1. Field of the Invention
The present invention relates, in general, to temperature detectors. More particularly, the invention relates to temperature detectors including A/D converters.
2. Description of the Prior Art
A temperature detector is known which outputs a digital signal to a control device, such as a microcomputer, in accordance with a temperature detected by a temperature sensor. The temperature detector includes an A/D converter to convert an analog signal corresponding to the temperature into the digital signal. An example of such an A/D converter is disclosed in the user's manual of TLCS-42,47,470 published by TOSHIBA CORP. in Mar., 1986. In this user's manual, an A/D converter has input terminals V.sub.AREF and V.sub.ASS across which the reference analog voltage is applied, and an analog input terminal AIN to which the analog voltage to be converted is applied. The A/D converter also includes resistors connected in a ladder arrangement and a comparator. The reference analog voltage is divided into a voltage value corresponding to one bit by the ladder resistors. A/D conversion is performed with the comparator comparing the input analog voltage with the divided voltage value sequentially.
A conventional temperature detector including the A/D converter will be explained referring to FIG. 2.
The A/D converter 3 includes circuit 5 fabricated as an IC. The voltage applied to an analog voltage input terminal AIN is input to an inverting input terminal (-) of comparator 7. A voltage dividing circuit 9 includes two hundred and fifty-seven resistors connected in series. The two ends of voltage dividing circuit 9 are connected to reference voltage input terminals V.sub.AREF and V.sub.ASS, respectively. The voltage at each voltage division point of voltage dividing circuit 9 is input into the analog input terminal of one of two hundred and fifty-six analog switches, respectively, together forming a switch group 11. The analog output terminals of these analog switches are mutually connected and further connected to a non-inverting input terminal (+) of comparator 7. A decoder 13 produces a high level output on one of its two hundred and fifty-six output terminals (labeled OUT255 to OUT0) in accordance with an eight-bit digital signal that is applied to input terminals IN7 to IN0. The output from each of terminals OUT255 to OUT0 of decoder 13 is applied to the digital input terminal of one of the analog switches, respectively. An eight-bit binary counter 15 starts to count in synchronism with a clock signal 17 which is input to a clock input terminal CLOCK thereof from a clock terminal CLK of A/D converter 3. Counter 15 is reset by a leading edge of a start signal 19 which is input to a start-counting terminal START thereof from a start-conversion terminal STC of A/D converter 3. An eight-bit count signal 21, which is output from output terminals OUT7 to OUT0, is incremented in synchronism with clock signal 17.
When a stop signal 23 input into a stop-count terminal STOP becomes high, counter 15 stops counting, and count signal 21 at this time is latched. The stop signal 23 is provided by the output of comparator 7 and is input to the stop-count terminal STOP of counter 15. The stop signal 23 is also output from an end-conversion terminal EDC of A/D converter 3. The eight-bit count signal 21 from counter 15 is applied to input terminals IN7 to IN0 of decoder 13, and is also output, as parallel data, from output terminals DATA 7 to DATA 0 of A/D converter 3. Resistors 25, 27 and 29 are externally connected to integrated circuit 5 of A/D converter 3. Resistor 25 is connected between the two reference voltage input terminals V.sub.AREF and V.sub.ASS. The reference voltage input terminal V.sub.AREF is connected to a DC power supply Vc through resistor 27. The reference voltage input terminal V.sub.ASS is connected to ground through resistor 29.
When start signal 19 applies a rising edge to counter 15, the count of counter 15 is reset, and then counter 15 starts to count in synchronism with clock signal 17, and eight-bit count signal 21 is successively incremented. Decoder 13 makes respective output terminal voltages thereof successively high in accordance with count signal 21. For example, when count signal 21 is zero, only output terminal OUT0 is made high, and when count signal 21 is one, only output terminal OUTl is made high. Consequently, the analog switches of switch group 11 are successively closed, one at a time, causing the voltage which is input to the non-inverting input terminal (+) of comparator 7 to be progressively increased in a stepwise manner from a value which is practically equal to the voltage applied to the reference voltage input terminal V.sub.ASS towards a value which is practically equal to the voltage applied at the reference voltage input terminal V.sub.AREF. When the stepwise voltage input to the non-inverting input terminal (+) of comparator 7 becomes larger than the voltage which is applied to the analog voltage input terminal (-) of comparator 7, the output of comparator 7 is inverted from a low level to a high level. When the output of comparator 7, i.e., stop signal 23 becomes a high level, counter 15 ceases counting and count signal 21 at this time is latched. Since the output of comparator 7 is also given to the end-conversion terminal EOC, the voltage of EOC also becomes a high level, and the termination of A/D conversion is thereby detected. Count signal 21 latched at this time is output from the data output terminals DATA 7 to DATA 0 as an eight-bit digital signal, whose value corresponds to the analog voltage which is applied to the input terminal AIN.
When A/D converter 3 is used in the temperature detector of, for example, a refrigerator, the analog voltage input terminal AIN is connected to a connection point of a temperature sensor 31 and a resistor 33. Temperature sensor 31 is provided in a freezer compartment of the refrigerator and may consist of a thermistor having a negative temperature characteristic. The other ends of temperature sensor 31 and resistor 33 are connected to the DC power supply Vc and ground, respectively. As the temperature detector of a refrigerator, the circuit must respond to temperatures around -20.degree. C. in the freezer compartment. The temperature resolution of about 0.1.degree. C. is also required in the A/D conversion. Therefore, conventionally, a temperature in a range having a width of twenty degrees from -30.degree. C. to -10.degree. C. is converted to eight-bit digital data so that the temperature resolution of about 0.08.degree. C. can be realized. The temperature in the freezer compartment, which is detected by temperature sensor 31, is converted to the voltage Vth at the connection point of temperature sensor 31 and resistor 33, and the converted voltage Vth is input to the terminal AIN. Therefore, for example, when the DC power supply Vc is 5.0 volts, the temperature in the range having a width of twenty degrees from -30.degree. C. to -10.degree. C. is converted to the voltage Vth which can range from 1.5 volts to 4.0 volts. The resistance of resistors 25, 27 and 29 are, therefore, determined so as to provide respective voltages of 4.0 volts and 1.5 volts to be applied to the two reference voltage input terminals V.sub.AREF and V.sub.ASS. Since the voltage Vth is then converted to the eight-bit digital data in the range from 1.5 volts to 4.0 volts, the resolution for this voltage Vth is about 10 millivolts corresponding to 0.08.degree. C.
In the conventional temperature detector mentioned above, since the voltage applied to the two reference voltage input terminals V.sub.AREF and V.sub.ASS is simply determined by the voltage dividing circuit consisting of resistors 25, 27 and 29, the input voltage range for which A/D conversion is possible is fixed. Consequently, in order to achieve the required resolution with a small bit number, the voltages applied to the two reference voltage input terminals have to be restricted. So, it is not possible to detect malfunctions of temperature sensor 31, such as short circuiting or open circuiting, which causes the voltage of the terminal AIN to be DC power supply voltage or zero.