Accurate persistent nodes
A timing circuit that can function as an accurate persistent node in an RFID tag includes a power capture circuit for capturing power from a power source, and a counter circuit that provides a count representing a progression of time. The count can then be compared to a reference value representing a time constant of the circuit.
1. A timing circuit, comprising:
a power capture circuit for capturing energy from a power source; and
a counter circuit coupled to the power capture circuit, the counter circuit providing a count that represents a progression of time,
wherein the power capture circuit includes a differential amplifier and a capacitance,
wherein the differential amplifier compares a voltage level of the power source to a voltage level of a node formed on the capacitance; wherein if the voltage level of the power source is greater than a specified voltage level, the differential amplifier clamps the node to the power source.
2. The circuit of claim 1 , wherein the power capture circuit includes a capacitor and a diode.
3. The circuit of claim 1 , wherein the specified voltage level is based on a voltage level of the node.
4. The circuit of claim 1 , wherein if the voltage level of the power source drops below a level of the capacitance stored on the node, the node is isolated from the power source.
5. The circuit of claim 1 , wherein the power source generates a current from a wireless carrier signal.
6. The circuit of claim 1 , wherein when the power source is equal to or less than a specified voltage level, the differential amplifier isolates the node from the power source.
7. The circuit of claim 1 , further comprising a subcircuit for comparing a voltage level of the power source to a voltage level of a node formed on the capacitance; wherein if the voltage level of the power source is greater than a predetermined voltage level, the differential amplifier clamps the node to the power source, wherein if the voltage level of the power source drops below a level of the capacitance stored on the node, the node is isolated from the power source.
8. The circuit of claim 7 , wherein the power source generates a current from a wireless carrier signal.
9. The circuit of claim 1 , wherein the differential amplifier is deliberately imbalanced.
10. The circuit of claim 1 , wherein the counter circuit includes a reference oscillator for generating the count, wherein the reference oscillator is powered by power directed thereto from the power capture circuit.
11. The circuit of claim 10 , wherein the reference oscillator functions for at least a predetermined amount of time after the power source ceases to provide power to the power capture circuit.
12. The circuit of claim 11 , wherein the predetermined amount of time is greater than a maximum frequency hopping interval of a remote wireless device in wireless communication with the circuit.
13. The circuit of claim 12 , wherein the predetermined amount of time is greater than a maximum frequency hopping interval for Radio Frequency Identification (RFID) devices as specified by the Federal Communications Commission (FCC) of the United States of America.
14. The circuit of claim 10 , wherein the reference oscillator is a voltage controlled oscillator.
15. The circuit of claim 10 , wherein the reference oscillator operates at substantially a constant frequency when operating.
16. The circuit of claim 10 , further comprising a calibration matrix of digitally controlled diodes for calibrating a bias current of the oscillator.
17. The circuit of claim 16 , wherein the calibration matrix is automatically calibrated upon detection of an event.
18. The circuit of claim 17 , wherein the circuit self-calibrates from a single reference frequency.
19. The circuit of claim 18 , wherein the reference frequency is derived from a signal from a remote device.
20. The circuit of claim 10 , wherein the reference oscillator operates continuously.
21. The circuit of claim 10 , wherein the reference oscillator operates on a current of less than about one microampere (μA).
22. The circuit of claim 10 , wherein the reference oscillator operates on a current of less than about 1 nanoampere (nA).
23. The circuit of claim 10 , wherein the reference oscillator operates on a current of less than about 10 picoamperes (pA).
24. The circuit of claim 10 , further comprising a switched capacitor precision resistor for controlling a bias current to the reference oscillator.
25. The circuit of claim 10 , further comprising an accumulating counter for generating the count based on input received or derived from an output of the reference oscillator, wherein the count is compared to a reference value representing a time constant of the circuit.
26. The circuit of claim 25 , wherein the accumulating counter is reset by a command from a remote device.
27. The circuit of claim 1 , wherein a reference value against which the count is compared is programmable by a remote device, the reference value representing a time constant of the circuit.
28. The circuit of claim 1 , wherein the gate to source voltage across one or more transistors in the circuit is less than that transistor's threshold voltage.
29. The circuit of claim 1 , wherein the gate to source voltage across one or more transistors in the circuit is between about 100 mV below a threshold voltage of the transistor and about 100 mV above a voltage at a lowest leakage point of the transistor.
30. The circuit of claim 1 , wherein the circuit operates at current levels at least about 10 times lower than a threshold voltage thereof, wherein the circuit operates at current levels at least about 10 times higher than at a lowest leakage point thereof.
31. The circuit of claim 1 , wherein the circuit is embodied on a Radio Frequency Identification (RFID) tag.
32. An RFID system, comprising:
an RFID tag implementing the circuit of claim 1 ; and
an RFID reader in communication with the RFID tag.
33. A timing circuit, comprising:
a power capture circuit for capturing power from a power source, wherein the power capture circuit further comprises a subcircuit for comparing a voltage level of the power source to a voltage level of a node formed on the capacitance; wherein if the voltage level of the power source is greater than a predetermined voltage level, the differential amplifier clamps the node to the power source, wherein if the voltage level of the power source drops below a level of the capacitance stored on the node, the node is isolated from the power source; and
a counter circuit coupled to the power capture circuit, the counter circuit providing a count that represents a progression of time, wherein the count is compared to a reference value representing a time constant of the circuit.
34. A method for generating a timing signal, comprising:
generating a count that represents a progression of time using a counter circuit coupled to a power capture circuit, the power capture circuit being for capturing energy from a power source, wherein the power capture circuit includes a differential amplifier and a capacitance, wherein the differential amplifier compares a voltage level of the power source to a voltage level of a node formed on the capacitance; wherein if the voltage level of the power source is greater than a specified voltage level, the differential amplifier clamps the node to the power source;
comparing the count to a reference value representing a time constant of the circuit; and
outputting a timing signal if the count matches the reference value.
35. The method of claim 34 , further comprising capturing energy from a wireless signal, wherein the energy powers a mechanism for generating the count.
36. The method of claim 35 , wherein generating the count continues at least until the count matches the reference value even when no energy is being captured from the wireless signal.
37. The method of claim 34 , wherein the count is reset upon receiving a command from a remote device.
38. A circuit, comprising:
an input node;
a storage node for storing an analog signal;
a coupling mechanism for selectively coupling the input and storage nodes to each other;
a control mechanism for controlling the coupling means responsive to the signal difference between the input node and the storage node;
a power capture circuit for capturing energy from a power source; and
a counter circuit coupled to the power capture circuit, the counter circuit providing a count that represents a progression of time,
wherein the power capture circuit includes a differential amplifier and a capacitance,
wherein the differential amplifier compares a voltage level of the power source to a voltage level of a node formed on the capacitance; wherein if the voltage level of the power source is greater than a specified voltage level, the differential amplifier clamps the node to the power source.
39. The circuit of claim 38 , wherein the control mechanism includes a differential amplifier with one input responsive to the input node and another input responsive to the storage node.
40. The circuit of claim 38 , wherein the analog signal is a voltage level.
41. The circuit of claim 38 , wherein the circuit is a sampling circuit.
42. The circuit of claim 38 , wherein the circuit is a peak detector circuit.