WO2005031375A1

WO2005031375A1 - Method and measuring device for determining the capacitanceof a capacitive electrical component connected to an integrated circuit

Info

Publication number: WO2005031375A1
Application number: PCT/NL2004/000661
Authority: WO
Inventors: Derk André KORT
Original assignee: Jtag Technologies B.V.
Priority date: 2003-09-26
Filing date: 2004-09-23
Publication date: 2005-04-07
Also published as: NL1024386C2

Abstract

A method and device for determining the capacitance of a capacitive electrical component (CX) connected to an integrated circuit. The circuit is provided with analog terminals (AT1, AT2) which can be connected by means of switches (S5, S6, SB1, SB2) to the component (CX) that is to be measured. The switches (S5, S6, SB1, SB2) are switched for exchanging electric charge via the analog terminals (AT1, AT2) between the component (CX) to be measured and a capacitive electrical reference component (CR) having a known capacitance, until the voltages across the component (CX) to be measured and the reference component (CR) are substantially equal. By subsequently measuring the voltage across the reference component (CR), the capacitance of the component (CX) to be measured is determined from the known capacitance of the reference component (CR) and the measured voltage across the reference component (CR).

Description

Ffethod and measuring device for determining the capacitance of a capacitive electrical component connected to an integrated circuit.

