Photon2 needs reset to boot

Hi, on my board the photon 2 does not start at power up, it needs a key press on the reset button to boot.
Does anyone know why so it can be solved?
thanks in advance

Hi @why

Is this on a custom PCB?

Regards, Friedl

yes it is.

It has a socket for photon and electron, both working for years. A receptacle for photon 2 fits on the socket. I see tree behaviors:

  1. exceptionally, it just runs fine.

  2. I need to power it up two to three times before photon 2 starts running.

  3. Photon 2 does not start at all: it needs a rst key press and then works fine.

The later one is the problem as users need to open the box to start it.

The power is supplied by a dc/dc TPS63070RNMR generating 5V which seems to drive a photon, an electron and a BSOM, but photon 2 does not start.

Advise is welcome.

Hi @why -

Is it possible for you to share the schematic of the carrier board?

Many thanks! Friedl.

Photon2/P2 are not 5V tolerant !


@why -

As per the warning from @armor - hence why it would be really helpful to see your schematic.

Should be supplying 5V to the device either via VBAT or VUSB pin you should be ok, providing there is no other 5V signals going to logic pins or power rails. This is why your schematic would help a great deal to troubleshoot the problem :wink:


the 5V is only connected to VUSB. Yes for some off our applications photon2 can not be used as it is not 5V safe.

Although connecting pins from photon to photon2 is no rocket sience, showing the schematics is currently not an option.

As the photon 2 starts when powered by a li-po or by the usb connector, what is the difference with powering by VUSB?

Some ideas:
Can it help by adding a capacitance on VUSB?
Can the "EN"-pin cause trouble?
Can the "rst"-pin cause trouble?

Is there a "best practice" to avoid weired things?

thanks n advance,

hi Erik -

While I can appreciate that this is not rocket science, I doubt your problems lie in this area. Hence the preference to see the schematic but I can understand if it is proprietary information.

Quite a bit CAN be different actually. In the first instance, you are plugging the USB or Battery into the Photon2 board directly whereas in the second the power to the Photon VUSB pin is supplied via your own PMIC on your board. Unless I am misunderstanding your message.

I doubt it. I have done a tutorial on the Photon 2 including PCB files couple of days ago. There are output caps on the LDO but that is it.

I would think only if your are purposefully pulling it low. Or if there is significant amount of noise/interference on your board causing it to drop low. Same goes for the RST pin.

Without seeing the design, my guess would be to. have a closer look at the PMIC. Having said that, it is strange then that the same PMIC works 100% with Electrons?
Can you maybe measure Voltage on the VUSB pin while the Photon 2 is plugged into and powered by your board? Preferably measure on the top side on the pin itself. Also measure voltages on EN, 3V3 and RST pins?


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Hi Friedl,

this morning I looked at the PMIC and got it more or less working:

  1. powered by our standard supply the photon 2 seems to start now.
  2. powered by an external 5V Li-po battery pack you sometimes needs to plug the power twice. NOT OK, but for the time being we can live with it.

Things changed on the main pcb ( not the receptable ):

  1. removed a ferrite beat between PMIC and VUSB.
  2. added 22nF on VUSB to reduce higher frequencies.
  3. yes in some rare cases the electron does start but does not connect to the cloud, adding 4700uf on the power supply does help in those cases. For photon 2, 470uF turned out to be working in our case.

I have 2 questions for you:

  1. is there a photon2 reason why your car-parking pmic only uses 3.7V at VUSB?
  2. have you any idea on what is ment by limitations in the datasheet sentence: "Power out (when powered by USB) 5 VDC at 1A maximum. Power in with limitations" ?

Thanks for directing me to the pmic on our main board (will need to be changed in the future).

Best regards,

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Hi Erik -

This is good news :slight_smile:
Not sure about why you need to plug in the LiPo twice before it starts though, still seems something is a little amiss :thinking:

There are lengthy debates on the use of ferrite beads in power rails.. some love them and swears by them, some think they are redundant. IMHO... I remove them from PMICs more often than not... actually, always :grin:

as a filter?

It sounds like your regulator is having a hard time supplying the needed current. I cannot see why such a large output capacitor is needed. In example, in SA we require 2A of current to start the GSM on B-Som devices... this is ±8 times more current than the Photon 2 requires. I have never using more than 22uF output capacitors between regulators and B-Som. See example below, this is the LDO section of a PMIC I am adding to a design now. This part will be responsible for delivering up to 2A of current to start the GSM. You will notice a mere 10uF between the LDO and Device.

The 5V input is supplied either by the LiPo charger (sorry Proprietary bit so cannot show that).

Haha no, not at all. As you can see it the same section as used in the screen grab. this is one of my go-to LDO's and has been operating on about 50k+ devices over the last 5 years+ The output voltage can be stepped up by adjusting either of the resistors on the SENSE PIN.

My guess would be that the power rail on the Photon will only be able to supply 1A of current total when you are powering the device via USB. This is most likely a limiting factor of the onboard LDO but will have to be confirmed by one of the Particle engineers.

Glad I could help in some way or the other! In my experience over the years as a PCB designer, PMIC's are more often than not overlooked and probably the area where clients (and engineers) are most likely to try and cut costs. Almost as if they believe all regulators are equal and if they measure 3V with a MM, they seem to be happy. I have seen this to be cause of many headaches and losses for clients. My guess is that if you sort out your PMIC you will have no further problems in this regard.

Regards, Friedl.