Datasheet AD7814 (Analog Devices) - 5

AD7814 CIRCUIT INFORMATION
25 µs later. Reading from the device before conversion is com- The AD7814 is a 10-bit digital temperature sensor. The part plete will provide the same set of data. houses an on-chip temperature sensor, a 10-bit A/D converter, a reference, and serial interface logic functions in a SOT-23
TEMPERATURE VALUE REGISTER
package. The A/D converter section consists of a conventional The temperature value register is a read-only register that stores successive-approximation converter based around a capacitor the temperature reading from the ADC in 10-bit twos comple- DAC. The parts are capable of running on a 2.7 V to 5.5 V ment format. The temperature data format is shown in Table I. power supply. This shows the full theoretical range of the ADC from –128°C The on-chip temperature sensor allows an accurate measurement to +127°C, but in practice the temperature measurement range of the ambient device temperature to be made. The working is limited to the operating temperature range of the device (–55°C measurement range of the AD7814 is –55°C to +125°C. to +125°C). A typical performance curve is shown in Figure 7.
CONVERTER DETAILS Table I. Temperature Data Format
The conversion clock for the part is internally generated so no
Digital Output
external clock is required except when reading from and writing
Temperature DB9 . DB0
to the serial port. In normal mode, an internal clock oscillator runs the automatic conversion sequence. A conversion is initi- –128°C 10 0000 0000 ated approximately every 350 µs. At this time, the part wakes up –125°C 10 0000 1100 and performs a temperature conversion. This temperature con- –100°C 10 0111 0000 version typically takes 25 µs, at which time the part automati- –75°C 10 1101 0100 cally shuts down. The result of the most recent temperature –50°C 11 0011 1000 conversion is available in the serial output register at any time. –25°C 11 1001 1100 Once the conversion is finished, an internal oscillator starts –0.25°C 11 1111 1111 counting and is designed to time out every 350 µs. The AD7814 0°C 00 0000 0000 then powers up and does a conversion. Please note that if the +0.25°C 00 0000 0001 CS is brought low every 350 µs (± 30%), then the same tempera- +10°C 00 0010 1000 ture value will be output onto the DOUT line every time with- +25°C 00 0110 0100 out changing. It is recommended that the CS line is not brought +50°C 00 1100 1000 low every 350 µs (± 30%). The ± 30% covers process variation. +75°C 01 0010 1100 The CS should become active (high to low) outside this range. +100°C 01 1001 0000 +125°C 01 1111 0100 The device is designed to autoconvert every 350 µs. If the +127°C 01 1111 1100 AD7814 is accessed during the conversion process, an internal signal is generated to prevent any update of the temperature value register during the conversion. Otherwise the user could
SERIAL INTERFACE
read back spurious data. The design of this feature results in this The serial interface on the AD7814 consists of four wires CS, internal lockout signal being reset only at the start of the next SCLK, DIN, and DOUT. The interface can be operated in autoconversion. Therefore, if the CS line goes active before the 3-wire mode with DIN tied to Ground, in which case the inter- internal lockout signal has been reset to its inactive mode, the face has read-only capability, with data being read from the data internal lockout signal will not be reset. To ensure that no lock- register via the DOUT line. The DIN line is used to write the part out signal is set, bring the CS low at a greater or less time than into standby mode, if required. The CS line is used to select the 350 µs (± 30%). This will mean that the AD7814 is not inter- device when more than one device is connected to the serial rupted during a conversion process. The AD7814 can be placed clock and data lines. in a shutdown mode, via the control register, in which case the The part operates in a slave mode and requires an externally on-chip oscillator is shut down and no further conversions are applied serial clock to the SCLK input to access data from the initiated until the AD7814 is taken out of shutdown mode. The data register. The serial interface on the AD7814 is designed to conversion result from the last conversion prior to shutdown can allow the part to be interfaced to systems that provide a serial still be read from the AD7814 even when it is in shutdown mode. clock that is synchronized to the serial data, such as the 80C51, In the automatic conversion mode, every time a read or write 87C51, 68HC11, 68HC05, and PIC16Cxx microcontrollers, as operation takes place, the internal clock oscillator is restarted at well as DSP processors. the end of the read or write operation. The result of the con- A read operation from the AD7814 accesses data from the version is typically available 25 µs later. Similarly, when the temperature value register, while a write operation to the part part is taken out of shutdown mode, the internal clock oscil- writes data to the control register. lator is restarted and the conversion result is typically available REV. E –5– Document Outline FEATURES FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS SPECIFICATIONS TIMING CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE PIN FUNCTION DESCRIPTIONS PIN CONFIGURATIONS CIRCUIT INFORMATION CONVERTER DETAILS TEMPERATURE VALUE REGISTER SERIAL INTERFACE Read Operation Write Operation MICROPROCESSOR INTERFACING AD7814 to MC68HC11 Interface AD7814 to 8051 Interface AD7814 to PIC16C6x/7x Interface AD7814 to ADSP-21xx Interface MOUNTING THE AD7814 SUPPLY DECOUPLING OUTLINE DIMENSIONS Revision History