The MCU chip is equipped with necessary external components, generally including power supply and power switch, reset circuit, crystal oscillator, input and output circuit, etc. to form a minimum system.
The MSP430F149 chip is an ultra-low-power microprocessor from TI, USA. It has 60KB+256 bytes of FLASH and 2KB of RAM, including basic clock module, watchdog timer, 16 bits with 3 capture/compare registers and PWM output. Timer, 16-bit timer with 7 capture/compare registers and PWM output, 2 8-bit parallel ports with interrupt function, 4 8-bit parallel ports, analog comparator, 12-bit A/D converter, 2 A serial communication interface and other modules. The MSP430F149 chip has the following features:
1) Low power consumption: When the voltage is 2.2V, the clock frequency is 1MHz, the active mode is 200μA; when it is off mode, it is only 0.1A, and it has 5 energy-saving working modes.
2) High-efficiency 16-bit RISC-CPU, 27 instructions, 8MHz clock frequency, instruction cycle time is 125ns, most instructions are completed in one clock cycle; 32kHz MSP430 microcontroller execution speed is higher than typical at 32kHz clock frequency The execution speed of the 8-bit microcontroller at 20MHz clock frequency.
3) Low voltage power supply, wide operating voltage range: 1.8 ~ 3.6V;
4) Flexible clock system: two external clocks and one internal clock;
5) Low clock frequency enables high-speed communication;
6) With serial online programming capability;
7) Powerful interrupt function;
8) The wake-up time is short, and it takes only 6μs to wake up from the low-power mode;
9) ESD protection, strong anti-interference ability;
10) Operating environment temperature range is -40 ~ +85 ° C, suitable for industrial environments.
The control of all peripheral modules of MSP430 series MCU is realized by special registers, so the program is relatively simple to write. Programming and development, through a dedicated programmer, you can choose assembly or C language programming, IAR company developed a dedicated C430 language for the MSP430 series of microcontrollers, can be directly compiled and debugged through WORKBENCH and C-SPY, flexible and simple to use.
1 overall system designThe minimum system is composed of the basic circuits necessary to ensure the reliable operation of the processor, mainly including the power circuit, the clock circuit, the reset circuit, the communication interface circuit, and the data storage circuit. The hardware block diagram is shown in FIG. 1 .
1.1 Power circuit
This system needs to use +5V and +3.3V DC regulated power supply, of which MSP430Fl49 and some peripheral devices need +3.3V power supply, and other parts need +5V power supply. In this system, the +5V DC voltage is used as the input voltage, and +3.3V is directly linearly stepped down by +5V. The principle of the power circuit is shown in Figure 2.
1.2 crystal oscillator circuit
The MSP430 series MCU clock module includes three clock sources such as a digitally controlled oscillator (DCO), a high speed crystal oscillator and a low speed crystal oscillator. This is to solve the contradiction between the fast processing data requirements and low power requirements of the system. By designing multiple clock sources or designing different working modes for the clock, the clock requirements of real-time applications of some peripheral components, such as low-frequency communication, can be solved. LCD display, timer, counter, etc. The digitally controlled oscillator DCO has been integrated into the MSP430. In the system, only the high-speed crystal oscillator and the low-speed crystal oscillator are designed.
The low speed crystal oscillator (LFXTl) meets the requirements for low power consumption and the use of a 32.768 kHz crystal. The LFXTl oscillator works in the low frequency mode by default, that is, 32.768 kHz. It can also work in the high frequency mode through an external 450 kHz to 8 MHz high speed crystal oscillator or ceramic resonator. In this circuit, we use the low frequency mode, and the crystal oscillator is connected to two 22pF. The capacitor is connected to the MCU via XIN and XOUT.
The high-speed crystal oscillator, also known as the second oscillator XT2, provides the clock for the MSP430F149 operating in high-frequency mode, with XT2 up to 8MHz. In the system, XT2 uses a 4MHz crystal, and XT2 externally connects two 22pF capacitors to the MCU through XT2IN and XT2OUT. The principle is shown in Figure 3.
1.3 Reset circuit schematic
Manual reset is a commonly used function of the minimum system. This system uses the dedicated reset chip IMP811 to achieve manual reset. The principle is shown in Figure 4.
1.4 Communication interface circuit
The communication interface is responsible for tasks such as data exchange with the peripheral serial host and support for printing. Serial communication requires only a small number of ports to achieve interoperability between the microcontroller and the PC, with unparalleled advantages. There are two ways to communicate serially: asynchronous mode and synchronous mode. The MSP430 series has a USART module for serial communication. In this design, the USART0 module of the MSP430F149 communicates with the peripheral serial host via the RS232 serial port.
The EIA-RS232 standard is a serial data transmission bus standard developed by the Electronic Industries Association (EIA). In the early days, it was applied to computers and terminals for long-distance data transmission via telephone lines and modems. With the development of microcomputers and microcontrollers, this communication method was adopted not only at a long distance but also at a close distance. In the short-range communication system, the telephone line and the MODEM are no longer used, and the end-to-end connection is directly performed. The RS232 standard uses a negative logic mode. The standard logic "1" corresponds to a -5V to -15V level, and the standard logic "O" corresponds to a +5V to +15V level. Obviously, the communication between the two must be converted by the signal level.
This system is realized by a dedicated level conversion chip MAX3232. The MAX3232 chip is a level-shifting chip from MAXIM that includes two receivers and drivers for reliable performance. The principle is shown in Figure 5.
1.5 data storage circuit
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