JPH10150365A - Microcomputer with built-in a/d converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow a microcomputer to vary precision per each bit of a digital signal after A/D conversion. SOLUTION: Control data are set to a control register 38 based on a decoding result of program data read from a ROM in the case of executing AD conversion. Anyone of a p-channel type MOS transistor(TR) 37 and operational amplifiers 24, 25, 26, 27 is turned on corresponding to the control data. Anyone of those outputs is applied to a non-ground side terminal of a series resistance circuit network 13 and then after AD conversion, the precision per each bit of the digital signal can be varied and improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microcomputer having a built-in AD converter suitable for improving the accuracy per bit of a digital signal obtained by the AD converter.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a conventional microcomputer having a built-in AD converter. In FIG.
(1) is a microcomputer and has a built-in AD converter (2) for performing analog-to-digital conversion. The AD converter (2) generally includes a successive approximation type and a batch comparison type. In both cases, an analog voltage and a reference voltage are compared to obtain a digital value of logic "1" or "0". For this purpose, a series resistor network for generating the reference voltage is provided. (3) is the series resistance network. The microcomputer (1) has a power terminal (4) and a ground terminal (5) for the microcomputer (1),
An analog input terminal (6) for the converter (2) and terminals (7) (8) for applying a predetermined voltage to both ends of the series resistor network (3) are provided. Then, the power supply voltage VDD is applied to the power supply terminal (4), and the ground terminal (5)
Is grounded. An analog voltage to be AD-converted is applied to the analog input terminal (6). Further, terminals (7) and (8) have voltages AV + and AV necessary to generate a desired reference voltage from each connection point of the series resistance network (3).
-Is applied. In FIG. 2, the terminal (8) is grounded (AV- = 0 volt). (9) is a regulator which outputs a stable power supply voltage VDD (for example, 5 volts). The microcomputer (1) operates by receiving the supply of the power supply voltage VDD.
Although not shown, the microcomputer (1) has a ROM storing program data,
It includes a CPU that performs a logical operation based on the result of decoding the OM read data, a RAM that writes and reads the operation data of the CPU, and the like.

If it is desired to use a voltage across the series resistor network (3) of 5 volts, the output of the regulator (9) may be connected to the terminal (7). Thus, a reference voltage obtained by dividing 5 volts by the resistance ratio is obtained from each connection point of the series resistance network (3). Then, by comparing the analog voltage and the reference voltage, a digital signal having a predetermined number of bits can be obtained.

[0004]

By the way, the accuracy of each bit of the digital signal after AD conversion is improved as the potential difference between both ends of the series resistor network (3) is reduced. That is,
The above accuracy improves as the input voltage of the terminal (7) is set lower. Depending on the use of the AD converter, there is a case where the above-mentioned improvement in accuracy is required. In this case, conventionally, a new regulator (10) is inserted between the output of the regulator (9) and the terminal (7), and a voltage lower than the power supply voltage VDD (for example, 3 volts) is supplied to the terminal (7). I was doing it.

However, since the regulator (10) is an external component of the microcomputer (1), there has been a problem that the price increases as the number of external components increases. Further, during operation of the microcomputer (1), the regulator (10) cannot be removed so as not to hinder the AD conversion operation of the AD converter (2). Therefore,
Even if a request to change the precision of each bit of the digital signal after AD conversion occurs during the operation of the microcomputer (1), the request cannot be dealt with and there is a problem that the application is not useful.

Therefore, the present invention can improve the accuracy of each bit of a digital signal after AD conversion without providing external components.
It is a further object of the present invention to provide a microcomputer with a built-in AD converter capable of changing the accuracy during operation of the microcomputer.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and is characterized by a series resistor network and a reference voltage obtained from the series resistor network. And a comparator for comparing an analog signal with an analog signal, and an AD converter for converting the analog signal to a digital signal of a predetermined bit, the register having a register for storing data of “1” or “0” obtained from the comparator And a control register in which data for varying a voltage applied to at least one end of the series resistance network is set based on a result of decoding the program. And applying one of the voltages obtained from the voltage generator to at least one end of the series resistance network based on the setting contents of the control register. In that with the switching unit, the.

