wpballa1
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What is the J2 header for on the Model 4B?

Tue Jun 25, 2019 12:57 am

Could it be a power/reset button??? That would be great.

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RiccardoMoretti
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Re: What is the J2 header for on the Model 4B?

Tue Jun 25, 2019 6:32 am

I suspect it gives more than a reset,
All the Pi Devices have a reset button with exception of the Rev 1Model B as far as I know,
In all the Pi's I've added a reset switch I have connected the 2 pins together, however the Pi 3 , I found that I needed to connect the Run to Ground to perform the reset,

I see the latest J2 header on the Model 4b has 3 pins, Im not sure if one is a ground, but i suspect connecting the run to ground would perform a reset,

I would like to find better documentation myself on what the 3 pins are, and the "proper" way to perform a reset..

Hopefully someone here can give us a clear answer?

hippy
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Re: What is the J2 header for on the Model 4B?

Thu Jun 27, 2019 10:33 am

RiccardoMoretti wrote:
Tue Jun 25, 2019 6:32 am
I would like to find better documentation myself on what the 3 pins are
The J2 header is included in the reduced schematics.

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RaTTuS
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Re: What is the J2 header for on the Model 4B?

Thu Jun 27, 2019 11:43 am

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bensimmo
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Re: What is the J2 header for on the Model 4B?

Thu Jun 27, 2019 12:30 pm

I assume it's the same as the one on the 3B+

hippy
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Re: What is the J2 header for on the Model 4B?

Thu Jun 27, 2019 1:08 pm

bensimmo wrote:
Thu Jun 27, 2019 12:30 pm
I assume it's the same as the one on the 3B+
No, the pinout and circuitry is different, and routed to a different variant of PMIC.

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rschneider
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Re: What is the J2 header for on the Model 4B?

Thu Nov 28, 2019 2:37 am

I have the pin-out assignment AND a suggested use for the J2 header.
Raspberry Pi 4B
"J2" – 3-pin HEADER PIN-OUT
————————————————
Pin 1 (square; "Global_EN): ---- 5V
Pin 2 (round; middle): ---------- Gnd
Pin 3 (round; "RUN"): ----------- 3.3V

The J2 Header connector CAN BE USED FOR A CASE-FAN CONNECTOR, after soldering wire pins to empty J2 header holes on on PCB...

NOTE: The center connector is direct-connected to Ground pins (Gnd) on the GPIO header, since Ground ("Gnd") is always common circuit-wide, whether a device is powered or not. However the outside J2 Header pins, 1 and 3, do NOT have ohmic continuity to GPIO power voltage pins - but the J2 header's 3.3V and 5V pins ARE active when the Pi is powered. This does not lessen their usefulness as fan-connector pins, obviously. Use of the J2 header to connect a case fan is preferable to using GPIO pins for a case fan, if you intend to use a 40-pin cable to connect the GPIO header outside the case, and still have a case fan running. I personally would like to know what the J2 header is intended for, and am continuing to research this; it might not have wide applicability, since no header wires are connected to the empty J2 PCB holes. In the mean time, I'm planning to add header wire pins to the J2 PCB holes and use it to power my case fan (pre-installed in a Vilmos case), which is intended to run on both 5V (fast) and 3.3V (low) speeds. The Vilmos case fan comes with female header sockets attached to the fan wires, for plugging into GPIO pins. GPIO is NOT IDEAL for connecting a case fan, since I have a 40-pin female header with attached cable that takes GPIO outside the case (for breadboarding, etc).
[NOTE: The PoE header (Power-over-Ethernet) does NOT provide 3.3V and/or 5V power pins; they connect to transformer center-taps in the Gigabit Ethernet circuit block.]

REFERENCE: See the "Rasberry Pi 4 Model B (Reduced)" Schematic Diagram, available as a 1-page PDF download (my download was dated June 18, 2019):

www.raspberrypi.org/documentation/hardw ... educed.pdf

hippy
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Re: What is the J2 header for on the Model 4B?

Thu Nov 28, 2019 12:46 pm

rschneider wrote:
Thu Nov 28, 2019 2:37 am
I personally would like to know what the J2 header is intended for
It is I imagine provided for cases where someone wishes to add external power management or control to the Pi. The RUN signal allows the Pi SoC to be reset, the EN allows the PMIC to powered up and down.
rschneider wrote:
Thu Nov 28, 2019 2:37 am
The J2 Header connector CAN BE USED FOR A CASE-FAN CONNECTOR, after soldering wire pins to empty J2 header holes on on PCB...
That really isn't recommended. And I would be surprised if it even works with the EN pin connected to 5V via a 100K 63mW resistor, RUN to 3V3 via 10K 63mW.

If you do try it; don't be surprised if it doesn't work, damages your Pi, or causes unreliable operation.

trejan
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Re: What is the J2 header for on the Model 4B?

Thu Nov 28, 2019 3:01 pm

rschneider wrote:
Thu Nov 28, 2019 2:37 am
The J2 Header connector CAN BE USED FOR A CASE-FAN CONNECTOR, after soldering wire pins to empty J2 header holes on on PCB...
No. This is isn't going to work. As hippy mentioned, the GLOBAL_EN and RUN_PG2 pins have resistors on them as weak pull ups on them which will significantly limit the amount of current available. You have less than 1mA available on both pins which is far lower than what a fan needs. Connecting a fan is likely to cause the Pi to reset as the fan tries to start up which causes the voltage on the two reset pins to droop that then resets the SoC or PMIC.

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