But you could use a timer interrupt in order to turn on and off the carrier at the correct intervals. I haven't personally used the data signal modulator module in the 12F1840 but it sure sounds like the "correct" way forward. The 12F1840 also allows you to run at 32MHz using the internal oscillator and 4x PLL which would allow you to get higher output frequency using the CCP module than on the 12F683 which tops out at 8MHz.

/Henrik.