Functional characteristics of integrated digital temperature sensor TC77 and design of circuit interface
Время публикации: 2020-07-01 18:45:08
TC77 is a 13-bit serial interface integrated digital temperature sensor produced by Microchip. Its temperature data is converted by the thermal sensing unit. The TC77 contains a 13-bit ADC with a temperature resolution of 0.062 5°C/LSB. Under normal operating conditions, the quiescent current is 250μA (typical). The communication between other devices and TC77 is realized by SPI serial bus or Microwire compatible interface. The bus can be used to connect multiple TC77 to realize multi-region temperature monitoring. The SHDN bit in the configuration register CONFIG activates the low-power shutdown mode. At this time, the current The consumption is only 0.1μA (typical). TC77 has the characteristics of small size, low assembly cost and easy operation, which is an ideal choice for system thermal management.
2. The internal structure and pin function of TC77
Figure 1 shows a schematic diagram of the internal structure of TC77. TC77 is composed of CMOS junction temperature sensor, 13-bit sigma-delta A/D converter with sign bit, temperature register, configuration register, manufacturer ID register and three-wire serial interface.
Figure 1 Schematic diagram of the internal structure of TC74
The pins are defined as follows:
SI/O: Serial data pin
SCK: serial clock
CE: Chip select terminal (active low)
VDD: power supply voltage (6.0 V)
3. The working principle of TC77
The digital temperature sensor TC77 obtains the temperature from a solid-state (PN junction) sensor and converts it into digital data. The converted digital temperature data is stored in its internal register and can be read at any time through the SPI serial bus interface or Microwire compatible interface. TC77 has two working modes, namely continuous temperature conversion mode and shutdown mode. The continuous temperature conversion mode is used for continuous temperature measurement and conversion, and the shutdown mode is used for power-sensitive applications that reduce the supply current.
3.1 Power on and reset of TC77
At power-on or voltage reset, TC77 is in continuous temperature conversion mode. The first valid temperature conversion at power-on or voltage reset will last approximately 300 ms. After the first temperature conversion is completed, the second bit of the temperature register is changed. Set to logic "1", and during the first temperature conversion, the second bit of the temperature register is set to logic "0", therefore, the first temperature conversion can be judged by monitoring the status of the second bit of the temperature register Whether it ends.
3.2 TC77's low-power shutdown mode
After getting permission from TC77, the host can put it into a low-power shutdown mode. At this time, the A/D converter is suspended and the temperature data register is frozen, but the SPI serial bus port is still operating normally. By setting the SHDN bit in the configuration register CONFIG, the TC77 can be placed in low-power shutdown mode: that is, when SHDN=0 is set to normal mode; when SHDN=1 is low-power shutdown mode.
3.3 TC77 temperature data format
TC77 uses 13-bit two's complement to represent temperature. Table 1 lists the relationship between TC77's temperature, two's complement and hexadecimal code. The least significant bit (LSB) in the table is 0.062 5 ℃, the last two LSB bits (that is, bit 1 and bit 0) are tri-stated, and the table is "1". At the end of the first temperature conversion after a power-up or voltage reset event, bit 2 is set to a logic "1".
Table 1 TC77 temperature digital output
3.4 TC77 serial bus
The serial bus of TC77 includes chip select signal line CE, serial clock signal line SCK and serial data signal line SI/O, following SPI or Mi-crowire interface standard protocol. When there are multiple TC77 connected to the serial clock and serial data signal line, CE is used to select one of the TC77 devices. When CS is logic "0", it is used to write or read data from the device. Synchronization; when CS is logic "1", SCK is disabled. The falling edge of CS starts the communication between devices, and the rising edge of CS stops the communication between devices. Figure 2 shows the timing of reading the temperature register.
Figure 3 is the TC77 multi-byte communication operation sequence, including the read operation of the temperature register and the write operation of the configuration register. The 16 SCK pulses of the first group are used to transfer the temperature data of the TC77 to the microprocessor, and the 16 SCK pulses of the second group are used to receive the instructions of the microprocessor, so that the TC77 enters the shutdown mode or the continuous conversion mode. The data written to the TC77 configuration register should be all 0s or all 1s, corresponding to the continuous conversion mode or the shutdown mode, respectively. When C0～C7 of the configuration register are all 1, it is the shutdown mode, when C0～C7 has one When 0 is written, it changes to continuous conversion mode.
4. Interface between TC77 and AVR microcontroller
4.1 Hardware interface between TC77 and AVR microcontroller
Figure 2 TC77 temperature register operation timing
Figure 4 is a schematic diagram of the interface hardware connection between TC77 and AVR microcontroller. The figure uses a synchronous serial three-wire SPI interface, which can be easily connected to a peripheral device adopting the SPI communication protocol or another AVR single-chip microcomputer to achieve short-distance high-speed synchronous communication.
ATmega128's SPI uses hardware to achieve byte-oriented full-duplex 3-wire synchronous communication, supporting master, slave and two different polarity SPI timing. The internal SPI interface of ATmega128 MCU can also be used for program download and upload of program memory and data E2PROM. However, it should be noted that the MOSI and MISO interfaces of the SPI no longer correspond to the PB2 and PB3 pins, but are converted to the PE0 and PE1 pins (PDI, PDO).
Figure 4 TC77 and AVR microcontroller hardware interface
4.2 Software interface between TC77 and AVR microcontroller
The interface software between TC77 and AVR MCU includes main program and interrupt service program. In the main program, we must first initialize the hardware SPI of ATmega128. During initialization, the MOSI, SCLK, and SS pins of PORTB should be used as the output, and the MISO pin should be used as the input, and the pull-up resistor should be turned on. Then initialize the SPI registers and read SPSR (SPX Status Reg-ister, SPI status register) and SPDR (SPI Data Register SPI, data register) once, so that the ISP is idle to wait for data to be sent. The AVR's SPI consists of a 16-bit cyclic shift register. When the data is moved out of the host, the data of the slave is also moved in at the same time, so the transmission and reception of the SPI can be completed in the same interrupt service routine. In the SPI interrupt service program, first read a received byte from the SPDR and store it in the receive data buffer, then take a byte from the transmit data buffer and write it into the SPDR, which is sent to the slave by the ISP. Once the data is written to SPDR, the ISP hardware begins to send the data. The next time the ISP is interrupted, it indicates that the transmission is completed and a data is received at the same time. In the program, putSPIchar() and getSPIchar() are the underlying interface functions of the application, and two data buffers are also used to form a circular queue.
TC77 can be directly connected with MCU with SPI or the same interface. For MCU without SPI interface, SPI operation can be synthesized through software programming. TC77 is very suitable for low-cost and small-scale applications of temperature measurement and control, such as the thermal protection of computer hard drives or other peripheral devices of PC, and also suitable for lower temperature measurement and control systems.Тег: TC77