Read state retention circuit and method

1. A read state retention circuit, applied in radio frequency identification (RFID), the read state retention circuit comprising:

a charge storage unit;

a charging circuit, coupled to the charge storage unit for charging the charge storage unit, the charging circuit having an NMOS transistor, the NMOS transistor being configured to charge the charge storage unit and having a drain terminal coupled to an operation voltage; and

a sensing circuit, coupled to the charge storage unit for sensing a voltage level of the charge storage unit.

2. The read state retention circuit according to claim 1 , further comprising a state indicator, coupled to the sensing circuit for generating an indication signal in response to the voltage level.

3. The read state retention circuit according to claim 2 , wherein the state indicator is a flip-flop.

4. The read state retention circuit according to claim 3 , being applied to an RFID tag, wherein the RFID tag responds to a reader whether the RFID tag has been read according to the indication signal.

5. The read state retention circuit according to claim 4 , wherein a reading signal is asserted to trigger the sensing circuit and the state indicator to sense the voltage level of the charge storage unit and generate the indication signal in response to the voltage level, wherein the indication signal is asserted to indicate that the RFID tag is read.

6. The read state retention circuit according to claim 1 , wherein the charge storage unit is a capacitor and charge on the charge storage unit represents a read state.

7. The read state retention circuit according to claim 1 , wherein the sensing circuit comprises a p-type metal oxide semiconductor (PMOS) transistor, an n-type metal oxide semiconductor (NMOS) transistor and a NOR gate, the PMOS transistor has a drain coupled to a drain of the NMOS transistor and the NOR gate, and the NMOS transistor has a gate coupled to the charge storage unit in order to sense the voltage level of the charge storage unit.

8. The read state retention circuit according to claim 1 , wherein the read state retention circuit is implemented in a single semiconductor chip.

9. The read state retention circuit according to claim 1 , wherein the charging circuit comprises a voltage doubler, coupled to the NMOS transistor, wherein the voltage doubler receives a setting signal and doubles a voltage level of the setting signal to control the NMOS transistor to charge the charge storage unit.

10. The read state retention circuit according to claim 9 , wherein the voltage doubler comprises a first inverter, a second inverter and a capacitor successively connected in series.

11. A read state retention method, comprising:

asserting a read signal;

raising a voltage level of the read signal; and

charging a capacitor by an NMOS transistor in response to the raised voltage level of the read signal, the NMOS transistor having a drain terminal coupled to an operation voltage.

12. The method according to claim 11 , wherein the raising step doubles the voltage level of the read signal by a voltage doubler.

13. The method according to claim 12 , further comprising a step of sensing a voltage level stored in the capacitor.

14. The method according to claim 13 , further comprising a step of generating an indication signal to indicate a read state in response to the sensed voltage level of the capacitor.

15. The method according to claim 11 , wherein charge on the capacitor represents a read state.

16. The method according to claim 15 , wherein the charging step charges the capacitor by the NMOS transistor in response to the raised voltage level of the read signal whereby the read state is retained.