[0008] Further, the voltage generator is composed of a plurality of operational amplifiers, and outputs a plurality of different voltages from the plurality of operational amplifiers. Further, the plurality of operational amplifiers are of a voltage follower type, and different reference voltages divided by resistance are applied to one input terminal of each of the plurality of operational amplifiers.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be specifically described with reference to the drawings. FIG. 1 is a circuit block diagram showing a microcomputer incorporating an AD converter according to the present invention. In FIG. 1, reference numeral (11) denotes a microcomputer, which incorporates a successive approximation type AD converter (12) for converting an analog signal into a digital signal having a predetermined number of bits (for example, 8 bits). In the embodiment of the present invention, the successive approximation type AD
Although a converter is used, a batch comparison type AD converter may be used without being limited to this.

In the AD converter (12), (1)
3) is a series resistor network, which has the same resistance value.
6 (= 2 ↑ 8) resistors are connected in series.
(14) is an analog input switching circuit, which has eight analog input terminals A provided in the microcomputer (1).
An analog signal applied to any one of D0 to AD7 is switched and output. (15) is a 3-bit channel register, which has analog input terminals AD0 to AD7.
Is set via a data bus (16) in accordance with the result of decoding program data read from a ROM (not shown) when performing A / D conversion when selecting one of the three. is there. The analog input switching circuit (14) is in a decoding format in which any one of the analog input terminals can be selected according to the contents of the channel register (15). For example, the contents of the channel register (15) are “000”, “001”, “010”, “011”, “1”.
At the time of 00, 101, 110, and 111, the analog input terminals AD0, AD1, AD2, AD3, and AD
The internal wiring of the analog input switching circuit (14) is connected so that 4, AD5, AD6, and AD7 are selected.
(17) is a comparator, which is a series resistor network (1).
3) The reference voltage appearing at the predetermined connection point is compared with the voltage value of one analog signal output from the analog input switching circuit (14). When the analog signal is larger than the reference voltage, a logic "1" is output. When the analog signal is smaller than the reference voltage, a logic "0" is output. (18) is an 8-bit data register, which converts eight digital values of "1" or "0" sequentially obtained from the comparator (17) from the most significant bit (MSB) to the least significant bit (LSB). It is set toward. (19) is a successive approximation circuit for receiving the output of the comparator (17). The successive approximation circuit (19) is controlled by an AD conversion instruction register (20). That is, ROM
The program data read out from the memory is a content for executing the A / D conversion. The data for starting the A / D conversion in the A / D conversion instruction register (20) via the data bus (16) based on the result of decoding the program data. Is set, the successive approximation circuit (19) starts operating according to the contents of the AD conversion instruction register (20) at this time. That is, the successive approximation circuit (19) operates so as to sequentially set the digital value output and applied one bit at a time from the comparator (17) from the most significant bit to the least significant bit of the data register (18). I do. An 8-bit digital value is applied from the comparator (17) to the successive approximation circuit (19), that is, the data register (18).
When the 8-bit digital signal after the AD conversion of the analog signal is set in the A / D converter, data indicating that the AD conversion operation has been completed is output from the successive approximation circuit (19) and set in the AD conversion instruction register (20). The contents of the AD conversion instruction register (20) at this time are decoded by the CPU via the data bus (16). The data register (1
The 8-bit digital signal set in 8) is transferred to the data bus (16) and logically operated by the CPU for a predetermined purpose. (21) is a series resistance network (13)
And a control circuit for controlling which connection point generates a reference voltage. A series resistor network (13) corresponding to the content of each bit set in the data register (18).
Is selected. Specifically, the voltage AV− at one end of the series resistance network (13) is grounded,
Assuming that the voltage applied to the other end is V, in the initial state in which the AD conversion operation is performed, the midpoint voltage of both ends of the series resistor network (13), that is, V / 2 is output as the reference voltage, and the analog signal is output. It is compared whether it is larger or smaller than V / 2. For example, when the analog signal is larger than V / 2, "1" is output from the comparator (17), and the digital value "1" is output to the uppermost position of the data register (18) via the successive approximation circuit (19). Set to a bit. The control circuit (21) detects that the most significant bit of the data register (18) is set to "1", determines that the analog signal is between V / 2 and V, and sets a serial resistance network. 3V / 4 which is the medium voltage of V / 2 and V in (13)
Is selected, and 3V / 4 instead of V / 2 is selected.
Is output from the series resistance network (13) as a reference voltage. Then, the comparator (17) compares the magnitude of the analog signal with the reference voltage 3V / 4. For example, when the analog signal is smaller than 3V / 4, "0" is output from the comparator (17), and this digital value "0" is output to the upper part of the data register (18) via the successive approximation circuit (19). Set to the second bit. The control circuit (21) detects that "0" is set in the upper 2 bits of the data register (18), determines that the analog signal is between V / 2 and 3V / 4, and In the resistance network (13), a connection point for generating 5V / 8, which is the medium voltage between V / 2 and 3V / 4, is selected, and 5V / 8 is used as a reference resistor instead of 3V / 4 as a series resistor. Network (1
Output from 3). Then, the comparator (17) compares the magnitude of the analog signal with the reference voltage 5V / 8. Hereinafter, the successive approximation operation is performed until a digital value is set in the least significant bit of the data register (18). Thus, the AD converter (12) is configured.

