Sensor circuit and method

Patent metadata

Field	Value
Publication number	US-10119839-B2
Application number	US-201615135533-A
Country	US
Kind code	B2
Filing date	Apr 21, 2016
Priority date	Apr 23, 2015
Publication date	Nov 6, 2018
Grant date	Nov 6, 2018

Abstract

A sensor circuit and method. The circuit includes a first subcircuit that includes a first sense capacitor, a first integration capacitor, and a first clock input for receiving a first digital clock signal for initiating discharge of the first integration capacitor at time T. The circuit includes a second subcircuit that includes a second sense capacitor, a second integration capacitor, and a second clock input for receiving a second digital clock signal for initiating discharge of the second integration capacitor at time T+T d . A rate of discharge of the first and second integration capacitors is at least partly determined by a capacitance of the first and second sense capacitor, respectively. At time T eval , after initiation of discharge of the first and second sense capacitors, the extent to which the first and second integration capacitors have discharged is compared. A digital signal indicating the result of the comparison is outputted.

First claim

The invention claimed is:

1. A sensor circuit comprising:
a first subcircuit comprising:
a first sense capacitor;
a first integration capacitor, and
a first clock input for receiving a first digital clock signal for initiating discharge of the first integration capacitor at time T, wherein a rate of discharge of the first integration capacitor is at least partly determined by a capacitance of the first sense capacitor;

a second subcircuit comprising:
a second sense capacitor;
a second integration capacitor, and
a second clock input for receiving a second digital clock signal for initiating discharge of the second integration capacitor at time T+T d , wherein a rate of discharge of the second integration capacitor is at least partly determined by a capacitance of the second sense capacitor,

wherein the circuit is operable:
at time T eval , after initiation of discharge of the first and second integration capacitors, to compare the extent to which the first and second integration capacitors have discharged, thereby to compare the capacitance of the first sense capacitor to the capacitance of the second sense capacitor; and
to output a digital signal indicating the result of the comparison,
wherein the sensor circuit further comprises a delay generator for varying T d in a plurality of measurements for comparing the capacitance of the first sense capacitor to the capacitance of the second sense capacitor.

2. The sensor circuit of claim 1 , wherein the first integration capacitor comprises a first decoupling capacitor connected to discharge through a first node of the first subcircuit, wherein the second integration capacitor comprises a second decoupling capacitor connected to discharge through a second node of the second subcircuit, and wherein the sensor circuit further comprises a latch having inputs connected to said nodes, wherein the latch is operable to set or reset according to the voltage at the first node compared to the voltage at the second node.

3. The sensor circuit of claim 2 , wherein the latch comprises a pair of cross-coupled field effect transistors, wherein a first of said transistors has a gate connected to the first node and wherein a second of said transistors has a gate connected to the second node.

4. The sensor circuit of claim 1 , wherein:
the first subcircuit further comprises a first inverter having an input for receiving the first digital clock signal and an output connected to the first sense capacitor, wherein the first integration capacitor is connected to a power supply input of the first inverter; and
the second subcircuit further comprises a second inverter having an input for receiving the second digital clock signal and an output connected to the second sense capacitor, wherein the second integration capacitor is connected to a power supply input of the second inverter.

5. The sensor circuit of claim 4 , wherein the first inverter and the second inverter each include a transmission gate.

6. The sensor circuit of claim 1 , wherein one of said first and second sense capacitors is a reference capacitor for sensing a reference capacitance to be compared to the capacitance of the other of said first and second sense capacitors.

7. The sensor circuit of claim 1 , further comprising logic connected to receive the digital signal indicating the result of said comparison from said output, wherein the logic is operable to:
control the delay generator for varying T d in a plurality of measurements for comparing the capacitance of the first sense capacitor to the capacitance of the second sense capacitor, and
to convert the results of the measurements into a digital word.

8. The sensor circuit of claim 7 , wherein said logic comprises successive approximation logic.

9. A sensor array comprising a plurality of sensor circuits according to claim 1 .

10. A secure integrated circuit comprising the sensor array of claim 9 , wherein the sensor circuits are operable to detect a change in capacitance in their sense capacitors associated with a tampering of the integrated circuit.

11. A method for comparing the capacitance of a first sense capacitor to the capacitance of a second sense capacitor of a sensor circuit, the method comprising:
using a first digital clock signal to initiate, at time T, discharge of a first integration capacitor, wherein a rate of discharge of the first integration capacitor is at least partly determined by the capacitance of the first sense capacitor;
using a second digital clock signal to initiate, at time T+T d , discharge of a second integration capacitor, wherein a rate of discharge of the second integration capacitor is at least partly determined by the capacitance of the second sense capacitor;
at time T eval , after initiation of discharge of the first and second integration capacitors, comparing the extent to which the first and second integration capacitors have discharged, thereby to compare the capacitance of the first sense capacitor to the capacitance of the second sense capacitor; and
outputting a digital signal indicating the result of the comparison, wherein the method further comprises performing a plurality of measurements by repeating the steps of:
initiating discharge of the first integration capacitor;
initiating discharge of the second integration capacitor, and
comparing the extent to which the first and second integration capacitors have discharged,

for a plurality of values of T d , to evaluate the capacitance of the first and/or the second capacitor.

12. The method of claim 11 , wherein the outputted digital signal is indicative of which of the first sense capacitor and the second sense capacitor has the largest capacitance.

13. The method of claim 11 , comprising converting the results of the plurality of measurements into a digital word.

14. A sensor circuit comprising:
a first subcircuit comprising:
a first sense capacitor;
a first integration capacitor, and
a first clock input for receiving a first digital clock signal for initiating discharge of the first integration capacitor at time T, wherein a rate of discharge of the first integration capacitor is at least partly determined by a capacitance of the first sense capacitor;

a second subcircuit comprising:
a second sense capacitor;
a second integration capacitor, and
a second clock input for receiving a second digital clock signal for initiating discharge of the second integration capacitor at time T+T d , wherein a rate of discharge of the second integration capacitor is at least partly determined by a capacitance of the second sense capacitor,

wherein the circuit is operable:
at time T eval , after initiation of discharge of the first and second integration capacitors, to compare the extent to which the first and second integration capacitors have discharged, thereby to compare the capacitance of the first sense capacitor to the capacitance of the second sense capacitor; and
to output a digital signal indicating the result of the comparison, wherein:
the first subcircuit further comprises a first inverter having an input for receiving the first digital clock signal and an output connected to the first sense capacitor, wherein the first integration capacitor is connected to a power supply input of the first inverter, and
the second subcircuit further comprises a second inverter having an input for receiving the second digital clock signal and an output connected to the second sense capacitor, wherein the second integration capacitor is connected to a power supply input of the second inverter.

15. The sensor circuit of claim 14 ,

Assignees

• Nxp Bv

Inventors

• Shrestha Rameswor
• Widdershoven Franciscus

Classifications

• H03K2217/960745

  Capacitive differential; e.g. comparison with reference capacitance · CPC title

• H03K17/9622

  using a plurality of detectors, e.g. keyboard · CPC title

• G01D5/24 Primary

  by varying capacitance · CPC title

• G06F3/044

  by capacitive means · CPC title

• H03K2217/96074

  Switched capacitor · CPC title