I am reading the power guidance and there is this note:
If both the USB1 and 5V (as input) have power to supply to the board at the same time without battery attached, the 5V power will supply to VBAT (simulates battery).
I am not sure what simulates battery means in this context.
Second, my intent is to have a HAT like board connected to the Tachyon that is providing the 5V to power it. Reading that, is it save to assume it is “safe” to have 5V from the HAT and USB providing power? I don’t look at this as a production use case, but for development purposes I am curious what protections might be recommended on the HAT itself.
Lastly, I think I read that the need for having a battery to use the device will not be required forever, is that correct? I do not think I need it if powering from the 5V pins, but this is a question in general.
When you power by 5V from the HAT connector, the battery is not required. If you are not using a battery:
If USB1 has PD or DCP, power will come from USB
If USB1 does not have PD or DCP, power will come from 5V and USB1 will be data only. This is the USB-A laptop to USB-C cable scenario.
The battery simulation comes into play because of the limited number of internal power paths. In some no-battery scenarios, the 5V is regulated and essentially fed into the battery input, along with supplying the necessary battery identification signals to the system. The main reason this is mentioned is that if you query the system for what the power supply is, it will respond with battery, because that's the power input that is being used from the point of view of Linux, even though it's really the 5V HAT power input.
It is a safe and supported configuration to have both HAT 5V power in and USB1 connected at the same time.
If powering from the HAT the battery is not required, given it shares a power path, I would assume it should NOT be connected? Or is that fine? This is more just out of curiosity and if I forget
And just to clarify, you mentioned USBA-A to USB-C, What if it is a USB-C to USB-C / PD connection - I think it would still be fine if powered via the HAT at the same time based on the battery path.
My question on if the battery will be required long term is more general, without powering from the HAT, right now both USB and battery are needed, I think I read that in the future USB will be enough?
If you have all three connected (USB, HAT 5V in, Battery), basically the HAT is ignored because it can't use the simulated battery power path because the battery is using it. But it's safe to connect all three.
If using a USB-C to USB-C it mostly depends on whether PD/DCP is negotiated as to the power coming from USB or HAT 5V in. It will automatically switch as necessary in real time.
The reason the battery or HAT 5V is required when using USB PD mode is that the current drops too low during negotiation. The battery or HAT 5V can keep the device running until sufficient power can be supplied by USB.
In USB-C DCP mode, there is enough current the whole time, so you don't need battery or HAT.
When using USB-A to USB-C, the USB port can never supply enough current, so battery or HAT 5V is always required.