DESCRIPTION The invention relates to a method and a measuring device for determining the capacitance of a capacitive electrical component connected to an integrated circuit, which circuit is provided with analog terminals which can be connected by means of switches to the component that is to be measured. In practice, printed circuit boards comprising integrated semiconductor circuits and discrete electrical components, for example, are very difficult to test. . Digital circuits can be tested by using "boundary scan" techniques, which are known in practice, provided that the integrated circuits are arranged for that purpose. To enable such testing, digital components forming a chain through which digital signals can be passed under the control of a suitable control module are connected between the external and internal terminals of an integrated circuit. Specific functions that the circuit on the printed circuit board is to perform are simulated and tested by means of said signals. This testing technique is also known by the name of IEEE Std. 1149.1 in practice. For testing analog signals, which may occur in mixed digital and analog integrated circuits, for example, i.e. integrated circuits comprising digital as well as analog components, the so-called "mixed signal circuits", e.g. for use in telecommunication systems, the well-known IEEE Standard 1149.1 has been extended with testing facilities for analog signals. This new standard is known by the acronym IEEE 1149.4, or simply "dot 4". This extension concerns the addition to the digital test facilities of an analog module provided with analog terminals, which can be connected to analog terminals of the integrated circuits via analog switches, i.e. semiconductor switches, and to power supply terminals for supplying a suitable supply voltage to the integrated circuit. Furthermore, an analog test bus interface circuit has been provided, which can be connected to the analog terminals via switches. The whole is controlled by a test control circuit in the form of a so-called "test access port" (TAP) controller, as known per se from the IEEE Std. 1149.1. By suitably switching the switches in the integrated circuit, the connections between the analog terminals and the components of the integrated circuit, the so-called "core", can be broken and test signals can be supplied via the analog test bus interface circuit to the electrical component or components connected to the integrated circuit, and signal measurements can be carried out on the component or components in question. A number of techniques for measuring the capacitance of capacitive electrical components are known in practice. With the so- called "roll -off point" or -3dB method, the capacitive component to be measured is connected, via the switches, into an RC low-pass filter configuration, to the input of which an adjustable frequency AC voltage is presented. The -3dB point of the RC filter is found by varying the frequency of the AC voltage and measuring the effective value of the voltage across the unknown capacitive electrical component that is to be measured. The accuracy of said measurement strongly depends on the accuracy of the available information regarding the contact resistance values of the various switches and the wiring, which partially determine the resistance of the RC filter, and on parasitic capacitances and the stray capacitance of the measuring environment, in this case the printed circuit board. In particular in the case of small capacitances in the order of less than 100 pF, the measuring accuracy is less than 10%, which is unacceptable for practical applications. Another technique that is known in practice is the so- called I-V method, in which the capacitive component to be measured is connected in series with a known resistive element via the switches and the test bus interface circuit. The voltage across the resistive element can be calculated from the applied voltage and the measured voltage across the capacitive component to be measured. In the case of a- purely capacitive component, the voltage across the resistive element and the voltage across the capacitive component include an angle of 90°. Subsequently, the voltage across the resistive element can be calculated by means of a simple vector calculation, and because the resistance value in- question is known, the current through the capacitor can be determined therefrom. Since the frequency of the applied AC voltage is known, the voltage across the capacitor is measured and the current through the capacitor can be determined from the aforesaid vector calculation, it is possible to calculate the capacitance of the capacitive component from said data. This measuring method, too, suffers from the fact that variations occur in the contact resistance values of the switches and the analog test bus, which variations seriously affect the accuracy of the measurement, in particular in the case of relatively small capacitance values. Furthermore, the frequency of the applied AC voltage must be carefully selected on account of bandwidth limitations in the switches and in the integrated circuit itself. Consequently it is the object of the invention to provide a novel and improved method for determining the capacitance of a capacitive electrical component connected to an integrated circuit, which circuit is provided with analog terminals that can be connected by means of switches to the component to be measured . According to the invention, this object is accomplished in that the switches are switched for exchanging electric charge via the analog terminals between the component to be measured and a capacitive electrical reference component having a known capacitance, until the voltage across the component to be measured and the voltage across the reference component are substantially equal, and subsequently measuring the voltage across the reference component, wherein the capacitance of the component to be measured is determined from the known capacitance of the reference component and the measured voltage across the reference component. The invention is based on the perception that the influence of unknown resistive elements in the chain between the capacitive components, such as contact resistances of the switches, resistances of the terminals and the wiring, can be effectively eliminated by effecting a transfer of charge between the capacitive components in question until the voltages across the components have equal values. When said values are equal or substantially equal, no current will flow through the resistive elements in question, or a current that is negligible to the extent that the voltage loss across said resistive elements is zero or negligibly small. Consequently, the resistances in question do not affect the eventual transfer of charge between the capacitive components, so that the voltage across the capacitive components in question is only a function of their capacitance. Since the voltage across the reference component can be calculated very accurately by means of techniques that are known in practice, the novel method according to the invention makes it possible to measure capacitances of capacitive electrical components in the order of 10 pF or higher with a ery high degree of accuracy (> 95%). In a preferred embodiment, the method according to the invention is characterized by the steps of: a) charging the component to be measured from an electrical voltage source to a voltage that is substantially equal to as the voltage of the voltage source; b) transferring charge from the component to be measured to the reference component until the voltage across the component to be measured and the voltage across the reference component are substantially equal; c) measuring the voltage across the reference component; and d) calculating the capacitance of the component to be measured from the product of the known