Simple USB Interface Data Acquisition System, The Easy Data Acquisition System Base ON USB Interface
Keywords: USB data acquisition, data acquisition circuit, MAX197, CH371
The data acquisition system with the USB interface is simple and convenient to use, no external power supply, can also utilize powerful calculation capabilities of the PC. Such systems generally do interface control with single-chip microcomputer, and people who do not use microcontrollers are a problem. Here is a USB data acquisition system without a single-chip microcomputer, as long as you use VB programming.
First, the system's hardware composition
The electrical principle of this system is as shown in the figure above. The CH371 is a USB interface chip, the MAX197 multi-channel AD converter performs data acquisition, and the computer is processed.
CH371 is a simple and easy-to-use USB interface chip of Nanjing Qinheng Electronics, which includes a universal driver of the chip itself and a computer side. The CH371 implements the entire process of USB communication protocol transmission control with hardware logic. The general driver provides a device-level interface to the computer application layer through software, so users do not have to consider USB communication protocols, firmware programs, drivers, underlying data transfer processes, etc. Various USB interface devices can be designed.
CH371 has a variety of ways of working, in addition to data transmission mode, it can also operate in a master, even without connecting the microcontroller, DSP controller, can also output control signals and input data. The CH371's D7 ~ D0, A3 to A0 are both bidirectional pins, which can be divided into two groups, which are set for input or output, respectively. Connecting the CH371 A3 to A0 from the MAX197 CS, WR, RD, HBEN input control line, connect the parallel data ports of D7 to D0 and MAX197, and can directly control the operation of MAX197 by software.
MAX197 is the 12-bit AD converter launched by Maxim, single 5 volt power supply, built-in 4.096 volt voltage reference, the peripheral circuit is simple, only a few capacitance can be worked. MAX197 has two internal and external clocks and sampling modes, analog input range and polarity optional, 0 ~ 5 volts, 0 to 10 volts, ± 5 volts, and ± 10 volts. There are 8 analog input channels. All of this can be selected by software, ie, write to the max197 to the control word, and the functions of the control word are shown in the table below:
Position
PD1
PD0
ACQMOD
RNG
BIP
A2
A1
A0
Function
Internal and external clock and power management mode
Internal and external control sampling mode
Analog signal scale
Analog signal polarity
Simulation input channel selection
Second, the software design of the system
1. Software interface of the CH371 computer
The CH371 provides three levels of software interfaces on the computer, and the highest layer is an application layer interface. The application layer interface is an API function-oriented application-oriented API function provided by the CH371 dynamic link library DLL. Each function implements a specific function, and uses a simple and easy-to-use API parameter instead of the data format required in the device level interface, all APIs have an operational status after the call is returned, and the user can achieve the required specific API function. Function.
The API functions provided by the CH371 dynamic link library DLL include several classes such as device management, data transmission, interrupt query, I2C operation, and direct control. We use two direct control functions.
Ch371GetStatus (Byval Iindex As Long, ByRef Idatareg As Long) AS Boolean
Its function is to enter the data and status directly through the CH371, where IIndex specifies the CH37 device serial number, and ISTATUS points to a double word unit for saving status data. Bit7 ~ bit0 corresponds to the D7 ~ D0 pin of CH371. IDataReg points to a double word unit for saving data of the data register.
CH371SETCONTROL (byval Iindex as long, byval ICTRDATA AS BYTE, BYVAL ICTRLADDR AS BYTE, BYVAL IDATA0E As Boolean, BYVAL IADDR0E As Boolean) AS Boolean
Its function is to output control signals directly through the CH371. ICTRDATA is data that is ready to write to D7 ~ D0 data registers, and ICTRLADDR is data that is ready to write A3 to A0 data registers, which must output data when outputting. Idata0e Control D7 ~ D0 output enable, 1 output, 0 input. Iaddr0e Controls A3 to A0 output enable, 1 output, 0 input.
2. VB application
The application is written in VB. Its core is to call the above two API functions, write the control word to the MAX197 through the D7 to D0 and A3 to A0 of CH371, start the analog number conversion, and finally read the conversion result. VB To use the API function in the CH371 dynamic link library, you must first declare these functions in the code segment of the DECLARE statement, CH371 chip vendor in the VB Demo provided, has two module already contains all API functions of CH371 Users only need to join them in their own project. Through these two function calls, write the control word to the max197, set the mode of operation, select the input channel, start the analog number conversion, finally read the conversion result, store the computer, and further process.
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