JP3082682B2 - Test circuit for D / A converter and microcomputer provided with the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a test circuit for a D / A converter and a microcomputer provided with the test circuit.

[0002]

2. Description of the Related Art Conventionally, as a test circuit for testing the conversion characteristics of a D / A converter, there are those disclosed in Japanese Patent Application Laid-Open Nos. 1-240017 and 2-135923. In Japanese Patent Application Laid-Open No. 1-240017, as shown in FIG. 6, both the output of the D / A converter 101 and the output of the D / A converter 101 after passing through the filter 102 are compared in an analog manner. A technique is described in which a comparison is made by a comparator 103 and a comparison output difference is compared by a comparator 104, and when the output of the D / A converter deviates significantly from that in a normal case, it is determined to be defective.

In Japanese Patent Laid-Open Publication No. Hei 2-135923, as shown in FIG.
A digital code obtained by bit-inverting the digital code for inspection by an inverter 113 is input to a reference D / A converter 112 provided separately from the D / A converters 111 and 112.
Are added to the analog output voltage of the adder 114 in the adder 114. The outputs of the comparators 115,
A comparison circuit 116 determines whether a constant added output voltage is obtained for all digital code inputs.
7 describes a technique for judging the quality of the D / A converter to be inspected by making a judgment.

[0004]

However, according to the technique disclosed in Japanese Patent Application Laid-Open No. 1-240017, when a D / A converter output is not generated, that is, a defective product in which a digital code input which is in a high impedance output state exists, It may be judged as good. This is because in the high impedance output state, the voltage of the D / A converter output line and the filter output line holds the voltage generated at the last digital code input, and there is no difference between the two. In the technique disclosed in JP-A-2-135923, when a test circuit such as a reference D / A converter and an adder is realized on the same semiconductor integrated circuit as the D / A converter to be measured, the element area is reduced. This increases the manufacturing cost. This is because the scale of the test circuit such as the reference D / A converter and the adder provided separately is large.

SUMMARY OF THE INVENTION It is an object of the present invention to be able to judge a defective product having a digital code input whose D / A converter output is in a high-impedance output state as defective, and to realize this with a minimum scale. An object of the present invention is to provide a test circuit and a microcomputer including the same.

[0006]

A test circuit for a D / A converter according to the present invention comprises: a D / A converter; current detection means for detecting a current flowing through an output line of the D / A converter; storing the detection result, and comprises a quality determination signal output means for outputting the stored detection result as the quality judgment signal, the D / a
The converter includes a resistor for dividing the first potential and the second potential.
And any of the resistors corresponding to the input digital code
A switch element group that derives a voltage dividing point to an output line.
The current detecting means includes a pull-up connected to the output line.
A resistor circuit and a pull-down resistor circuit, and output to the output line
A voltage determination circuit for determining a voltage to be applied,
The pull-up resistor circuit and pull-down resistor circuit
For the digital code, the switch element group is
When deriving the voltage dividing point within the range of the midpoint potential from the second potential
The pull-up resistor circuit is connected to the output line but before
The pull-down resistor circuit is not connected and the input digital
For the code, whether the switch element group is the midpoint potential
When deriving a voltage dividing point within the range of the first potential from
Although a pull-down resistor circuit is connected to the power line,
The resistor circuit is not connected, and the voltage determination circuit
Open fault in the path from the line of force to the voltage divider in the resistor
Output when the path is normal and the path is normal
Outputs the result by judging the difference from the voltage
It is characterized by.

Further, the microcomputer of the present invention comprises:
A test circuit for the D / A converter and a register for holding a pass / fail judgment signal output from the test circuit are provided, and the data held in the register is read and output to an output port. I do. In a microcomputer equipped with a successive approximation type A / D converter having a voltage output type D / A converter, current detection means for detecting a current flowing through an output line of the D / A converter; The D / A converter includes a resistor for dividing the first potential and the second potential, and a resistor in the resistor corresponding to an input digital code. A voltage output type D / A converter including a switch element group for leading an arbitrary voltage dividing point to an output line, wherein the current detecting means is configured to output a first potential or a second potential of the D / A converter during a test. Output with the potential terminal open
A third potential is applied to the line, a current flowing through the output line is detected, and the pass / fail judgment signal output means outputs a pass / fail judgment signal based on an open fault of the switch element group based on the current flowing through the output line during the test. The A / D converter is configured to read the pass / fail judgment signal from the CPU and output the same to an output terminal, so that a missing code failure mode of the A / D converter caused by an open failure of the switch element of the D / A converter is provided. Is detected. The means for detecting the current of the output line of the D / A converter is constituted by a current sense amplifier circuit.

