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Hot!Switching power on AVdd

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mpittman
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2020/05/21 07:56:46 (permalink)
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Switching power on AVdd

Hello,
 
I'm using a PIC24FJ128GC010.
 
I have two separate linear regulators for the analog and digital portions of my circuit. One reg will power the analog pins of my PIC and the other reg will power everything else on the PIC. The datasheet (Section 2.1) states that the AVdd pin must be powered even if the analog is not being used.  I understand that this may be requried under "normal" operation but are there cases where you can get away without doing this?  Is this true for just putting the PIC to sleep and then waking? What I would like to do is use a PIC IO to shutdown the linear reg powering the analog circuitry to minimize sleep current. This would mean that power will be removed from the AVdd pin while sleeping, but will be turned back on immediately after waking.  Is this possible?
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    NKurzman
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    Re: Switching power on AVdd 2020/05/21 08:19:08 (permalink)
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    Since the Manufacturer does not say what you can "get away with"  Then you are guessing and hoping to get lucky.  If it fails in the Future it is on you.
    #2
    rpg7
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    Re: Switching power on AVdd 2020/05/21 08:28:35 (permalink)
    5 (2)
    On the PIC32MX, it fails to 'come alive' if AVdd is not kept within .3V of DVdd
    #3
    mpittman
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    Re: Switching power on AVdd 2020/05/21 08:44:51 (permalink)
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    Thanks rpg7.
     
    @NKurzman
    I'm not hoping to "get away with" anything.  As everyone knows, datasheets are often vague at best and do not cover every situation.  When the datasheet says AVdd must be powered, I'm just asking if that is for the general RUN state or is there a situation or exception to that requirement such as when the PIC is sleeping/waking.  If I were just "guessing or hoping to get lucky" I wouldn't be asking the question on here in the hope of getting some feedback from someone who has some experience doing this would I?
    #4
    RISC
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    Re: Switching power on AVdd 2020/05/21 09:34:42 (permalink)
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    Hi,
    I think datasheet is not "vague" regarding AVdd :
    In Absolute maximum rating section 37 page 429
    Voltage on AVDD and SVDD with respect to VSS .....................(VDD – 0.3V) to (lesser of: 4.0V or (VDD + 0.3V))
    If you don't follow this anything can happen...including no oscillation, spurious behaviour.
    On dsPIC30 it was simply not operating...
    Regards
    #5
    JackG
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    Re: Switching power on AVdd 2020/05/21 09:38:14 (permalink)
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    See my post on this subject:
    Re:Connect AVdd to what? Thursday, December 26, 2013 2:19 PM (permalink)
    The net is that on the pic24 that I refered to actually had AVdd connected to Vdd thru 2 ohms of metal on the chip
    You should measure your chip to see  if they are also connected.  -JackG
      
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    mpittman
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    Re: Switching power on AVdd 2020/05/21 09:51:22 (permalink)
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    Thanks for your feedback on the operation of the dsPIC30.
     
    Maybe vague was a bad choice of words, but all I meant was just looking at electrical specs for a pin doesn't tell the whole story of their operation.  You can remove power from the PIC completely (Vdd & AVdd) and as long as VBAT is powered you still have some "function" (RTCC and some RAM retention).  I was just asking if there is any mode of "function" with AVdd not powered.
     
    Thanks again.
    #7
    mpittman
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    Re: Switching power on AVdd 2020/05/21 10:01:51 (permalink)
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    @JackG
    Thanks for this.  Great post.  I'll check it out.
     
    Unfortunately though, this just adds confusion.  If Vdd and AVdd are essentially connected together, then why would the PIC not function if AVdd pin was not powered?
    post edited by mpittman - 2020/05/21 10:04:10
    #8
    RISC
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    Re: Switching power on AVdd 2020/05/21 12:04:26 (permalink)
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    Hi
    As you may imagine there are some "interconnects" between analog and digital cells inside the microcontroller. Not powering the analog part of the device will lead to currents flowing in analog area improperly powered through internal signals. I'm not expert in microcontroller design but making a Vbat pin is very difficult because you have similar issues...how do you make interfaces between powered parts and unpowered parts without having parasitic  currents flowing.
    Bottom line : if you don't follow the specification it is not predictable (unless you are the designer of the chip) what could potentially happen...
     