capacitance and the measured voltage divided by the difference between the voltage of the voltage source and the measured voltage. Charging of the capacitive component to be measured to the voltage of the voltage source, so that contact resistances in the connection between the voltage source and the component to be measured will not affect the amount of charge that is eventually stored in the component to be measured, is advantageously used in this embodiment as well. In another embodiment, the method according to the invention is characterized by the steps of : a) charging the reference component to a predetermined reference voltage from an electrical power supply source; b) transferring charge from the reference component to the component to be measured, until the voltage across the component to be measured and the voltage across the reference component are substantially equal ; c) measuring the voltage across the reference component; and d) calculating the capacitance of the component to be measured from the product of the known capacitance and the measured voltage divided by the difference between the reference voltage and the measured voltage. This embodiment has the advantage that the reference component can be fed both from an electric voltage source and from an electric current source, as in the latter case the reference voltage across the capacitive reference component can be measured with a sufficient degree of accuracy by means of a suitable high-ohmic measuring circuit. When an electric voltage source is used, the reference component in question can also be charged until the voltage across the reference component is equal to the voltage of the electric voltage source, so that the influence of contact resistances on the amount of charge that is transferred can be effectively eliminated again. Preferably, the one component from which charge is transferred to the other component has a lower capacitance than the other component. In practice this means that in the preferred embodiment the reference component preferably has a higher capacitance than the component to be measured. In the further embodiment, in which the reference component is charged first, the reference component will preferably have a lower capacitance than the component to be measured. If, in the case of a single transfer of charge from one component to the other, the proportion or ratio between the capacitance of the one component and that of the other component is too large, the voltage across the other component will be small, and this voltage will be difficult to measure. In yet another embodiment of the method according to the invention, in order to obtain a sufficiently accurate measurement in such a situation, the steps of charging the one component and transferring charge to the other component are repeated a number of times if the ratio between the capacitance of the one component, from which charge is transferred, wherein the capacitance of the other component is relatively small, and the capacitance of the component to be measured is calculated from the voltage that is measured after charge has been transferred a number of times and from the number of times that charge has been transferred. It will be understood that the larger the ratio between the capacitance of the one component and that of the other component, the larger the number of times that charge is transferred will be. A sufficiently long period of time for transfer of charge will have to be reserved for equalising the voltages across the capacitive components in question in accordance with the inventive idea. In yet another embodiment of the method according to the invention, the period of time for transferring charge is determined from the RC time constants of the capacitive components in question and the contact resistance values of the switches, the terminals and the wiring in the charge transfer chain. According to yet another embodiment of the method according to the invention, the RC time constants of the component are estimated from one or more tests measurements of the capacitance of the component to be measured if the contact resistance values are unknown. The RC time constants thus determined provide an adequate perception of the period of time that is required for transferring charge for the purpose of equalising the voltages across the components. An adequate estimate of the RC time constants is necessary because the switches must not be switched to a charge transfer position for such a long period of time that charge starts to leak from one component, or from both, which will introduce an inaccuracy into the measurement. Since the switches are controlled in the rhythm of a clock or timer that is connected to the IEEE Std. 1149.1 control circuit, the clock frequency thereof must be selected such that there will be sufficient time for the transfer of charge between the components in question. In a preferred embodiment, in which the analog test bus interface circuit has a first ATI bus and a second AT2 bus, the reference component is connected to the AT2 bus. The invention also provides a measuring device for determining the capacitance of the capacitive electrical component connected to an integrated circuit, which circuit is provided with analog terminals which can be connected to the component to be measured by means of switches, characterized in that the measuring device is arranged for switching the switches for exchanging electric charge via the analog terminals between the component to be measured and a capacitive electrical reference component having a known capacitance, until the voltage across the component to be measured and the voltage across the reference component are substantially equal, and furthermore comprises means for measuring the voltage across the reference component and means for calculating the capacitance of the component to be measured from the known capacitance of the reference component and the measured voltage across the reference component. In a preferred embodiment, the measuring device is arranged for controlling the switches of an IEEE Std. 1149.4 test circuit. The invention also provides control software for use in a measuring device arranged for controlling the switches of an IEEE Std. 1149.4 test circuit, which device is provided with a suitable control processor. The invention also provides a printed circuit board provided with one or more integrated circuits and at least one reference capacitor having a known capacitance for use in the method and measuring device as described in the foregoing. The invention in particular provides a printed circuit board provided with at least one integrated circuit comprising an IEEE Std. 1149.1 test circuit, in which the reference capacitor is directly connected to the test bus interface circuit. The invention will be explained in more detail hereinafter with reference to the appended drawings. Fig. 1 schematically shows an integrated circuit provided with a boundary-scan test circuit according to the IEEE Standard 1149.4. Fig. 2 schematically shows part of the structure of an analog boundary-scan module of the circuit that is shown in Fig. 1. Fig. 3 schematically shows an embodiment of a measurement of the capacitance of a capacitive electrical component connected to the integrated circuit of Fig. 1. Fig. 4 shows a simplified diagram of the circuit that is shown in Fig. 3. Fig. 5 schematically and graphically shows the repeated performance of the method according to the invention. Fig. 6 schematically shows a printed circuit board and the measuring device according to the invention that is connected thereto. The invention will be explained hereinafter on the basis of an application in an integrated circuit provided with a boundary-scan test circuit according to IEEE Std. 1149.4. Detailed information about this standard are to be found in "IEEE Standard for Mixed-Signal Test