A regulator (22) outputs a power supply voltage VDD for the microcomputer (11). The power supply voltage VDD is supplied to a power supply terminal (2).
3) is applied. (24) (25) (26) (27)
Is an operational amplifier of the voltage follower type, and the output of any one of these operational amplifiers (24), (25), (26) and (27) is the voltage AV + on the non-ground side of the series resistance network (13).
Will be used selectively. Here, the reason that the operational amplifiers (24), (25), (26), and (27) are used as a voltage follower type is that a constant output voltage is always maintained with respect to the fluctuation or variation in the resistance value of the series resistance network (13). A
This is because it has a characteristic that can be output as V +. The + terminal of each of the operational amplifiers (25), (26) and (27) has a resistor (2) built in the microcomputer (11).
8) Voltages obtained by dividing the power supply voltage VDD by (29), (30), (31), (32), and (33) are applied as reference voltages. For example, operational amplifiers (25) (26)
The (+) terminal of (27) has 4VDD / 5 and 3VDD, respectively.
/ 5 and VDD / 2 are applied so that the resistors (28) to (3)
The resistance value of 3) is set. The operational amplifier (2
5) The (-) terminal of (26) and (27) is connected to the output terminal, and further, the operational amplifiers (25), (26) and (27)
Are connected in common and the series resistor network (1
3) is connected to the non-ground side terminal.

The + terminal of the operational amplifier (24) is connected to a terminal (34) provided in the microcomputer (11).
A predetermined divided voltage appearing at a connection point of the resistors (35) and (36) connected in series between the output of the regulator (22) and the ground is applied to the terminal (34) as a reference voltage, and the operational amplifier (24) ) Is applied to the + terminal. The negative terminal of the operational amplifier (24) is connected to its output terminal, and its output terminal is commonly connected to the output terminals of the operational amplifiers (25), (26) and (27) and the series resistance network (13).
Is connected to the terminal on the non-ground side. That is, the operational amplifier (24) allows the user of the microcomputer (11) to generate another reference voltage from the operational amplifiers (25), (26), and (27) generated inside the microcomputer (11). It is used when it is necessary to generate an external resistor (35) (3
The user can freely determine the resistance ratio of 6).

The source / drain path of the P-channel MOS transistor (37) is connected to the + of the operational amplifier (24).
It is connected between the terminal and the output terminal, and is turned on to short-circuit the operational amplifier (24). In FIG. 1, a series resistor (35) (3
The connection point of 6) is connected, but the regulator (22)
Is directly connected to the terminal (34), the voltage VDD can be supplied to the non-ground side terminal of the series resistance network (13) via the P-channel MOS transistor (37). Since the terminal (34) is shared with the operational amplifier (24) and the P-channel MOS transistor (37), the number of terminals is not increased as compared with the conventional case.

