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Design of PWM Controller in a MCS-51 Compatible MCU
Abstract
This paper presents a design of Pulse-Width Modulated (PWM) controller module in a MCU based on MCS-51structure. The design can generate 2-channel Programmable periodic PWM signals. These output PWM signals from MCU can be used for a variety of applications including motor control. The function of the design allows users to select independent or complementary inversion timing relationships between 2 PWM wave forms. The latter mode selection also includes optional dead time function to support driving H-bridges and inverters. Therefore, users can control the output PWM signals through setting the duty-cycle registers. After the successful simulation at the front end, practical experiments made on a NIOS development board verify the design. 1. Introduction
PWM technology is a kind of voltage regulation method by controlling the switch frequency of DC power with fixed voltage to modify the two-end voltage of load. This technology can be used for a variety of applications including motor control, temperature control and pressure control and so on. In the motor control system shown as Fig. 1, through adjusting the duty cycle of power switch, the speed of motor can be controlled. As shown in Fig. 2, under the control of PWM signal, the average of voltage that controls the speed of motor changes with Duty-cycle ( D = t1/T in this Figure ), thus the motor speed can be increased when motor power turn on, decreased when power turn off.
Therefore, the motor speed can be controlled with regularly adjusting the time of turn-on and turn-off. There are three methods could achieve the adjustment of duty cycle: (1) Adjust frequency with fixed pulse-width. (2) Adjust both frequency and pulse-width. (3) Adjust pulse-width with fixed
frequency. Generally, there are four methods to generate the PWM signals as the following: (1) Generated by the device composed of separate logic components. This method is the original method which now has been discarded. (2) Generated by software. This method need CPU to continuously operate instructions to control I/O pins for generating PWM output signals, so that CPU can not do anything other. Therefore, the method also has been discarded gradually. (3) Generated by ASIC. The ASIC makes a decrease of CPU burden and steady work generally has several functions such as over-current protection, dead-time adjustment and so on. Then the method has been widely used in many kinds of occasion now. (4) Generated by PWM function module of MCU. Through embedding PWM function module in MCU and initializing the function, PWM pins of MCU can also automatically generate PWM out signals without CPU controlling only when need to change duty-cycle. It is the method that will be implemented in this paper. In this paper, we propose a PWM module embedded in a 8051 microcontroller. The PWM module can support PWM pulse signals by initializing the control register and duty-cycle register with three methods just mentioned above to adjust the duty cycle and several operation modes to add flexibility for user.
The following section explains the architecture of the PWM module and the architectures of basic functional blocks. Section3 describes two operation modes. Experimental and simulation results verifying proper system operation are also shown in that section. Depending on mode of operation, the PWM module creates one or more pulse-width modulated signals, whose duty ratios can be independently adjusted. 2. Implementation of PWM module in MCU 2.1 Overview of the PWM module
A block diagram of PWM module is shown in Fig.3. It is clearly from the diagram that the whole module is composed of two sections: PWM signal generator and dead-time generator with channel select logic. The PWM function can be started by the user through implementing some instructions for initializing the PWM module. In particular, the following
power and motion control applications are supported: ? DC Motor
? Uninterruptablel Power Supply (UPS)
The PWM module also has the following features: ? Two PWM signal outputs with complementary or independent operation
? Hardware dead-time generators for complementary mode ? Duty cycle updates are configurable to be immediated or synchronized to the PWM
2. 2 Details of the architecture 2.2.1 PMW generator
The architecture of the 2-output PWM generator shown in Fig.3 is based on a 16-bit resolution counter which creates a pulse-width modulated signal. The system is synthesized by a system clock signal whose frequency can be divided by 4 times or 12 times through setting the value of T3M for PWM0
or T4M for PWM1 in the special register PWMCON as shown in Fig.4. To PWM0 generator, the clock to 16-bit counter will be pre-divided by 4 times by default when T3M is set to zero. And the clock will be divided by 12 times when T3M is set to 1. This is also true for PWM1. The other bits in
PWMCON are explained in detail in Table 1.
2.2.2 Channel-select logic
The follow Fig. 5 shows the channel-select logic which is useful in Complementary Mode. From this diagram, it is clear to know that signal CP and CPWM control the source of PWMH and PWML. And the details about the two control signals will be discussed in the section 3, and the architecture of dead-time generator will also be discussed in section 3.1 for the continuity of Complementary Mode.
3. Operation Mode and Simulation Results
The design has two operation modes: Independent Mode and Complimentary Mode. By setting the corresponding bit CPWM in register PWMCON shown in Fig. 4, user can select one of the two operation modes. When CPWM is set to zero, PWM module will work in Independent Mode, whereas,
PWM module will work in Complimentary Mode. In the following of this section, the two operation mode will be explained respectively in detail and the simulation results of the PWM module from the Synoposys VCS EDA platform which verify the design will also be shown. 3.1 Independent PWM Output Mode
An Independent PWM Output mode is useful for driving loads such as the one shown in
Figure 1. A particular PWM output is in the Independent Output mode when the corresponding CP bit in the PWMCON register is set to zero. In this case, two-channel PWM outputs are independent of each other. The signal on pin PWM0/PWMH is from PWM0 generator, and the signal on pin PWM1/PWML is from PWM0 generator. The separate case is achieved by the channel-select logic shown in Fig. 6. The PWM I/O pins are set to independent mode by default upon advice reset. The dead-time generator is disabled in the Independent mode. The simulation result is shown in Figure 4 as the following Fig.5. Tr4 and tr3 are run bits to PWM0 and PWM1, respectively. Actually, from this diagram, Pin P1[5]/ P1[4] of MCU is used for PWMH/ PWML or normal I/O ,alternatively.
基于MCS-51单片机兼容的PWM控制器的设计
摘要
本文介绍了一种基于MCS-51单片机的脉宽调制在单片机中的应用的设计。该设计可产生两个通道定期的PWM信号，而这些来自单片机中的输出PWM信号可用于很多场合，包括在电机控制中的应用。该设计的目的就是允许用户可以在两道PWM波形之间选择独立的一个通道，或者两个。后者的模式选择还有通过H桥和逆变器死区的功能。因此，用户可根据寄存器的占空比来控制输出的PWM信号。在前面成功的模拟之后，在Nios开发板上的实地实验论证了设计。 1．简介
PWM是一种用过改变固定的直流电源开关频率来改变负载两端电压的技术。包括电机控制，温度控制和压力控制等等。在电机控制系统中如图1所示，可以通过调整电源开关的占空比，电机转速来控制。如图2所示。在PWM信号的控制下，平均电压和占空比控制速度的变化（D = t1图T中），因此，当电机功率变大，电机转速提高，关闭电源，电机降速。
图1： PWM控制框图
图2：电枢电压和占空比之间的关系
因此电机的转速可以由定时调整开关的状态来控制，有三种方式可以实现占空比的调整：（1）调整固定脉冲宽度的频率。 （2）同时调整频率和脉冲宽度。 （3）调整固