Bus", IEEE Standard 1149.4-1999, IEEE 1999. It will be understood that the invention is not limited to use thereof with integrated circuits provided with such a boundary-scan test circuit, but that it can be used essentially with any integrated circuit provided with analog terminals that can be connected by means of switches to a capacitive electrical component to be measured and to a capacitive electrical reference component. For a clear understanding of the invention, an integrated circuit provided with a boundary-scan test circuit according to IEEE Std.

1149.4 will be briefly discussed first. In Fig. 1, numeral 1 indicates an integrated semiconductor circuit comprising a so-called core 2, in which the electrical components, such as transistors, resistors and the like that are needed to enable the integrated circuit 1 to perform its functions are present. Numeral 3 indicates a number of digital input and output ports, which are connected to the corresponding input and output terminals of the core 2 via so-called "Digital Boundary Modules" (DBM) 4. Besides digital input and output terminals, the integrated circuit 1 also comprises two analog input and output terminals 7, 8 in the illustrated embodiment, which are connected to the corresponding digital input and output terminals of the core 2 via Analog Boundary Modules (ABM) 5, 6, respectively. In practice circuits having fewer or more DBM's and ABM's may occur. The ABMs 5, 6 are connected to an analog Test Bus Interface

Circuit (TBIC) 9 via an internal test bus 10. The test bus interface circuit 9 is accessible from outside the integrated circuit 1 via an Analog Test Access Port (ATAP) 11 comprising terminals ATI and AT2, respectively. The DBMs 4, the ABMs 5, 6 and the test bus interface circuit 9 connect to Test Control Circuitry 12 via a so-called boundary- scan pad 13. A so-called Test Access Port (TAP) 14 is provided for supplying suitable control signals to the test control circuitry 12, said port comprising terminals TD1, TDO, TMS and TCK, which are accessible from outside the integrated circuit 1. The test control circuitry 12 inter alia comprises a TAP-controller, an instruction register and a decoder for supplying test signals to the core 2 of the integrated circuit 1 across the boundary-scan pad 13. More detailed information about the test control circuit 12 and the TAP 14 is to be found in the "IEEE Standard Access Port and Boundary-scan Architecture", IEEE Standard 1149.1a-1993, IEEE 1993. Fig. 2 is a schematic, more detailed view of a part of an ABM 5, 6, which shows only those components that are necessary for a clear understanding of the invention. The ABMs 5, 6 comprise a number of switches, in particular semiconductor switches, that can be controlled by the test control circuitry 12, which switches are represented as mechanical switches in Fig. 2 for the sake of simplicity. The switch SD functions to make and break the connection between the analog terminal 7 and the core 2. The switches SB1 and SB1 function to connect the analog terminal 7 to the conductors AB1 and AB2, respectively, of the internal test bus 10. The test bus interface circuit 9 is provided with switches S5 and S6, amongst other components, via which the terminals ATI and AT2 can be connected to the conductors AB1 and AB2 of the internal test bus 10. It is noted that the test bus interface circuit has several other switching possibilities, which are not relevant for a clear understanding of the invention, however. The terminal 7 can be connected to a first power supply terminal V_H of the integrated circuit 1, for example a positive supply voltage, via a switch SH, and to a second power supply terminal V_L, for example the signal earth of the integrated circuit 1, via the switch SL. The switch SG provides a possibility for measuring voltage on the terminal 7 via a terminal V_G. Fig. 3 shows an electric diagram based on the circuitry of Fig. 2 for determining, in accordance with the invention, the capacitance of a capacitive electrical component C_x connected between the analog terminals 7, 8 of the integrated circuit 1. In accordance with the invention, a capacitive electrical reference component C_R connects to the signal earth 15 of the circuit at the terminal AT2. In practice, said reference component C_R is a capacitor having a precisely defined capacitance value. The capacitive component C_x to be measured is connected to the positive supply voltage V_H of the circuit via the terminal 7 by means of the switch SH₅, i.e. the switch SH associated with the ABM 5. The capacitive component C_x is connected to the signal earth 15 via the switch SH₆, i.e. the switch SH of the ABM 6. It is assumed in this connection that the circuit 1 is connected to a supply voltage, the terminal V_L of which is connected to the signal earth 15. The capacitive component T_x can be connected to the capacitive reference component C_R via the switch SB2₅, i.e. the switch SB2 of the ABM 6, and the switch S6 of the test bus interface circuit 9. The other switches of the ABM 5, 6 and the test bus interface circuit 9 are in the non-conducting or open position for carrying out the method according to the invention. The connection between the terminals 7, 8 and the core 2 is interrupted via the respective switches SD. Fig. 4 shows a simplified diagram of the circuit of Fig. 3, which is used for explaining the preferred embodiment of the method according to the invention. Numeral 16 indicates a voltage source having a voltage V_H, whilst numeral 17 indicates a measuring instrument for measuring the voltage across the capacitive reference component C_R. Although the measuring instrument 17, for example a high-ohmic voltmeter, is shown to be directly connected to the reference component C_R, it will be apparent to those skilled in the art that the measuring instrument 17 only needs to be connected to the reference component in question during the time that voltages are actually being measured. In accordance with the preferred embodiment of the method according to the invention, the capacitive component C_x to be measured is charged to the voltage V_H from the voltage source 16 by closing the switch SH₅ with the switch SB1₅ in the open position. Note that once C_x has been charged to the voltage V_H, no current will flow through the switch SH₅ and the associated wiring and terminals any more, ^■ so that contact resistances in the switch and the wiring will not affect the eventual charge or the voltage across the capacitive component C_x. The charge Q_x stored in C_x equals:

