1. Field of the Invention
The present invention relates to a passive type RFID semiconductor device, an IC tag, and a control method. In particular, the present invention relates to a semiconductor device for generating a power supply voltage based on a received radio signal, an IC tag including the semiconductor device, and a control method for the IC tag.
2. Description of Related Art
In recent years, attentions have been paid to a technique regarding RFID (Radio Frequency IDentification) as a product automatic identifying technique for affixing an IC tag having product specific information written thereto, and scanning this information using a radio antenna to manage a product in real time, in merchandise logistics management at the factory and article management at a retail shop.
The above RFID IC tag (hereinafter simply referred to as “IC tag”) does not incorporate a battery since a power supply voltage is generated based on radio waves at the time of communicating data with a reader/writer by radio waves. Such an IC tag is a so-called “passive type”, and an internal circuit of the IC tag rectifies (shapes) a part of a carrier from the reader/writer to generate a power supply voltage necessary for the operation. The generated power supply voltage allows plural circuits in the semiconductor device of the IC tag to operate. The circuits include a logic circuit controlling the IC tag, a nonvolatile memory where product specific information is written, and a communication circuit necessary for data communication with a reader/writer.
FIG. 11 is a block diagram of a conventional passive-type IC tag that is denoted by 101. As shown in FIG. 11, the conventional IC tag 101 includes a power supply voltage generating circuit 111, a receiving circuit 112, a transmitting circuit 113, a control circuit 113, a charge-pump circuit 115, a nonvolatile memory 116, and an antenna 120.
An operation of the conventional IC tag 101 of FIG. 11 is described. A reader/writer (not shown) propagates radio waves including a frame pulse wave (pulse wave of a predetermined frequency) that can be recognized by the IC tag 101 in a given range thereof. When the IC tag 101 is situated in the range where the radio waves including the frame pulse wave can be recognized, the IC tag 101 receives the radio waves via the antenna 120. After receiving the radio waves, the IC tag 101 rectifies the received radio waves in the power supply voltage generating circuit 111 to generate a power supply voltage necessary for internal circuits of the IC tag 101 to operate. Further, a clock signal necessary for the internal circuits of the IC tag 101 to operate is generated based on a frequency of the pulse wave in the radio waves and in addition, the internal circuits are initialized in preparation for reception of a write or read command sent from the reader/writer.
When the IC tag 101 receives radio waves carrying a command and data sent from the reader/writer, the receiving circuit 112 demodulates signals of the command and data on the received radio waves. The control circuit 113 receives the decoded command and data to execute a processing designated by the command. For example, as regards a read command, the control circuit 113 reads data from a designated address of the nonvolatile memory 116 to send the read data to the transmitting circuit 113. The transmitting circuit 113 modulates the received data and then superimposes the data on a carrier and transmits the data from the antenna 120. As regards a write command, the received data is written to a designated address of the nonvolatile memory 116. In general, a high voltage of 14 V to 16 V is necessary for writing data to the nonvolatile memory 116. Therefore, as a voltage for writing data to the nonvolatile memory 116, a voltage obtained by boosting a power supply voltage generated by the power supply voltage generating circuit 111 by means of the charge-pump circuit 115 is used.
For example, a technique of generating a power supply voltage using radio waves received by the antenna 120 for operating the control circuit 113, the charge-pump circuit 115, and the nonvolatile memory 116 and writing received data to the nonvolatile memory 116 by radio is disclosed by Udo Karthaus et al. in “Fully Integrated Passive UHF RFID Transponder IC With 16.7-μW Minimum RF Input Power” in IEEE JOURNAL OF SOLID STATE CIRCUITS, VOL. 38, NO. 10, October, 2003, pp. 1602-1608.
When the reader/writer reads data from the nonvolatile memory 116 in the IC tag 101 in response to a read command, it is generally determined whether or not the data is normally read, based on whether or not the IC tag 101 sends back a response. If receiving the read data in accordance with the read command, the reader/writer determines that data was normally read. If the read data cannot be received after the elapse of a predetermined period, the reader/writer determines that data was not normally read.
The control circuit 113 for controlling operations of reading/writing data from/to the nonvolatile memory 116 and the transmitting circuit 113 for transmitting the read data normally operate if a power supply voltage generated by the power supply voltage generating circuit 111 is equal to or higher than the lowest possible voltage value at which a logic circuit can operate (limit voltage value for operating the logic circuit, referred to as “logic circuit operation marginal voltage value”) However, a power supply voltage should be equal to or higher than the lowest possible voltage value at which a reading circuit in the nonvolatile memory 116 can operate (limit voltage value for operating a memory read circuit, referred to as “memory read circuit operation marginal voltage value”), for normally reading data from the nonvolatile memory 116.
Prior to the explanation about the memory read circuit operation marginal voltage value, the structure of the nonvolatile memory is first described. FIG. 12 shows a typical structural example of the nonvolatile memory. The nonvolatile memory of FIG. 12 is an EEPROM that stores data by applying hot electrons to a floating gate of each memory cell. As shown in FIG. 12, this nonvolatile memory includes a memory cell array 210 for storing data, a row decoder 220 for decoding a row address, a column decoder 230 for decoding a column address, and a data input/output circuit 240 for inputting/outputting read data or written data.
