Choosing a PIC

Discussion in 'Embedded Systems and Microcontrollers' started by vwdevotee, Jul 20, 2009.

  1. vwdevotee

    Thread Starter New Member

    Jul 19, 2009
    8
    0
    Ok, well maybe not a PIC per se, another brand is totally acceptable. But I need a microcontroller that can generate several simultaneous PWM signals. For my project I need to drive 6 FETs to convert DC to 3 phase AC, and drive 4 FETs in a full bridge DC/DC converter. Maybe I'm over-complicating it, but it looks like I might need 10 signal trains. depending on the wiring, I might be able to eliminate two in the converter, but that's still up to 8 PWM signals. Can normal I/O channels be used for this, or do I need a chip intended to PWM? The most I've seen is 3 PWM outputs on one PIC, and that won't cut it for me. Does anyone know of a good PIC I could use? Thanks for everyones help!!
     
  2. mauro.laurenti

    Active Member

    May 8, 2009
    68
    0
    Hello,

    A normal I/O could be enough but the drawbacks are:


    • you do not have good resolution (good or not is up to your spec)
    • You have to handle the timing with the software an the timers.
    If you use the internal hardware you get better resolution and after you set the hardware there is no CPU overhead.

    If I do not remember wrong there are application notes from microchip for 3 phase motors. The serie 18F is a must the dsPIC serie could be better.
    There is also a dedicated web page section for the motor control topic.

    Ciao,

    Mauro
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    PIC24xxx and 30-33 series have 8 PWM outputs.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Atmel with AVRs has even more PWM channels on some AVRs.

    For example, ATmega1281 has 9 channels and and ATmega1280 has 16 PWM channels.

    Making PWM with not a built in PWM function is not a good idea because the PWM frequency will vary with processing speed and the flow of the code.
     
  5. mauro.laurenti

    Active Member

    May 8, 2009
    68
    0
    If you control the PWM using the timer and the interrupts, the code flow will not matter until a maximum PWM frequency that can not be as high as the hardware PWM frequency.

    In general, as mentioned, I would not use a software PWM.

    I'm currently developing a DC motor controller using a dsPIC33F and the LM18200 driver.

    Ciao,

    Mauro
     
  6. Tahmid

    Active Member

    Jul 2, 2008
    344
    25
    Hi,
    You could use dsPICs for internal hardware PWM. Or you could do so using software (delay loops and timers, etc).
    You could also look at the ATmegas as mik3 said.
     
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    We would need to know what the timing requirements are. Is the timing for the pwm relative ( on off ratio ) or absolute. I have a circuit for 16 pwm outputs in software that works fine ( on an 877 ). But its absolute timing is not critical.
     
  8. vwdevotee

    Thread Starter New Member

    Jul 19, 2009
    8
    0
    Can you point me to the motor control forum? I looked around the site and couldn't seem to find it. It was my understanding that once a dedicated PWM channel is set it just does it's thing with an oscillator, but for generating an AC wave form the duty cycle needs to vary within each AC cycle. That's why I wasn't sure about using normal PWM bits.

    Thanks again.
     
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