Qx (1)

wherein: C_x = the capacitance of C_x After C_x has been charged, the switch SH₅ is opened, i.e. the connection with the voltage source 16 is broken, and the switch SB1₆ is closed for transferring charge from the capacitive component C_x to be measured to the capacitive reference component C_R. The transfer of charge from C_x to C_R will continue until the voltages across C_R and C_x have equal values. No current will flow through the switch SB1₆, the wiring and the terminals at that point, so that the contact resistance in the switch and in the wiring will not affect the transfer of charge. Assuming that an amount of charge Q_t is transferred from C_x to C_R, the following obtains with regard to the voltage V_x across C_x and the voltage V_r across C_R:

(Q_x - Q_t)/C_x = Q_t/C_r = V_r = V_x (2)

wherein: Q_t = the amount of transferred charge from C_x to C_R C_r = the capacitance of C_R From (1) and (2) the unknown capacitance C_x of the capacitive component C_x can now be obtained from:

C_x - C_r • V_r/ (V_H - V_r) (3)

By measuring the voltage V_r across the reference component C_R after the transfer of charge between C_x and C_R has been completed, the capacitance C_x of the capacitive component C_x to be measured can be calculated from the measured voltage V_r, the known voltage V_H and the known capacitance C_r of the reference component C_R by means of equation (3). Instead of first charging the component C_x to be measured, it is also possible to carry out the method according to the invention by first charging the reference component C_R to a predetermined reference voltage. In the circuit of Fig. 3, the reference component C_R can be charged by closing the switches SH₅, SB2₆ and S6, with the switch SH₆ in the open (non-conducting) position, of course. Subsequently, charge can be transferred from the reference component C_R to the component C_x to be measured by opening the switch SH₅ and closing the switch SH_5- In the same manner as explained above, the unknown capacitance C_x of the component C_x to be measured can be calculated from:

C_x = C_r • (V„ - V_r)/ V_r (4)

Instead of using a voltage source for charging the reference component, it is also possible to use a current source, in which case the voltage across C_R is measured with the measuring instrument 17 prior to the transfer of charge from C_R to C_x. In equation (4), the value of the measured voltage will take the place of V_H in that case. When using the method according to the invention, the one capacitive component, which receives charge from the other capacitive component, preferably has a capacitance value which is lower than the capacitance value of the other component. If charge is transferred from

C_x to C_R, C_x must have a lower capacitance than C_R. Conversely, when charge is transferred from C_R to C_x, C_x will preferably have a higher capacitance than C_R. If the ratio between the smallest and the largest capacitance value is very large, only a relatively low voltage will remain across the two capacitive components after the transfer of charge, which voltage will be difficult to measure, at least in an accurate manner. In such a situation the method according to the invention can be carried out a repeated number n of times. That is, the one component is charged from the power supply source and subsequently transfers charge to the other component. For the circuit as shown in Fig. 4 it obtains that the voltage across the reference component C_R will increase in steps in that case, as is schematically shown in Fig. 5. After n steps, the voltage across the reference component C_R will reach a value close to a final value. For the circuit as shown in Fig. 4, it obtains now that after n repetitions the voltage v_r(n) across the reference component is in accordance with:

v» C. +C ^{v» +} c-+ v_r(n-l) (5)

v_r(n + 1) = V C„ + C ^H + c +^E—c. v_r(n) (6)

