Can a passive NFC tag harvest power from a smartphone when its screen is locked? In other words will there be a magnetic field when the screen of the phone is locked ? I understand that you cannot read an NFC tag or write on it when the screen is locked but the question here is about the power (i.e. the magnetic field)
thanks
It depends on the OS and power management of the phone device you are referring to. One of the main reason why Read operations are generally not enabled when the screen is locked is that the operation to perform on a tag in proximity is generally user driven. In the screen lock state user cannot provide such input. Also, one has to be sure which app of phone is processing the tag.
Talking about NCI 1.0 there is no command that directs the firmware to behave differently based on the Screen state. So, a discovery cycle started with screen on will continue to remain on in the screen locked state. But OEM's may have implemented proprietary commands/configs to achieve better control.
NCI 2.0 supports disabling Poll/Listen in certain states of Screen using a Set Config.