Design method and application of low power MCU system for portable equipment

1. Challenges of portable devices to processors With the popularity of electronic portable devices in the world, people are increasingly demanding electronic portable devices, and hope that products have more functions, such as mobile phone camera auto focus and mobile phone flash And pedometers; hope that the product power consumption is lower, such as wireless devices, handheld POS machines and home medical products; hope that the product size is smaller, such as sports watches; hope that the product's confidentiality is good; processing power, such as portable instruments and high Precision motion control; hope for lower prices and shorter development cycles.

However, the confusion is that many portable devices tend to have multiple requirements at the same time, but in reality it is difficult to achieve: at the same time meet high-speed processing, low power consumption and price? ARM high speed, but power consumption and high price; at the same time meet high speed processing and small package? I hope that the package area is as small as 3×3mm, without sacrificing speed; while satisfying small package and SOC? Need ADC, SPI and 12 I / O, and the size, preferably less than 5 × 5mm; development cycle, ARM performance is completely appropriate, but ARM often requires operating system support, long development cycle, and market opportunities fleeting.

To this end, this article will start from the development of these problems for electronic portable devices, how to design a low-power single-chip system and method and the advantages of MCU in low power consumption, and the application of small-size single-chip microcomputer as an example. Make an introduction.

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2, how to design a low-power single-chip system problem: Is the power consumption of the single-chip system only determined by the power consumption of the microcontroller? The answer is that the system with the single-chip microcomputer as the core, the total energy consumption of the system is composed of the energy consumption of the single-chip microcomputer and the energy consumption of its peripheral circuits. In order to reduce the power consumption of the entire system, in addition to reducing the operating power consumption of the microcontroller itself, it is also necessary to reduce the power consumption of the peripheral circuits.

2.1 How to design a low-power single-chip system?

To design a low-power microcontroller system, you need to start with both hardware and software.

2.11 hardware design * Select the CPU core as simple as possible. When choosing a CPU core, avoid blindly pursuing performance. The principle of choice should be "enough to use". If the 8-bit machine is enough, there is no need to use a 16-bit machine. In general, the faster the microcontroller runs, the greater the power consumption. A complex CPU has high integration and powerful functions, but there are many on-chip transistors and a large total leakage current. Even if it enters the STOP state, the leakage current can not be ignored. The simple CPU core not only has low power consumption but also low cost.

* Use a low voltage power supply system. Low voltage power supply can greatly reduce the operating current of the system. At present, the MCU is reduced from the TTL compatible 5V power supply to 3.3V, 3V, 2V or even 1.8V power supply. Lowering the power supply voltage of the MCU can effectively reduce its power consumption. The reduction of power supply voltage is also an important trend in the development of future microcontrollers.

* Select a system with a low power mode. The low power mode refers to the system's Idel, Stop, and Suspen modes. The power consumption of the microcontroller in this mode will be much smaller than the power consumption in the operating mode.

* Choose the right clocking solution. The choice of clock is quite sensitive to system power consumption. There are two aspects to be aware of:

First, the system bus frequency should be as low as possible. The total current consumption inside the microcontroller is divided into: running current and leakage current. The higher the integration degree of the single chip microcomputer, the higher the ambient temperature and the larger the leakage current. The operating current of the microcontroller is almost proportional to its clock frequency. By reducing the clock frequency, the power consumption of the microcontroller can be effectively reduced.
Second, about the clock scheme. Whether to use a phase-locked loop, whether to use the internal oscillator or an external oscillator. Modern single-chip microcomputers generally use phase-locked loop technology, so that the clock frequency of the microcontroller can be controlled by the program. The microcontroller uses an externally low oscillator and is controlled by software. The system clock can be adjusted over a wide range to get a higher bus clock. Using a phase-locked loop results in additional power consumption. In terms of clocking schemes, the use of an external crystal oscillator without the use of a phase-locked loop is one of the least power consuming. Some microcontrollers have an internal clock or an external clock. This allows dual clocks to be used according to the needs of the actual system: a high speed clock and a low speed clock. Use a high-speed clock when processing events and a low-speed clock when idle. This dual clock system can effectively reduce power consumption.