Subtraction of the equations (7) and (6) provides the following numerical equivalent of the differential equation:

v_r(n + 1) - v_r(n - 1) = 2 • ^ (8) dn

dv_r(n) 1 , C_x C_x . , . 1 ,C_X C_x . — ^L - + - - ( -^ + — ) • v (n) = - ^• (— + — ) • V_u (9) dn 2 C_r C_x + C_r ^{Λ i} 2 ^C_r C_x + C_r ^{H [ }}

Which is of the standard type: y'+p(x) ^• y = r(x) (10)

which provides a solution when the voltage at point in time t = 0 is likewise 0, which is the case with the method according to the invention, because the component C_x to be measured is supposed to be uncharged when the measurement is started. Consequently, the following applies: v_r(n) = (1 - e ^"* ^^ " ) ^• V_H (ii)

From which C_x can be deduced, resulting in:

It will be understood that if the reference component is charged first, a similar formula for calculating the capacitance of the component to be measured can be derived upon repeated transfer of charge from the reference component to the component to be measured. Although contact resistances in the switches, the terminals and the wiring do not affect the accuracy of the measurement when the method according to the invention is used, it will be apparent to those skilled in the art that on the other hand the accuracy of the measurement is influenced by the resistance paths to the signal earth, which cause leakage of charge stored in C_x and C_R. Consequently, switching must take place as soon as possible after the transfer of charge has been completed, that is, transfer of charge from the power supply source to a respective capacitive component and transfer of charge between the respective capacitive components themselves. In practice the point in time at which the transfer of charge., for example from C_x to C_R, is complete must be determined from the RC time constants of the respective capacitive components and the contact resistances of the switches, the terminals and the wiring in the charge transfer chain. If said time constants are unknown, an idea of the RC time constants can be formed by means of a number of test measurements, and the time for completing the transfer of charge can be roughly estimated therefrom. In the case of integrated circuits comprising boundary-scan modules, the duration of a charge transfer is determined by the length of the boundary-scan chain and the TCK clock rate. It is best therefore, to gear the clock rate to the time required for transferring charge. Fig. 6 schematically shows a measuring device 18 for carrying out the method according to the invention, which measuring device comprises a control processor 19 and a measuring instrument 17, for example a high-ohmic voltmeter or other suitable high-ohmic measuring instrument for measuring voltages, known to those skilled in the art. In the illustrated embodiment, the measuring instrument 18 is arranged for carrying out the method according to the invention with a circuit assembly consisting of a number of integrated circuits 1, one or more capacitive components C_x, resistors R and, if necessary, inductances L, on the printed circuit board 20, which components C_x, R and L are discrete components connected to one or more of the integrated circuits 1. The integrated circuits 1 are of the type provided with a boundary- scan test circuit IEEE Std. 1149.4 comprising a TAP 14 and an ATAP 11 connected to the measuring device 18. The reference component C_R may be directly connected to the printed circuit board 20, for example as a fixed component, for carrying out the method according to the invention, and/or be incorporated in the measuring instrument 18. In most practical applications, a capacitance value in the order of 1 μF will suffice for the reference component for carrying out the preferred method according to Fig. 4, depending on the capacitance value of the component to be measured. Depending on the magnitude of parasitic capacitances in the circuit, capacitances of up to 10 pF can be determined with an accuracy level of more than 95%, and capacitances of up to 100 pF can be determined with an accuracy level of 99% or higher. The invention also relates to software for enabling the measuring device 18 to deliver suitable control commands to the test control circuit 12 for controlling the switches of the ABM 5, 6 and the test bus interface circuit 9, which software can be loaded into the control processor 19.

Claims

1. A method for determining the capacitance of a capacitive electrical component connected to an integrated circuit, which circuit is provided with analog terminals which can be connected by means of switches to said component that is to be measured,, characterized in that said switches are switched for exchanging electric charge via said analog terminals between said component to be measured and a capacitive electrical reference component having a known capacitance, until the voltage across said component to be measured and the voltage across said reference component are substantially equal, and subsequently measuring the voltage across said reference component, wherein the capacitance of said component to be measured is determined from the known capacitance of said reference component and said measured voltage across said reference component.