A control register (38) controls the on / off of the P-channel MOS transistor (37) and the on / off of the operational amplifiers (24), (25), (26), and (27). Only 3-bit control data for turning ON only is set based on the result of decoding the program data read from the ROM. (3
9) is a decoder constituting the voltage switching section according to claim 1, and four AND gates (40) (41) (42).
(43), a NAND gate (44) and three inverters (45), (46) and (47). Specifically, the control data set in the control register (38) is “000”, “001”, “010”, “011”,
"100" (bit data on the left is the most significant bit MS
B, the right bit data represents the least significant bit LSB)
At the time of each, a P-channel type MOS transistor (3
7) The wiring is connected so that the operational amplifiers (24), (25), (26), and (27) are turned on. That is, when the control data is "000", the outputs of the AND gates (40) to (43) and the NAND gate (44) are all "0",
As a result, only the P-channel MOS transistor (37) is turned on. At this time, 5 volts is applied to the non-grounded terminal of the series resistance network (13). When the control data is “001”, the AND gate (4
Since (0) to (42) output "0" and the AND gate (43) and the NAND gate (44) output "1", only the operational amplifier (24) is turned on, and the user sets the series resistance ( 35) A reference voltage determined by the resistance of (36) is applied to the non-ground side terminal of the series resistance network (13). When the control data is "010", the AND gate (4
0) (41) and (43) output “0” and AND
Since the gate (42) and the NAND gate (44) output "1", only the operational amplifier (25) is turned on, and the non-ground side terminal of the series resistor network (13) has a reference voltage of 4VDD / 5. Is applied. The control data is “011”
, The AND gates (40), (42), and (43) are "0"
And the AND gate (41) and the NAND gate (44) output "1".
Only 6) is turned on, and a reference voltage of 3VDD / 5 is applied to the non-grounded terminal of the series resistance network (13). When the control data is “100”, the AND gate (4
1) (42) and (43) output “0” and AND
Since the gate (40) and the NAND gate (44) output "1", only the operational amplifier (27) is turned on, and the non-ground side terminal of the series resistance network (13) has a reference voltage of VDD / 2. Is applied.

As described above, according to the embodiment of the present invention,
The voltage applied to the non-ground side of the series resistance network (13) can be changed to a different voltage level based on the result of decoding the program data read from the ROM. That is, the accuracy of the digital signal after AD conversion per bit can be improved in the microcomputer (11) by the program of the microcomputer (11). It becomes unnecessary and cost increase can be prevented. Furthermore,
In a series of operations of the microcomputer (11), A
Even when a change in the precision per bit of the digital signal obtained by the D converter (12) is required, the precision can be easily changed by the program, and the AD conversion operation according to various applications can be performed. A microcomputer (11) which becomes executable and incorporates an AD converter (12)
As a result, the range of application will be expanded.

[0016]

According to the present invention, since the voltage applied to the series resistor network constituting the AD converter can be changed by a program, the accuracy of each bit of the digital signal after AD conversion in the microcomputer can be improved. Can be changed.
Therefore, cost increase can be suppressed without increasing external parts. Further, even when a change of the system is requested during a series of operations of the microcomputer, it is possible to easily cope with the change, so that there is obtained an advantage that the application range of the microcomputer is expanded.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram showing a microcomputer incorporating an AD converter according to the present invention.

FIG. 2 is a block diagram showing a conventional microcomputer with a built-in AD converter.

[Explanation of symbols]

 (11) Microcomputer (12) AD converter (25) (26) (27) AD converter (38) Control register (39) Decoder

Claims (3)

[Claims] A series resistor network, a comparator for comparing a reference voltage obtained from the series resistor network with an analog signal,
A microcomputer having a register in which data of “1” or “0” obtained from the comparator is stored, and incorporating an AD converter for converting the analog signal into a digital signal of a predetermined bit. A voltage generating unit that generates a voltage, a control register in which data for changing a voltage applied to at least one end of the series resistance network is set based on a result of decoding a program, and a setting content of the control register. And a voltage switching unit for applying any one voltage obtained from the voltage generation unit to at least one end of the series resistance network. 2. The microcomputer with built-in A / D converter according to claim 1, wherein said voltage generator comprises a plurality of operational amplifiers, and outputs a plurality of different voltages from said plurality of operational amplifiers. 3. The A / D converter according to claim 2, wherein the plurality of operational amplifiers are of a voltage follower type, and different reference voltages divided by resistance are applied to one input terminal of each of the plurality of operational amplifiers. Built-in microcomputer.