2.12 Application Software Design The importance of application software design for a low-power system is often overlooked. An important reason is that software defects are not as easy to find as hardware, and there is no strict standard to judge the low-power characteristics of a software. However, designers can avoid the "invisible" power loss if they try to reflect the low-power characteristics of the application in the software.

* Replace "query" with "interrupt". In the case where low power consumption is not required, it is not important that the program uses the interrupt mode or the query mode. However, in the case of low power consumption, the two methods are far apart. Using the interrupt mode, the CPU can do nothing, or even enter the wait mode or stop mode; in the query mode, the CPU must constantly access the I / 0 register, which will bring a lot of extra power.

* Replace "subprogram" with "macro". The push-pull operation of the subroutine call, to perform two operations on the RAM, will bring more power consumption. The macro expands at compile time and the CPU executes the instructions in order. Using macros will increase the amount of code in the program, but for applications that don't care about the large amount of program code, using macros will undoubtedly reduce the power consumption of the system.

* Minimize the amount of CPU computation. Reducing the computational workload of the CPU can effectively reduce the power consumption of the CPU. The work of reducing CPU operations can be started from many aspects: First, replace the real-time calculation with the look-up table method. Second, the inevitable real-time calculation ends when the accuracy is enough, avoiding the "excessive" calculation. Third, try to use short data types. For example, try to use character-type 8-bit data instead of 16-bit integer data, try to use fractional operations and avoid floating-point operations. Fourth, let the I/O module run intermittently, that is, unused I/O modules or intermittently used UO modules should be turned off in time to save power; unused I/O pins should be set to output or set to input, The pull-up resistor is pulled high.

3. The advantage of single-chip microcomputer (MCU) in low power consumption.

The problem facing today is that users or the market have strict requirements on the low power consumption of the microcontroller (MCU). How will the MCU be designed to meet the market? It should be said that the MCUs of many manufacturers today have the advantage of low power consumption. For example, only the Silabs MCU is used as an example to illustrate the advantages of the MCU in terms of low power consumption.

*The supply voltage is low. The MCU supply voltage is 2.0 to 5.25V. Low supply voltage can effectively reduce the power consumption of the entire microcontroller system.

* There are several low power modes. The MCU's low power modes are Idle mode and Stop mode. In order to further reduce the power consumption of the MCU and improve the market competitiveness, the MCUs that have been introduced since the second half of 2006 will have the Suspend mode. The power consumption in this mode is nanoampere.

* There are a variety of clock options to choose from. The MCU's built-in oscillator is available in high-speed oscillation mode and low-speed oscillation mode. There are several options for the frequency in each mode. It is also possible to connect an external oscillator. More importantly, these clock modes can be switched in real time while the MCU is running. This makes it easy for customers to control low power. For example, when processing data, the system runs at a high speed state; when idle, it runs at a low speed state.

* High-speed real-time interrupt response. The MCU responds to interrupts very quickly, typically requiring only 5 system clock cycles. The interrupt response is fast, and the CPU spends less time waiting, which saves a lot of waiting power.

* Flexible I/O settings. The I/O port of the MCU is rich in resources and flexible in configuration. There are three configurations: open drain, push-pull output, and weak pull-up. Users can disable or enable these modes by setting the relevant registers according to actual needs. The configuration of the port as an open drain is the most energy efficient way. In addition, other peripherals not used on the MCU chip can be turned off by software. In short, according to the requirements of the project, flexible use of various low-power characteristics of the MCU, through software control, can achieve low power requirements.

Using MIPS power consumption to measure the MCU's low power performance is relatively accurate. For example, to perform a task that requires 10K instructions, the working current of A MCU is 3mA, and the speed is 10MIPS. Then A MCU needs to work lmS to complete the task, consume 3mA*1ms*Vcc, and then A MCU can enter low power mode. . The operating current of the B MCU is 1 mA, and the speed is 2 MIPS, then the B MCU needs to work 5 mS to complete, so that the consumption of the B MCU to complete the task is 1 mA * 5 mS * Vcc. MCUs with large currents but fast speeds may save more power! 4. What is the design method of MCU in terms of low power consumption?