In the memory cell array 210, plural memory cells 211 are arranged in a row direction (horizontal direction) and a column direction (vertical direction). The memory cells 211 are floating gate type cells for storing data by controlling electrons in the floating gate. In the memory cell array 210, plural word lines 212 extend in the row direction, and bit lines 213 extend in the column direction. The memory cells 211 are arranged at intersections between the plural word lines 212 and the plural bit lines 213. Each of the memory cells 211 has a control gate connected with a corresponding word line 212, a drain connected with a corresponding bit line 213, and a grounded source.
In the case of writing data to the memory cells 211, for example, a row address is input to the row decoder 220 to select a corresponding one of the word lines 212. A column address is input to the column decoder 230 to select a corresponding one of the bit lines 213 and select a memory cell corresponding to the desired address from among the memory cells 211. Then, a writing voltage is applied to the selected word line 212 and bit line 213. The reading voltage is a high voltage boosted by the charge-pump circuit 115. By applying such a high voltage, a charge amount of a floating gate of the selected memory cell 211 is changed to write data to the memory cell 211.
In the case of reading data from the memory cells 211, similar to the writing operation, a memory cell corresponding to a desired address is selected from among the memory cells 211. Then, the selected word line 212 applies a reading voltage to a control gate of the memory cell 211, and the selected bit line 213 applies a reading voltage to a drain of the memory cell 211. The reading voltage is lower than the writing voltage, in other words, the reading voltage is not boosted by the charge-pump circuit 115. Hence, a threshold voltage varies depending on charges in the floating gate of the memory cell 211 (stored data), so data is read based on whether or not current flows between a drain and source. For example, in the case where a threshold voltage is increased in accordance with the data writing operation, a memory cell is out of conduction, which suggests that the data is being written.
Circuits such as the row decoder 220 or the column decoder memory cells 211 are composed of inverters 221 and 231 and other such transistors. Output terminals of the inverters 221 and 231 are connected with the word line 212 and the bit line 213. The transistors such as the inverters 221 and 231 receives high voltage upon the writing operation as mentioned above and are thus high-breakdown-voltage transistors. In general, a threshold value of the high-breakdown-voltage transistor is higher than that of any general transistor. That is, a power supply voltage necessary for operating the high-breakdown-voltage transistor is higher than that of any general transistor. For example, as for a circuit requiring a writing voltage of 15 V, a threshold value is about 1.5 V.
Therefore, in the case of writing data to the nonvolatile memory 116, a boosted voltage, i.e., a high voltage is necessary. In the case of reading data from the nonvolatile memory 116, a voltage that allows the high-breakdown-voltage transistor such as the row decoder 220 to operate is necessary. The minimum value of a reading operation voltage is the memory read circuit operation marginal voltage value.
FIG. 13 shows a relation between a power supply voltage that is generated using radio waves, and a logic circuit operation and the nonvolatile memory reading operation and the IC tag reading operation by the reader/writer.
If the generated power supply voltage is below the logic circuit operation marginal voltage value, data is normally read from neither the logic circuit nor the nonvolatile memory. That is, the control circuit for controlling the reading operation cannot operate in response to a read command from the reader/writer, so the IC tag cannot respond to the command from the reader/writer. Accordingly, the reader/writer fails to read data from the IC tag (in this case, the IC tag reading operation of the IC tag is denoted by a cross mark “x”).
If the generated power supply voltage is between the logic circuit operation marginal voltage value to memory read circuit operation marginal voltage value, the logic circuit can operate but data cannot be normally read from the nonvolatile memory. That is, the IC tag can operate in accordance with the read command from the reader/writer, but data is not normally read from the nonvolatile memory, with the result that wrong data is read and the read data is sent to the reader/writer. Therefore, although the reading operation for the IC tag ends in failure, the IC tag reading operation of the reader/writer appears to normally end (in this case, the IC tag reading operation of the reader/writer is designated by a triangle mark “▴”.
Further, if the generated power supply voltage is the memory read circuit operation marginal voltage value or higher, data can be normally read from both the logic circuit and the nonvolatile memory. That is, data is normally read from the nonvolatile memory in response to a read command from the reader/writer, so correct data is read in response to the read command. Accordingly, the reader/writer succeeds in reading data from the IC tag (in this case, the IC tag reading operation of the reader/writer is designated by a circle mark “∘”).
As mentioned above, in the IC tag, if the generated power supply voltage ranges from the logic circuit operation marginal voltage value to the memory read circuit operation marginal voltage value, although a normal reading operation is not executed in response to a read command from the reader/writer, the reader/writer determines that the IC tag reading operation normally ends.
That is, the power supply voltage generated by the IC tag ranges from the logic circuit operation marginal voltage value to the memory read circuit operation marginal voltage value, a reading circuit for the nonvolatile memory in the IC tag does not normally operate, but the logic circuit normally operates, so the IC tag responds to the read command. Thus, the reader/writer mistakes the wrong data for correct data.
Hence, there is a possibility that the data read from the IC tag is wrong, which reduces a reliability of the read data. If the data read from the IC tag is wrong, an erroneous operation is executed.