High-precision data acquisition device based on PIC16F87X series microcontroller

1ADS1210 Pins and Functions
ADS1210 is a new type of A/D converter with high precision, wide dynamic range, single 5V power supply and 24-bit resolution. The package types are 18-pin dual in-line and 18-pin surface mount. The pin function description is shown in Table 1.

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●Instruction Register (INSR)
The instruction register is an 8-bit register that indicates whether the system is performing a read or write operation, and determines the byte length of the read/write operation and the starting address of the register for the read/write operation. 
●Command Register (CMR)
The command register is the key to the ADS1210, and it controls all the features and functions of the ADS1210. Once the falling edge of the serial port clock writes the last bit of the data byte into the command register, the new mode takes effect. The command register controls the options and operating modes selected by the ADS1210, including the gain (G) setting of the programmable gain amplifier, the enhancement mode (TMR), the output data rate (Decimation), and the correction method, etc.
●Data Output Register (DOR)
The data output register stores the most recent conversion result. Before the "data ready signal" becomes low, the contents of the register are updated with a new result.
●Offset Register (OCR)
The offset register performs offset correction on the conversion result before it is placed in the data output register. In this application, the contents of the offset register may be the self-correction result or the system correction result, and the offset register can be read and written through the serial port. 
● Full-scale correction register (FCR)
The full-scale correction register performs full-scale correction on the conversion result before it is placed in the data output register. The actual offset register value and the full-scale correction register value change with changes in structure, temperature and power supply. Therefore, the actual value of the offset register and the full-scale correction register in any state cannot be accurately predicted, that is, the correction of a given system cannot simply obtain a certain result by measuring the external error and writing it into the register as a correction factor.
Communication with ADS1210 starts after the data conversion is completed, that is, when the level is low. Usually, the instruction register is written first to indicate the starting address and byte length of the register to be operated next, and whether the system is reading or writing, thereby determining what type of communication is to be performed next. After that, with the generation of the appropriate clock pulse (the number of cycles of the SCLK clock is determined by the byte length specified by the instruction register), the register specified by the instruction register is operated, and after completion, the next low level of the pin is waited for, and this is repeated until the data acquisition is completed. 
In actual measurement, power supply is an important factor affecting accuracy. In order to reduce its influence, the power supply must be stable and have low noise. Therefore, MAX666 is used as the voltage regulator chip of +5V voltage, and the voltage fluctuation is reduced by using the role of peripheral capacitors. In addition, ADS1210 can use its own correction register to calibrate the A/D conversion data through programming to further improve the measurement accuracy. Through the setting of CMR, the user can choose the appropriate correction mode. Under background correction, the correction is repeated at a fixed time interval. For other types, normal operation is restored once the correction operation is completed. This design adopts the background correction mode. 
The hardware system design of the PIC series microcontroller is simple, and the instruction system design is refined. The PIC microcontroller integrates a wealth of peripheral modules, which can realize the control of peripheral modules by operating the internal registers.
The serial expansion communication interface is an important channel for data exchange between the microcontroller and other computers or modules. The PIC16F87X series microcontroller is mainly equipped with two forms of serial communication modules, namely the master synchronous serial communication MSSP and the universal synchronous/asynchronous receiver and transmitter USART. Among them, the MSSP module is mainly used for short-distance serial communication within the system, such as SPI and I2C modes; the USART module is mainly used for long-distance serial communication between systems. [page]

SPI (Serial Peripheral Interface) is a synchronous serial expansion interface for microcontroller peripheral chips. Since ADS1210 has a flexible synchronous serial interface compatible with the SPI communication mode, all data exchanges between the PIC microcontroller and ADS1210 in this design adopt the SPI communication mode. The SPI module circuit of the PIC microcontroller consists of three main parts: the transmit buffer, the receive buffer, and the shift register.
The data format used by USART in asynchronous mode is 1 start bit, 8 or 9 data bits, and 1 stop bit, without a parity check code, and the commonly used data is 8 bits. The 8-bit baud rate generator BRG provided on the chip can use the system clock signal to generate a standard baud rate frequency for serial transmission. USART has independent transmitters and receivers, but the data format and baud rate they use are the same. This design uses the USART asynchronous transmission function of the microcontroller to transmit the collected data to the host computer after necessary level conversion through the MAX232 chip. 
The interface between ADS1210 and external devices includes two-wire, three-wire, four-wire and multi-wire. Here, the four-wire system is used to realize the interface with the microcontroller. The interface signals are data ready signal (), data input/output line (SDIO), data output line (SDOUT), and clock signal line (SCLK). See Figure 1 for details.

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