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LM74CITP-5 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM74CITP-5 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 15 page 1.0 Functional Description (Continued) munication will consist of 32 serial clocks. The first 16 clocks comprise the transmit phase of communication, while the second 16 clocks are the receive phase. When CS is high SI/O will be in TRI-STATE®. Communica- tion should be initiated by taking chip select (CS) low. This should not be done when SC is changing from a low to high state. Once CS is low the serial I/O pin (SI/O) will transmit the first bit of data. The master can then read this bit with the rising edge of SC. The remainder of the data will be clocked out by the falling edge of SC. Once the 14 bits of data (one sign bit, twelve temperature bits and 1 high bit) are transmit- ted the SI/O line will go into TRI-STATE. CS can be taken high at any time during the transmit phase. If CS is brought low in the middle of a conversion the LM74 will complete the conversion and the output shift register will be updated after CS is brought back high. The receive phase of a communication starts after 16 SC periods. CS can remain low for 32 SC cycles. The LM74 will read the data available on the SI/O line on the rising edge of the serial clock. Input data is to an 8-bit shift register. The part will detect the last eight bits shifted into the register. The receive phase can last up to 16 SC periods. All ones must be shifted in order to place the part into shutdown. A zero in any location will take the LM74 out of shutdown. The following codes should only be transmitted to the LM74: • 00 hex • 01 hex • 03 hex • 07 hex • 0F hex • 1F hex • 3F hex • 7F hex • FF hex any others may place the part into a Test Mode. Test Modes are used by National Semiconductor to thoroughly test the function of the LM74 during production testing. Only eight bits have been defined above since only the last eight trans- mitted are detected by the LM74, before CS is taken HIGH. The following communication can be used to determine the Manufacturer’s/Device ID and then immediately place the part into continuous conversion mode. With CS continuously low: • Read 16 bits of temperature data • Write 16 bits of data commanding shutdown • Read 16 bits of Manufacture’s/Device ID data • Write 8 to 16 bits of data commanding Conversion Mode • Take CS HIGH. Note that one complete temperature conversion period will have to pass before the LM74 Temperature register will contain the new temperature data. Until then, it will contain a "stale" temperature (the data that was in the register before going into shutdown mode). 1.3 TEMPERATURE DATA FORMAT Temperature data is represented by a 13-bit, two’s comple- ment word with an LSB (Least Significant Bit) equal to 0.0625˚C: Temperature Digital Output Binary Hex +150˚C 0100 1011 0000 0111 4B 07h +125˚C 0011 1110 1000 0111 3E 87h +25˚C 0000 1100 1000 0111 0C 87h +0.0625˚C 0000 0000 0000 1111 00 0Fh 0˚C 0000 0000 0000 0111 00 07h −0.0625˚C 1111 1111 1111 1111 FF FFh −25˚C 1111 0011 1000 0111 F3 87h −55˚C 1110 0100 1000 0111 E4 87h Note: The last two bits are TRI-STATE® and depicted as one in the table. The first data byte is the most significant byte with most significant bit first, permitting only as much data as neces- sary to be read to determine temperature condition. For instance, if the first four bits of the temperature data indicate an overtemperature condition, the host processor could im- mediately take action to remedy the excessive tempera- tures. 1.4 SHUTDOWN MODE/MANUFACTURER’S ID Shutdown mode is enabled by writing XX FF to the LM74 as shown in Figure 7c. The serial bus is still active when the LM74 is in shutdown. Current draw drops to less than 10 µA between serial communications. When in shutdown mode the LM74 always will output 1000 0000 0000 00XX. This is the manufacturer’s/Device ID information. The first 5-bits of the field (1000 0XXX) are reserved for manufacturer’s ID. As mentioned in Section 1.2, writing a zero to the LM74 con- figuration register will take it out of shutdown mode and place it in conversion mode. In other words, any valid code listed in Section 1.2 other than XX FF will put it in conversion mode. After leaving shutdown, but before the first tempera- ture conversion is complete, the temperature register will contain the last measured temperature which resided in the temperature register before entering shutdown mode. After the completion of the first conversion, the temperature reg- ister will be updated with the new temperature data. www.national.com 9 |
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