[0008]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a first embodiment of the present invention. The D / A converter 1 is connected to an output line 10 of the D / A converter 1 for detecting a current of the output line 10. The sense amplifier circuit 11 and the sense amplifier circuit 1
And a latch circuit 20 that latches the output 17 of the first signal and outputs a determination signal 21. The D / A converter 1 receives the digital input signal 8 in the decoder 2 and turns on one of the switching transistors 4 to 7 in accordance with the input data. Switching transistor 7
Are connected between each node of the resistor array 3 connected between the reference voltage signal 9 and the ground potential and the D / A converter output line 10. Therefore, a voltage determined by the input data and the voltage value of the reference voltage signal 9 is output to the output line 10.

In the sense amplifier circuit 11, the Pch transistors 13 and 14 constituting the current mirror type sense amplifier are connected to the D /
It is connected to the output line 10 of the A converter 1. The resistor 12 is a load resistor of the sense amplifier. Nch transistor 1
The AND gate 16 to which the test mode signal 19 and the clock signal 18 are input is connected to the gate electrode 5. In the test mode, the Nch transistor 15 is turned on in synchronization with the clock signal 18, a current flows through one of the Pch transistor 14, the Nch transistor 15 and the switching transistors 4 to 7, and the output of the sense amplifier circuit 11 is output. At 17, a predetermined output voltage is generated. The latch circuit 20 operates in synchronization with the clock signal 18, holds the output voltage, and outputs this voltage as the determination signal 21.

Next, the operation of the first embodiment will be described in detail with reference to FIGS. FIG. 2 shows an example of the operation in the non-test mode. When the digital input signal 8 changes, as shown in FIG.
Since the levels of 2,... Are changed and the switching transistors 4 to 7 are selectively turned on accordingly, the voltage of the output line of the D / A converter output line 10 becomes as shown in FIG. Changes depending on the digital value of the digital input signal 8.
Here, for example, at the time of a failure when the switching transistor 5 is not turned on, or at the time of abnormal operation such as disconnection of the wiring connecting the switching transistor 5 and the resistor array 3 or the D / A converter output line 10. , D / a converter output line becomes high impedance state at the timing t ₃ when the switching transistor 5 is operated,
Accordingly, the potential generated at t ₂ is held by the capacitance added to the output line 10 of the D / A converter as shown by the broken line in FIG. The original normal operating voltage higher than the voltage switching transistor 4 is generated at the timing t ₂ is turned on as in chain line in FIG. (B) is generated.

FIG. 3 shows an operation example in the test mode when such a defective state occurs. In this test mode, the reference voltage signal 9 is open. In the figure, t ₁₂ is timing when the switching transistor 6 is turned on, current flows through the output line 10 via the Pch transistor 14 and Nch transistor 15 and the switching transistor 6 in the sense amplifier circuit 11, a sense amplifier A predetermined potential is generated at the output 17 of the circuit 11 at the timing when the clock signal 18 is at the H level, and the determination signal 21 output from the latch circuit 20 is at the H level.
It is determined to be good. Meanwhile, the t _13, the switch transistor 5 during normal turned on, t ₁₂ same operation are performed. However, the defective state described above occurs and D
When the output line 10 of the / A converter is in a high impedance state, no current flows through the sense amplifier circuit 11, and the output 17 is at the ground potential. Therefore, the determination signal 21 which is the output of the latch circuit 20 becomes L level and is determined to be defective.

As described above, in the first embodiment, D
It is possible to easily detect a failure mode in which the output of the / A converter 1 becomes a high impedance output state by using the sense amplifier circuit 11 having a simple configuration. Therefore, a test circuit that detects a failure mode in a high impedance output state without increasing the manufacturing cost can be configured.

Next, a second embodiment of the present invention will be described with reference to FIG. In the figure, a D / A converter 1
Is the same as in the first embodiment, except that the resistance value of the resistor array 3 is r, the switching transistor 4 is connected to the power supply Vdd side from the middle point, and the switch is connected to the ground side than the middle point. Transistors 5, 6, 7 are connected. The output line 1 of the D / A converter 1
As means for detecting a current of 0, here, a configuration is provided that includes a pull-up resistance circuit, a pull-down resistance circuit, and a voltage determination circuit.

The pull-up resistor circuit includes a resistor 50 having a resistance value r interposed between the power supply Vdd and the output line 10, a Pch transistor 53, and a NAND gate 55 connected to the gate of the Pch transistor 53. The input of the NAND gate 55 includes the most significant bit Pn signal of the digital input signal 8 via the inverter 54,
The test signal 19 is input. The pull-down resistor circuit includes a resistor 51 and an Nch transistor 52 having a resistance value r interposed between the ground and the output line 10.
The AND gate 56 is connected to the gate of the channel transistor 52. The input of the AND gate 56 receives the most significant bit Pn signal of the digital input signal 8 and the test signal 19.