In general, the higher the speed at which the MCU operates, the higher the supply voltage and the higher the power consumption. To reduce the power consumption of the microcontroller system, it is necessary to reduce the power supply voltage of the microcontroller system and reduce the frequency of the MCU operation.

For example, the customer should make a wireless timing product and use the battery as the power supply. The average power consumption should not exceed 200uA. The product works intermittently: it is activated when data is received, and the data is processed quickly; when it is idle, it goes into sleep state to reduce power consumption. Many manufacturers have reached 150μA to meet the requirements of Manlan customers. For example, the C8051F333 type MCU.

4.1 Look at the electrical parameters of the C8051F333 to know the * normal mode, the CPU fetches instructions from the Flash.

IDD (when Vdd=3.6v F=25MHz) is 10.7mA-11.7mA; IDD (when Vdd=3.0v F=25MHz) is 7.mA-8.3mA; IDD (when Vdd=3.6v F=1MHz) ) is 0.38 mA; IDD (when Vdd = 3.0 v F = 80 Hz) is 31 μA.

*Idel mode, the CPU stops working.

I DD (when Vdd=3.6v F=25MHz) is 4.mA-5.2mA; IDD (when Vdd=3.0v F=25MHz) is 3.8mA-4.1mA; IDD (when Vdd=3.6v F=1MHz) Time) is 0.2 mA; IDD (when Vdd = 3.0 v F = 80 Hz) is 16 μA.

Looking at the data of the above two modes, certainly not. Because it is necessary to meet the operating speed of the system, and to meet the frequency of low power consumption ≥ 1MHZ, the power consumption must exceed the customer's requirements. The correct answer is: Without reducing the operating speed of the MCU (the operating frequency of the MCU processing data is 24.5MHZ), the customer successfully achieved the low power requirement using the C8051F333: lower than 150μA, it is really a fish and bear's paw. Both.

4.2 How is it implemented?

See Figure 1. Both internal and external crystals are used. The internal high-speed crystal oscillator is used for 24.5MH2, and when it is idle, it switches to the external low-speed crystal oscillator 32.768KH2 and enters the Idle mode. And it's as simple as turning off all unused peripherals. It can be seen that power consumption is a system problem. The power consumption of the MCU system is determined by the power consumption of the MCU and its peripheral circuits. Low power consumption is saved by countless details.

5, the application of small-size single-chip microcomputer in portable devices Faced with the challenges presented by today's portable devices, how to deal with the challenges? The use of small-size single-chip microcomputer is an ideal measure. The C8051F small-size single-chip microcomputer is taken as an example to analyze its application in portable devices. To this end, we will introduce the application characteristics of the C8151F small-size single-chip microcomputer.

The application characteristics of 5.1C8051F small-size single-chip microcomputer are summarized as follows: small package, low power consumption, wide voltage working range (2.7V-3.6V), high speed, high integration and high confidentiality.

5.2 The application uses TFT screen backlight management and flash letter application as an example.

5.21C8051F30X Application in Color TFT Screen Backlight With the popularity of portable electronic devices, people are increasingly demanding their color display screens, and LEDs are being gradually applied to LCD backlights. The traditional LED backlight uses white LED as the backlight. It has the following disadvantages: Generally, the backlight needs multiple white LEDs, but the white LEDs have individual differences, and the consistency is not good, which may lead to uneven color and distortion of the color display. The white LED is prone to aging, which reduces the brightness of the color display. If the product has these problems, it is easy to leave a bad impression on the consumer and weaken the competitiveness of the product.

