What volts can I supply the VBAT pin with?

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  • I have not done the FET tutorial. This is consistent on every PICO I have tried thus far. Its a little concerning because the current project is similar to what your doing. I am actually creating a wireless sprinkler control system where I have solar panels built into the in ground sprinkler boxes connected to a 12 volt battery. I can add a lm7810 to the circuitry but was hoping to get away without having to. I did try cleaning the back up with alcohol and still no change.

    https://goo.gl/photos/qfNoU1xSMsBseRoM9

  • I just tried it here and got the same disconnection as you (although I can still program the board with power applied). Honestly I'm a bit confused about what's going on.

    The power supply circuit is here: http://www.espruino.com/Pico

    Looking at VBat, it's doing what we'd expect, and the 3.3v pin still seems stable at 3.3v - the regulator is rated for 16v normal (and 18v max). I guess potentially the voltage regulator might somehow shut off when the input voltage rises suddenly?

    But once power is applied, I can write some code, unplug and replug USB, and everything stays working fine... So in your case as well (sprinkler where power would stay connected) it should be ok?

    But that doesn't explain why you can't reconnect? Are you using Windows? I can't tell from the video but it seems likely.

    I wonder whether the issue is actually because of a Chrome bug. I know if you just unplug a USB device while Chrome is connected, when you reconnect you can get a problem like you're experiencing where you can't send or receive data. It could be that the disconnect when power is applied is causing that to happen.

    If you disconnect the Pico from USB, shut down the Web IDE and start it again, it might work next time you connect.

    I guess the way to stop it is to start the Pico powered off 14v, and to then connect USB and keep it running off the 14v power supply - that should work.

  • Yeah I am doing that now and its very unreliable for some reason. Very few times have I been able to reconnect. Never over 14 volts have I been able to. I am using the Dell Chromebook. I tried it on my windows computer and nothing changed.

  • When it's not working, is Vcc actually at 3.3v? Measure it...

    If you have a scope, put the scope on it. There's no black magic, and the only connection between the regulator and the Pico is through power and ground - so what's it doing to the power?

  • So I checked it over extensively and here is where I am at. I have a NRF905 connected exactly as follows:

    var data, csn = B10, dr = B1, txen = A6, ce = A7;
    SPI2.setup({sck:B13, miso:B14, mosi:B15});
    var nrf = require("NRF905").connect(SPI2,csn,dr,tx­en,ce);
    var myAddress = 0x17;
    var masterAddress = 0x27;
    
    function rx_callback(data){
      console.log("got "+JSON.stringify(data));
    }
    
    E.on('init', function() {
      clearWatch();
      clearTimeout();
      nrf.init(myAddress, rx_callback);
    });
    
    save();
    

    When I have the PICO connected to the NRF905 with the computer powering everything works fine. When it is connected to the the battery without the NRF905 I can still talk to it with the computer. When connected with the battery and NRF905 then all connection is lost and am unable to reconnect. The pico led blinks very fast.

    The pins seem to read as follows (connected to computer only, or connected to battery without NRF905):
    Bat in is 13V, B15 is 3.3v, B14 is floating, B13 is 0v, B10 is 3.3v, B1 is floating, A7 is 3.3, A6 is 0v, A5 is floating.

    With the battery they are similar but seem to be resetting over and over.

    GND is 0v, VBAT is 13V, VDD is 3.3v, B3 is floating, B4 is floating, B5-A8 are 0v.

    I do have an oscilloscope but its currently not working properly..... And not sure where to even begin even if it were working......

  • So I actually tore into my oscilloscope and found that a wire on the horizontal deflection plate was disconnected. Anyways, I can say that when less than about 9.5 volts the pico keeps working. 10 volts will cause a slow resetting of the pico while at 13 volts it will reset quite a bit faster. Also at 10 volts it takes a few seconds before it started resetting where as 13 volts it's immediate. The USB side of the Schottky diode appears to be pulsing and the BAT side is steady. So at BAT 12 volts the USB side is jumping between around 3.3 volts and 8 volts. FET and B0 are not connected and when checked for resistance do not register at all. The supply side of the regulator is steady at BAT Volts and GND is steady 0v where 3.3 pulses to 0v

  • I'm not really sure what's up - I asked around and all we could think of is that the diode breaks down with a certain reverse voltage. Which revision board do you have? I did change the diode for a better one on the rev 1.4.

    Now, when we checked, the reverse voltage was within the allowed limits - 30v or so, but I wonder whether if the diode has been stressed - especially on the 1v3 (even a momentary short) - then it might break down at a lower voltage than rated.

    If you're ok just powering it off 12v, how about just removing that diode? It should be relatively easy to take off (or just lift one side) with a soldering iron, and should totally fix your problems - although after that you'd have to make sure the Pico was powered externally to program it.

    Sorry about that, but it's about the only thing I can think of at the moment.

  • Just to add, if USB is jumping to 8v that'll definitely be causing the reset - and you should try and avoid doing anything that makes that happen as it'll eventually kill the GPIO line attached to it, which will stop you from being able to connect over USB.

    But that does mean that the only thing that could be causing the voltage is that diode...

    I just checked and the rev 1.3 has a 1PS76SB40, which has a reverse voltage of 40v so should also be fine.

  • So this Pico I am currently using is 1.4. And remember this does NOT happen when the nrf905 is NOT connected. So i suspect it's something to do with current?

  • How much power does the nRF905 draw? Maybe you could try putting a capacitor across the power lines to it.

    Is anything hot on the Pico? The voltage regulator has over-temperature protection

    There's also a polyfuse for overcurrent - is it possible that you connected the nRF905 to the 'Vout' pin of the Pico - which would be 13v? That would then massively over-volt the nRF905, which would probably put high voltages on the IO lines, which would go back into the Pico's STM32 and might also raise the voltage on the USB line. Finally the polyfuse would trip and cut the voltage - but it's self-resetting so it would just keep doing it over and over.

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What volts can I supply the VBAT pin with?

Posted by Avatar for DavidSchoen @DavidSchoen

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