Further, the voltage judgment circuit includes an AND gate 58 for inputting the most significant bit Pn and the output line 10, respectively, and the most significant bit Pn via an inverter 57.
NAND gate 59 for inputting n and output line 10 respectively
And an OR gate 60 that receives the AND gate 58 and the NAND gate 59 as inputs and outputs a determination signal 61. Here, the threshold voltage (V
i) is set to 0.4 × Vdd or less, and the input threshold voltage (Vi) of the NAND gate 59 is 0.6 × Vd
d or more.

According to this configuration, when the most significant bit Pn of the digital input signal 8 is at L level, the Pch transistor 53 is turned on, and the pull-up resistor circuit is connected to the output line 10. Therefore, the switching transistors 5 to 7
In the case of the normal operation in which either of them is ON, the D / A
The potential generated on the converter output line 10 becomes 0.6 × Vdd or less as shown in (Equation 1).

(Equation 1) As described above, since the input threshold voltage (Vi) of the NAND gate 59 is set to 0.6 × Vdd or more, the output of the NAND gate 59 becomes H level, and the judgment signal 61 which is the output of the OR gate 60 becomes H and is determined to be good.

On the other hand, during a defective operation in which the switching transistors 5 to 7 are not turned on, the potential of the output line 10 of the D / A converter is Vdd, the output of the NAND gate 59 is at L level, and the output of the OR gate 60 is A certain determination signal 61 becomes L level and is determined to be defective.

When the most significant bit Pn of the digital input signal 8 is at the H level, the Nch transistor 52 is turned on, and a pull-down resistor circuit is connected to the output line 10. Therefore, in a normal operation in which the switching transistor 4 is turned on, the potential generated on the D / A converter output line 10 is 0.4 × Vdd or more.

(Equation 2) As described above, since the threshold voltage (Vi) of the AND gate 58 is set to 0.4 × Vdd or less, the output of the AND gate 58 becomes H level, and the judgment signal which is the output of the OR gate 60 becomes It becomes the H level and is determined to be good.

On the other hand, during a defective operation in which the switching transistor 4 is not turned on, the output line 10 of the D / A converter
Is the ground potential, and the output of the AND gate 58 is L
Level, and the judgment signal 6 which is the output of the OR gate 60
1 is at the L level and is determined to be defective.

In the second embodiment, no current mirror type sense amplifier circuit is used. Therefore, at the time of designing, there is no need to determine the optimum sense amplifier operating point based on the resistance value of the resistor array 3 and the current supply capability of the transistors constituting the sense amplifier circuit.
It has the advantage of easy design. In addition, there is no need to consider variations in the operating point of the sense amplifier due to manufacturing variations, and there is an advantage in circuit characteristics that the device operates stably with a wide range of power supply voltages.

Further, an example of a microcomputer to which the test circuit of the D / A converter according to the first embodiment of the present invention is applied will be described with reference to FIG. In this application example, the test circuit of the present invention is applied to a successive approximation D / A converter built in a microcomputer. 1, a sense amplifier circuit 85 as shown in FIG. 1 is connected to the output line 10 of the D / A converter 1, and the sense amplifier circuit 85 detects the output high impedance failure mode of the D / A converter. The result is transmitted to the data bus 88 via the mode register 86 and the read buffer 87 and output to the output terminal 90 via the output port 89.

On the other hand, the output line 10 of the D / A converter 1 is input to the negative input of the comparator 82. The voltage of the analog input signal 80 input to the input terminal ANin, which is held by the sample and hold circuit 81, is input to the positive input of the comparator 82. Then, the comparator 82
Is input to the successive approximation register 83, from which the successive approximation register output 84 is output. This successive approximation register output 84 is input to the D / A converter 1 as a digital input signal 8.

In the microcomputer including this application example, the output high impedance failure mode of the D / A converter 1 constituting a part of the A / D converter can be detected in advance. Failure analysis can be easily performed. Further, since the output high-impedance defective product of the D / A converter 1 can be removed in advance, it is possible to remove in advance the missing code defective product (defective mode in which a certain A / D conversion output digital value does not exist) as the A / D converter. Therefore, there is an advantage that a high-precision measuring instrument is not required for testing the A / D converter.