2. A method according to claim 1, characterized by the steps of: a) charging said component to be measured from an electrical voltage source to a voltage that is substantially equal to the voltage of said voltage source; b) transferring charge from said component to be measured to said reference component until the voltage across said component to be measured and the voltage across said reference component are substantially equal ; c) measuring the voltage across said reference component; and d) calculating the capacitance of said component to be measured from the product of said known capacitance and the measured voltage divided by the difference between the voltage o said voltage source and said measured voltage.

3. A method according to claim 1, characterized by the steps of: a) charging said reference component to a predetermined reference voltage from an electrical power supply source; b) transferring charge from said reference component to said component to be measured, until the voltage across said component to be measured and the voltage across said reference component are substantially equal ; c) measuring the voltage across said reference component; and d) calculating the capacitance of said component to be measured from the product of said known capacitance and said measured voltage divided by the difference between said reference voltage and said measured voltage.

4. A method according to claim 3, characterized in that said electrical power supply source is an electrical voltage source having a predetermined reference voltage, wherein said reference component is charged to a voltage that is substantially equal to said voltage of said voltage source.

5. A method according to claim 3, characterized in that said electrical voltage source is a current source, wherein said reference voltage is determined by measuring the voltage across said reference component prior to the transfer of charge to said component to be measured.

6. A method according to any of the preceding claims, characterized in that, if the ratio between the capacitance of the one component, from which charge is transferred, and the capacitance of the other component is relatively . small , the steps of charging said one component and transferring charge to said other component are repeated a number of times, and wherein the capacitance of said component to be measured is calculated from the voltage that is measured after charge has been transferred a number of times and from said number of times that charge has been transferred.

7. A method according to claim 6, characterized in that the larger the ratio between the capacitance of said one component and the capacitance of said other component, the larger the number of times that charge is transferred will be.

8. A method according to any of the preceding claims, characterized in that said switches are switched to a position for transferring charge until the extent to which voltage leakage from a component is negligible within the accuracy of said measurement.

9. A method according to claim 8, characterized in that the period of time for transferring charge is determined from the RC time constants of said capacitive components and contact resistance values of said switches, terminals and wiring in the charge transfer chain.

10. A method according to claim 9, characterized in that said RC time constants are estimated from one or more test measurements of the capacitance of said component to be measured.

11. A method according to any of the preceding claims, characterized in that said integrated circuit comprises a so-called IEEE Std. 1149.4 test circuit comprising controllable switches, analog terminals, an analog test bus interface circuit and a control circuit for controlling said switches for connecting an electrical component to said terminals, the test buses and power supply terminals, wherein said method is performed by connecting said reference component to said test bus interface circuit and switching said switches for transferring charge from and to a respective component.

12. A method according to claim 11, characterized in that said analog test bus interface circuit has a first ATI bus and a second AT2 bus, wherein said reference component is connected to said AT2 bus.

13. A method according to claim 11 or 12, characterized in that said switches are controlled in the rhythm of a timer that is connected to said control circuit.

14. A measuring device for determining the capacitance of a capacitive electrical component connected to an integrated circuit according to any of the claims 1-10, which circuit is provided with analog terminals which can be connected by means of switches to said component that is to be measured, characterized in that said measuring device is arranged for switching said switches for exchanging electric charge via said analog terminals between said component to be measured and a capacitive electrical reference component having a known capacitance, until the voltage across said component to be measured and the voltage across said reference component are substantially equal, and comprises means for measuring the voltage across said reference component and means for calculating the capacitance of said component to be measured from said known capacitance of said reference component and said measured voltage across said reference component.

15. A measuring device according to claim 14, characterized in that said measuring device is arranged for controlling the switches of an IEEE Std. 1149.4 test circuit for carrying out the method according to any of the claims 11-13.

16. A measuring device according to claim 15, characterized in that said measuring device is provided with a suitably programmed control processor for controlling said switches of said IEEE Std. 1149.4 test circuit.

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17. Control software for use with a measuring device according to claim 16 for carrying out the method according to any of the claims 11-13 when loaded in the main memory of said control processor of said measuring device.

18. A data carrier provided with control software according to claim 17.

19. A printed circuit board provided with one or more integrated circuits and at least one reference capacitor having a known capacitance for use in said method and said measuring device according to any of the claims 1-16.

20. A printed circuit board provided with at least one integrated circuit comprising an IEEE Std. 1149.4 test circuit, wherein a reference capacitor is directly connected to said analog test bus interface circuit.