    If you want to "explore" similar issues apply digital signals to a PIC on the I/Os without applying the supply (typical issue in some designs which do not follow a proper power supplies startup sequence. the MCU may seem to operate sometimes but it is powered through the I/Os (in past ICs, latchup was occuring destroying most of the time the IC...)
    Regards
     
    #9
    mpittman
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    Re: Switching power on AVdd 2020/05/21 12:16:38 (permalink)
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    Sure.  Fair enough - I cans see that.
     
    So accepting that AVdd ALWAYS needs to be powered (in any/all modes), the question becomes: 
    Is there really any point in powering the digital and analog parts of the PIC separately anyway (if Vdd and AVdd are connected internally)?
    #10
    NKurzman
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    Re: Switching power on AVdd 2020/05/21 12:24:35 (permalink)
    5 (1)
    Connected leaves a lot of Room.  There is Not a wire there.  If there was it would not matter.
     If Power is a big concern, then use a Filter to connect Vdd and AVdd. The Question is will your design care.  Is a little more Noise OK?  Can you design a better filter?  Does it fit and not cost too much?
    #11
    RISC
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    Re: Switching power on AVdd 2020/05/21 12:59:47 (permalink)
    5 (1)
    Hi,
    AVdd must simply "follow" Vdd +/- 0.3V.
    The reason for filtering AVdd from Vdd generally using an inductor is to remove the digital noise so that A/D converter has proper performance ...
    A good example on how to design the power supply can be seen in the PIC24F "GC" Starter Kit for Intelligent Analog schematics.
    Look to the demoboard user manual appendix A.
    Regards
    post edited by RISC - 2020/05/21 15:26:20
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    crosland
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    Re: Switching power on AVdd 2020/05/21 13:03:53 (permalink)
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    JackG
    Re:Connect AVdd to what? Thursday, December 26, 2013 2:19 PM (permalink)
    The net is that on the pic24 that I refered to actually had AVdd connected to Vdd thru 2 ohms of metal on the chip
    You should measure your chip to see  if they are also connected.  -JackG

    No link and the forum search doesn't find it easily.
     
    merlepittman
     (if Vdd and AVdd are connected internally)?




    How do you think they are connected? What do you think that "2 ohms" really is?
    #13
    JackG
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    Re: Switching power on AVdd 2020/05/22 03:57:42 (permalink)
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    I think it really is metal. It's hard to get as low as 2 ohms with any other material on the chip.
    #14
    NKurzman
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    Re: Switching power on AVdd 2020/05/22 07:12:29 (permalink)
    4 (1)
    2 Ohms for a pieces of metal that small?
    Be careful roaming out electronic circuits.
    A multi meter bias can power them. That may affect the readings. Swap the leads to see if you get the same reading.
    #15
    JackG
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    Re: Switching power on AVdd 2020/05/22 10:31:44 (permalink)
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    The resistance of a resistor is pretty much independent of its physical size, it only depends on its length to width ratio. If Microchip’s process implements their chip wiring with aluminum material, its resistivity is somewhere around 50 miliohms per square. So to produce 2 ohms, a 1 mil wide line would be about 40 mils long.
    Back then, I did measure the resistance with both polarities and both readings were about the same but I don’t remember if I accounted for the multimeter lead resistance. Probably not since I got about the same readings between Vdd pins. In any case the 2 pads on that particular chip were connected and whether it was by 1 ohm or 10 doesn’t change the application problem very much.  I still wonder if they connected those 2 power pads on all of their designs. 
    Link to ref thread: https://www.microchip.com/forums/m768005.aspx
    -JackG
     
    post edited by JackG - 2020/05/22 14:36:28
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    NKurzman
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    Re: Switching power on AVdd 2020/05/22 11:00:29 (permalink)
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    Unless you are going to de-cap the chip and look at it. Or get a statement from Microchip.
    You can only go with what’s written in the data sheet. If you guess wrong, then it’s on you.
    #17
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