How should we solve these problems? The C8051F30x color TFT screen backlight solution solves these problems well. Its functional block diagram (shown in Figure 2) is as follows:

*Technical features The backlight uses red, green and blue LEDs, and the emitted light constitutes white light. Because the LEDs of each color are connected in series, AMS high performance LED Driver AS3691 is used; the brightness of R, G and B colors is detected by Avago's CoIor Sensor chip HDJD-S722-QR999; C8051F30x according to HOJD-S722 -QR999 sends the signal, generates the corresponding PWM to control the brightness of the R, G and B colors, so that the effect of white light is always in the best shape; at the same time, the C8051F30x communicates with the main CPU through an interface to receive the brightness adjustment of the main CPU. control commands.

*The size of the MCU is now very compact, and the thickness is very thin. The size of the MCU is very high. The C8051F30x is only 3×3mm in size and only 0.9mm thick. It requires PWM output, multi-input ADC and communication. Interface; C8051F30x supports 3 PWM outputs, multiplexed AD input, supports UART and communication interface, and can provide 8 user I/O ports on 3×3mm package.

* The program features a good solution to the traditional white LED individual differences, so that the color of the color display is always in a uniform state; well solved the problem of brightness reduction caused by white LED aging; C8051F30X, AS3691 and HDJD-S722 -QR999 package is small, easy to make a module with TFT display; C8051F30X has two low-power modes, Idee and Stop, especially Stop mode, which consumes less than 0.1uA. When the system enters the Stop mode, the peripheral circuit can be turned off to enter the power-saving state, and when the display is required, the main controller wakes up.

5.22 Application of C8051F313 in mobile phone flash and pedometer With the popularity of mobile phones, people's functional requirements for mobile phones are getting higher and higher. Those mobile phones with their individuality are more and more popular among people, especially the young ones. The increase of mobile phone functions means the increase of the size of the mobile phone and the increase of power consumption. This is related to the small size of the mobile phone and the long standby time. The requirements have become contradictions; to solve these contradictions, mobile phone designers have to pursue the miniaturization, low power consumption and high performance of components; the application of MCUC8051F313 in mobile phone flash and pedometer is a typical example. Figure 3 is a schematic block diagram of the application of the C8051F313 in the mobile phone flash and pedometer functions.

*About the pedometer pedometer principle: the acceleration and time axis of a person walking or running is roughly a sine wave; using the acceleration, it is possible to calculate how many steps have been taken, and to estimate the distance traveled according to the stride.

The implementation of the pedometer function: using MCUC8051F313 and MEMS IC acceleration sensor MAX6500; C8051F313 sampling MAX6500 two-way acceleration sensor input, analyzing the acceleration, calculating how many steps have been taken; according to the walker's stride, you can also estimate the distance traveled.

*About the mobile phone flash letter mobile phone flash letter principle: use the flashing light on the mobile phone, in the dark light environment, by shaking the mobile phone quickly and left and right, using the human retina delay phenomenon, resulting in visual residual image, thus forming a continuous light and shadow signal, so The trustee visually produces continuous information.

Implementation of mobile phone flash function: use MCUC8051F313 and MEMSIC accelerometer MAX6500; C8051F313 receives the information to be displayed and drives the corresponding LED; MAX6500 detects the component change (force size and direction) of acceleration in X and Y axis, and sends The C8051F313; C8051F313 analyzes the acceleration signal detected by the MAX6500 and automatically flips the direction of the character to be displayed.

* Mobile phone flash and pedometer requirements for MCU are demanding: mobile phones are extremely demanding on MCUs, providing up to 1/0 of the smallest possible size; C8051F313 is available on 5 x 5mm packages 25 I/O and analog inputs 16 I/O ports are used to drive 16 LEDs; 2 analog inputs are used to sample the accelerometer output; and a standard 2-wire controller is used to communicate with the Baseband CPU.

Strict low-power design: power consumption is less than 1μA in STOP mode; good I/O port design ensures high I/O external impedance during standby, and effectively cuts off I/O port standby power consumption above megaohm level.

Perfect mass production requirements: Once produced, the number of programming will be very large, with a complete mass production programming solution, fast handheld programmer, no PC participation, no operator familiar with programming methods, boot program, automatic when the phone is turned on Load the application and program it through the C2 programming interface. It also has good security features.