[0024]

As described above, according to the present invention , a test circuit is constituted by a pull-up resistor circuit and a pull-down resistor circuit connected to a D / A converter output line, and a voltage judgment circuit to detect a failure mode. Is performed, the detection of the high impedance failure mode of the output of the D / A converter can be performed stably with a wide range of power supply voltage, and the circuit design becomes easy. In addition, the test circuit of the present invention may be a microcomputer.
As part of successive approximation A / D converter built in
To form a part of successive approximation type A / D converter
Output high impedance failure mode of the D / A converter
Can be detected in advance, and the successive approximation A / D conversion
The missing code failure mode of the exchanger can be easily detected,
A high-precision measuring instrument is required for testing the successive approximation type A / D converter.
There is an advantage that it is no longer necessary.

In the microcomputer of the present invention,
Input a digital signal to the voltage output type D / A converter
Current in the output line at the time of
Of the A / D converter caused by the open failure of the
Since a single code defect is detected, it can be used as a test circuit.
D / A conversion is possible by using a sense amplifier circuit with a simple configuration.
Easily detect high impedance failure mode of converter output
Inspection without increasing manufacturing costs.
The output circuit can be realized on a semiconductor integrated circuit.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of a test circuit of a D / A converter according to the present invention.

FIG. 2 is a time chart for explaining an operation of the D / A converter of FIG. 1;

FIG. 3 is a time chart for explaining an operation of the test circuit in FIG. 1;

FIG. 4 is a circuit diagram of a test circuit according to a second embodiment of the present invention.

FIG. 5 is a block circuit diagram of a successive approximation A / D converter to which the test circuit of the first embodiment is applied.

FIG. 6 is a block circuit diagram of an example of a test circuit of a conventional D / A converter.

FIG. 7 is a block circuit diagram of another example of a test circuit of a conventional D / A converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 D / A converter 2 Decoder 3 Resistance array 4-7 Switching transistor 8 Digital input signal 9 Reference voltage signal 10 D / A converter output line 11 Sense amplifier circuit 13, 14 Pch transistor 15 Nch transistor 16 AND gate 20 Latch circuit 50, 51 Resistance 52 Nch transistor 53 Pch transistor 55, 59 NAND gate 56, 58 AND gate 60 OR gate 80 Analog input signal 81 Sample hold circuit 82 Comparator 83 Successive comparison register 85 Sense amplifier circuit 86 Mode register 90 Output terminal

Claims (5)

(57) [Claims] And 1. A D / A converter, and a current detecting means for detecting a current flowing through the output line of the D / A converter, and stores the detection result, and the stored detection result as quality determination signal Output means for outputting a pass / fail judgment signal;
The A converter is a resistor for dividing the first potential and the second potential.
And any of the resistors corresponding to the input digital code
A switch element group that derives a voltage dividing point to an output line.
The current detecting means includes a pull-up connected to the output line.
A resistor circuit and a pull-down resistor circuit, and output to the output line
A voltage determination circuit for determining a voltage to be applied,
The pull-up resistor circuit and pull-down resistor circuit
For the digital code, the switch element group is
When deriving the voltage dividing point within the range of the midpoint potential from the second potential
The pull-up resistor circuit is connected to the output line but before
The pull-down resistor circuit is not connected and the input digital
For the code, whether the switch element group is the midpoint potential
When deriving a voltage dividing point within the range of the first potential from
Although a pull-down resistor circuit is connected to the power line,
The resistor circuit is not connected, and the voltage determination circuit
Open fault in the path from the line of force to the voltage divider in the resistor
Output when the path is normal and the path is normal
A test circuit for a D / A converter, which determines a difference from a voltage and outputs a result . Wherein the test circuit of the D / A converter according to claim 1, comprising a register for holding the quality determination signal output from the test circuit, the output reads data held in the register A microcomputer which outputs to a port. 3. A microcomputer including a successive approximation type A / D converter configured to include a test circuit of the D / A converter according to claim 1 or 2. 4. A microcomputer equipped with a successive approximation type A / D converter having a voltage output type D / A converter, wherein current detection means for detecting a current flowing through an output line of the D / A converter; A quality determination signal output unit that outputs a quality determination signal based on the detection result, wherein the D / A converter includes a resistor for dividing a first potential and a second potential;
A switch element group for leading an arbitrary voltage dividing point in the resistor to an output line in accordance with the input digital code. A third potential is applied to the output line while the terminal of the first potential or the second potential of the A / A converter is open, and a current flowing through the output line is detected. And outputting a pass / fail judgment signal due to an open failure of the switch element group based on the current flowing through the output line in
After reading out the data, the data is output to the output terminal.
A / A caused by the open failure of the switch element of the converter
A microcomputer for detecting a missing code failure mode of a D converter. 5. A method for detecting a current of an output line of the D / A converter.
5. The device according to claim 4, wherein the means for performing the operation includes a current sense amplifier circuit.
The microcomputer as described .