Pogo pins power clarification - reading schematics

[bokomaru]

I agree, no separation between the pogo pin #1 (DCIN) and the charger port is a real issue, but only when attaching an external battery.

The problem is more general than that. It's an issue no matter what you connect to pogo pin #1. Doesn't matter if it's an external battery or not, or even whether you're using pogo pin #1 as an input or as an output.

If you are providing power as an input to the Pinephone, then there is the possibility of two power sources being shorted together at DCIN. One from pogo pin #1, the other from the USB C connector.

If you are drawing power as an output from the Pinephone, then the presence/absence of that power at pogo pin #1 depends on (1) is a charger connected to the USB C connector, or (2) is the Pinephone in USB host mode.

As a side note, one more case should be worth trying:

4) Phone powered up, USB flash drive connected to the USB Type-C port (to turn the phone into a current source)

Sure, this is a similar test to the one we talked about before: controlling PD8-VCC5V_EN.

In the first case above, I still have some doubts about 4 V at USB-5V.  First, why is Q600 enabled by the PMIC while the phone is turned off?

Why not? :-P

The battery can still power something while the A64 is off. You can provide ACIN while the phone is off, too, e.g., you can charge the battery.

Hmm, something is either wrong there, or I'm missing something.  As we know, as soon as a charger is connected, the phone powers on, so it would be pretty much impossible to measure the effects of connecting a charger while the phone remains powered off.

Nothing wrong there. Connect the charger, let the phone power on, then shut it down from software. Leave the charger plugged in, and ta da! You're charging while the A64 is off.

Second, the battery is declared with 3.8 V as the nominal voltage, so why don't we see voltage lower than 3.8-4 V at USB-5V, which should be caused by the voitage drop across D600?

Two things.

(1) Sure, the "nominal" voltage is 3.8 V. But depending on how "full" it is, a lithium polymer/ion battery will range from around 3.4 V to 4.2 V, unloaded. When drawing current, battery voltage dips, and can go as low as 3.0 V or so. This is not specific to the PinePhone; this is based on my knowledge of hobby lithium polymer batteries for radio control electronics.

(2) We don't know where any voltage drop comes from, or what the voltage drop across D600 would/should be. And we haven't been carefully measuring voltage drops, which seem to be only ~ 0.15 V. D600 was only a guess/example for a possible explanation of a relatively minor, constant voltage difference in one or more scenarios.

Pretty much the same question about the voltage drop caused by D600 applies to the second case above.  Whenever the LP6226 isn't enabled, USB-5V should show VBAT minus the voltage drop across D600, which clearly isn't the case.  Thus, something doesn't add up, meaning that further investigation is required.

What should the voltage drop be across D600? We haven't looked it up.

The voltage drop across D600, which doesn't show up on USB-5V, is something that needs to be investigated further, if you agree.

Meh, the voltage drop could be investigated. But I don't necessarily think that knowing exactly where the ~ 0.15 V drop comes from is critical to understanding this. There's enough other evidence clearly showing that pogo pin #5's voltage is related to charger or battery voltage.

I cannot remember where (maybe in the PineBook Pro schematic?), but I've seen power switches as part of the USB circuitry that are used to cut off the bus power when a power source (not a power sink) is connected to a OTG-capable USB port.  Those power switches were described by the component manufacturer as made specifically for use with OTG USB ports.

Sure. I don't have a lot of experience designing/analyzing OTG hardware, and I haven't seen PineBook Pro schematics. Anyway, what you're describing is handled by the combination of the AXP803, the ANX7688S, the A64, the LPW5206, and the LP6226. If I remember correctly, there is actually an option (in hardware or in software) to select between the AXP803 or the ANX7688S as the "controller", who senses or decides, for example, when a particular type charger is connected.

So I'll stand by my interpretation that we shouldn't provide power _to_ pogo pin #5 as an _input_ to the Pinephone.

I really have nothing against that rule, but then what's the actual purpose of the LPW5206?

It has two purposes.

(1) Limit current flowing from USB-5V to DCIN.

(2) Enable/disable the ability for that current to flow at all.

[edit: And the larger purpose is for USB host mode, where the Pinephone supplies 5 V on DCIN, which goes through the USB C connector as an output. I trace the flow of current for that below, from PS to the USB C port.]

Unless there are actually different "USB-5V" power planes inside the PinePhone, instead of a single USB-5V plane, the schematic allows USB-5V to become a power source.  I know, it's strange and I don't see the actual purpose, but that's what the schematic shows to us.

No, it doesn't. Unless you can show that, for example, 5.3 V at USB-5V would not flow backwards across zener diode D600 to PS, a power source should not be connected as an input to the Pinephone at pogo pin #5.

That's exactly what I thought initially, because it makes perfect sense.  But, that would be the case only if the IN and OUT of the LPW5206 changed their places in the schematic.  With the IN becoming OUT and vice-versa, the LPW5206 would be taking DCIN and passing it to USB-5V, while limiting the current to 500 mA (or 1 A).  But, that's simply not the case currently, unfortunately.

No, LPW5206 _is_ connected properly for when the phone is acting as a USB host and is driving the USB C connector's VBUS. IN needs to be on USB-5V, and OUT needs to be on DCIN.

The name "DCIN" is confusing there. It is actually not _only_ a 5V input to the phone. When the phone is a USB host, DCIN is a 5V output from the Pinephone.

There is no need for a device like the LPW5206 to monitor current going into the phone. DCIN goes directly to the PMIC's ACIN/VBUS inputs.

I think that if you ignore pogo pin #5, it's exactly what the schematics tell us. Just because pogo pin #5 is on the input side of the LPW5206, it doesn't mean that we _have_ to provide power there, or even that we _can_ or should. We definitely can draw power from there, though like we said before, we don't know exactly how much current is safe.

Could you, please, explain how did you come to that conclusion, based on the schematic?  I agree that it would be logical, but that is not what's in the schematic, unfortunately.

The schematics tell us that pogo pin #5 is not for connecting a power source as an input to the Pinephone.

(1) If you supply power at pogo pin #5, current could travel backwards across the zener diode D600, connecting a power input the the PS output of the AXP803. This depends on the actual voltage provided at pogo in #5, and on the zener breakdown voltage of D600.

(2) When the Pinephone is a USB host, current flows from PS, through D600 and the LP6226 to USB-5V, through the LPW5206 to DCIN, through the 40PIN connector to the "USB-C small board", through an AW338XX, to the USB C connector.

The IN of the LPW5206 is on USB-5V, and the OUT is on DCIN, allowing current to flow in this direction. If IN and OUT were flipped, the Pinephone would not be able to act as a USB host by driving DCIN.

Well, yes, but not easily if there is also a charger connected to the phone's Type-C port.

Right, there's no isolation between pogo pin #1 and the USB C port; it's the same net. That's the only real functional/safety concern that I have with how the mainboard pogo pins are designed. [edit: Also, no isolation between a power source on pogo pin #1 and the Pinephone's USB host mode]

There is actually an AW338XX on the USB-C small board, which we've talked about before, but I don't think it fixes this problem.

Let's remember, there's no isolation between the power sources inside the phone.

I don't think you meant it that way, but this isn't literally true for all power sources. So to clarify:

There is separation and control for other power sources and power modes. For example, the battery is connected to the PMIC in a different place than the USB C port. And for example, there is sensing and control so that the Pinephone can either act as a USB _host_ by driving the USB C VBUS, or as a USB _device_ by drawing current from there.