Power management

1. A device comprising:

an antenna;

a power harvesting circuit for harvesting power from a radio frequency field received at the antenna in order to power functions of the device;

a communication unit comprising a first integrated circuit coupled to the antenna for transmitting and receiving signals by means of the antenna, the communication unit being configured to communicate according to a protocol in which a party to a communication session deems the session to have timed out if during a predetermined period it does not receive a signal from another party to the session; and

a module comprising a biometric reader with a biometric sensor and a processing circuit;

the device being configured to interrupt the operation of the module in response to receiving a signal from the communication unit, after which the communication unit transmits a signal by means of the antenna,

wherein the communication unit is configured to:

receive a request for identity verification from a first party to a communication session in which the device is also a party,

in response to receiving that request, signal the module to perform an identity verification process,

maintain the communication session until the identity verification process is complete by periodically transmitting a message to the first party,

receive a result of the identity verification process from the module, and

transmit the result of the identity verification process to the first party,

wherein the communication unit is configured to:

on transmitting the message to the first party, signal the module with a first additional signal, the first additional signal having a first format, and

wherein the module is configured to:

in response to receiving the first additional signal having the first format, resume operation, and

wherein the communication unit is configured to:

a predetermined time interval after transmitting the message to the first party, signal the module with a second additional signal, the second additional signal having a second format, and

wherein the module is configured to:

in response to receiving the second additional signal having the second format, interrupt the operation of the module, and

on interrupting operation, transmit a signal to the communication unit.

2. A device as claimed in claim 1 , wherein the communication unit is configured to maintain the communication session until the identity verification process is complete by periodically transmitting a wait time extension message to the first party.

3. A device as claimed in claim 1 , wherein the module is configured to, on receiving a signal of the first format, transmit an acknowledgement signal to the communication unit.

4. A device as claimed in claim 1 , wherein:

the first and second additional signals are represented by high and low states of a single signal line.

5. A device as claimed in claim 1 , wherein the module is configured to, on interrupting operation, store a state of at least one active component of the module, and set at least that component in a low power state.

6. A device as claimed in claim 1 , wherein the module is configured to, on beginning or resuming operation, sense an amount of power available from the power harvesting circuit and select for execution a task requiring not more than the amount of power available.

7. A device as claimed in claim 6 , wherein the module comprises a first subsystem having a relatively high power requirement and a second subsystem having a relatively low power requirement and the module is configured to, on beginning or resuming operation, sense the amount of power available from the power harvesting circuit and to, if a first task involving use of the first subsystem and a second task involving use of the second subsystem and not the first subsystem are both available for execution, select for execution the second task if insufficient power is available to meet the requirement of the first subsystem.

8. A device as claimed in claim 7 , wherein the module is configured to, if sufficient power is available to meet the requirement of the first subsystem, select the first task for execution.

9. A device as claimed in claim 6 , wherein the module is configured to, on beginning or resuming operation, sense the amount of power available from the power harvesting circuit and to, if a first task having a first power requirement is available for execution, to commence execution of that task only if sufficient power is available to complete the task.

10. A device as claimed in claim 1 , wherein the communication unit is implemented in the first integrated circuit and at least part of the module is implemented in a second integrated circuit.

11. A device as claimed in claim 10 , wherein the first integrated circuit is a secure element.

12. A device as claimed in claim 1 , wherein the device is in the form of a card.

13. A device as claimed in claim 1 , wherein the device is an identity verification device and the module is the biometric reader comprising the biometric sensor for sensing a biometric feature and the processing circuit is configured to process data sensed by the sensor to form processed data and compare the processed data with data defining a reference biometric feature so as to verify a sensed biometric feature.

14. A device as claimed in claim 13 , wherein the module is a fingerprint reader.