A4WP Signalling and Messaging Protocols

- the A4WP wireless charging system incorporates a data channel / control link to provide messaging and signalling between the receiver and transmitter for which a specific message and signalling protocol is used.

One of the key features of the A4WP wireless charging system is the bidirectional control link between the transmitting and receiving units.

This A4WP signalling control link enables accurate control of the charging and power deliver.

There are various elements to the signalling and control message protocol and structure.


A4WP PTU states

A key element of the A4WP signalling and control is the different states that are defined for the transmitting unit.

There are four states into which the PTU may exist. These functional states typically progress from one to the next:

  • Configuration:   This state occurs just after the power-on state has been initiated for the PTU. This state includes a self-check.
  • Power Save:   Once the A4WP charger has completed its power on routine, it typically enters its power save state. During this state it performs load detection routines and checks for the number of devices that may require power.
  • Low Power:   During this state, the A4WP system establishes its data link between the charger and receiver
  • PTU Power Transfer:   During this state the A4WP system transfers power from the charger or transmitter unit to the receiver unit. In addition to this the system monitors for state transitions, charge durations, exit times and fault conditions. Where required, information is passed over the control link using the defined protocols to enable the required actions to be taken.

A4WP PRU states

Like the charger or PTU, the receiver also moves through various states to control the relevant actions within the receiver.

Unlike the A4WP charger which has four states, the receiver only has three:

  • PRU Null:   This PRU state depends upon the level of the output from the PRU rectifier and therefore whether a specific level of power is being received.
  • PRU Boot :   During this phase the data link is established between the PRU and PTU to enable the control messages to be sent. It follows a defined protocol.
  • PRU On:   This is the phase during which power is transferred to the receiver unit. Also during this state, messaging is carried out between the transmitter and receiver to establish time of charge left, whether charging should cease, and fault conditions including over-voltage, over-current, over-temperature, etc.

A4WP signalling

A4WP uses the Bluetooth Low Energy, BLE profile for system control messaging and signalling.

This profile for the A4WP control messaging allows a variety of functions to be undertaken including:

  • PTU current adjustment by the PRU to gain the correct power transfer.
  • Transfer of PRU data to the PTU.
  • PTU specific data can be signalled to the PRU.
  • PTU can control the charge prioritisation.

In addition to this A4WP uses the Bluetooth 4 Generic Attribute framework, GATT, and this enables devices to pair without the need for manual intervention - just when they come close enough for charging to be initiated.

By Ian Poole


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