The role of MCUs and PSoCs in making home appliances smarter - Embedded.com

The role of MCUs and PSoCs in making home appliances smarter

Editor's note: In addition to discussing home appliance design with microcontrollers, this article describes a more integrated approach using programmable system-on-chip (PSoC) devices that incorporate software-programmable MCUs with hardware-programmable logic arrays.

In many home appliances such as washing machines, air conditioners, microwave ovens, vacuum cleaners, and refrigerators, microcontrollers (MCUs) are used for motor control, analog sensor measurements, front panel keypad control, and LED/LCD displays.

The home appliance industry uses 8-, 16-, and 32-bit microcontroller-based circuitry for motor control and TRIAC/ LED/ LCD drive applications. The MCU controls and manages all the functions and feature of the appliance. When the user presses the start button, the input goes to the microcontroller from the front panel keyboard and the MPU starts the three-phase brushless DC (BLDC) motor/permanent magnet synchronous motor (PMSM). Motor speed will be varied and controlled as per user inputs from the front panel keypad.

The MCU uses either an internal or external serial EEPROM (I2C/SPI based) to store old data. It uses a real-time clock (RTC) for displaying accurate time information. Temperature measurements are done using an onboard resistance temperature detector (RTD)-, thermistor-, or thermocouple-based temperature sensing device.

The MCU uses an external ADC and amplifiers for receiving analog inputs from sensors, temperature, and battery. It uses external signal conditioning, comparators, and gate driver circuitry for driving and controlling a 3-phase BLDC/ PMSM motor. The microcontroller can also receive remote control inputs through an IR receiver (at 38 kHz input).

External buffer driver circuitry is required to drive 7-segment LED/LCD/graphical displays. Typically a 7-segment LED/LCD/graphical display with backlight is used for showing temperature, battery input, speed value, and error/warning messages. The microcontroller also interfaces with onboard peripherals such as I2C/ SPI and external peripherals like UART/USB communications.

MCUs in washing machines
MCUs in washing machines include the following blocks:

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Figure 1: Washing machine block diagram using an MCU

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Figure 2: Washing machine block diagram using an integrated SoC

In a washing machine, the MCU receives analog inputs (water level sensor, water hardness sensor, humidity sensor, door open sensor, laundry load sensor, optical sensor, detergent density sensor, load Imbalance sensor, and volume sensor) through an external ADC. Temperature sensing is done by an onboard RTD and an external EEPROM is used to store data, such as customized wash program, memory backup, child lock, and favorites. The microcontroller allows the washing machine to adjust the water and cut power automatically.

The MCU also controls self-diagnostics, including water supply failure, spin failure, drainage failure, child lock, overflow protection, and door lid open. Clocks and timers are used to implement sleep mode and add delays (delay start condition) in operations. A buzzer (PWM-based) generates tones at different frequencies and also provides alert tones during overload conditions. The MCU also automatically turns off the machine after washing is completed, thus saving power.

MCUs in air conditioners
MCUs in air conditioners include the following blocks:

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Figure 3: Air conditioner block diagram using an MCU

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Figure 4: Air conditioner block diagram using an integrated SoC

In air conditioners, the MCU receives analog inputs (sensor input) through an external ADC. Temperature sensing is done by an onboard RTD and thermistor, and an external EEPROM is used to store data (set temperature value). The MCU controls external BLDC motor and fan using PWM and comparators. It receives various filter inputs, which are used for purification.

The MCU controls the motor and compressor based on the temperature set by the user. The MCU also uses a relay driver and TRIAC driver circuitry to switch off power line AC inputs to the system. It uses clock and timers to set up sleep mode, auto switch-off function, and 24-hr on/off timer function. It uses PWM-based buzzer for generating tones at various frequencies. The MCU also controls self-diagnostic features, and includes auto restart and over-current protection. During power failure, it automatically restores the air conditioners to previous settings.

MCUs in microwave ovens
MCUs in microwave ovens include the following blocks:

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Figure 5: Microwave ovens Block Diagram using an MCU

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Figure 6: Microwave ovens Block diagram using an SoC

In microwave ovens, an MCU receives analog inputs from weight sensor, humidity sensor, volume sensor, CT current sensor through an external ADC. Temperature sensing is done by an onboard RTD and thermistor, and an external EEPROM is used to store data, such as child lock and reprogramable cooking data.

The MCU controls self-diagnostic functions and includes auto restart, auto deodorizer, auto protection, and overflow protection. During power failure, it automatically restores the microwave oven to the previous settings

The MCU uses a clock and timers to implement sleep mode and add delays in operation. It uses a PWM-based buzzer for generating tones at different frequencies. It also automatically turns off the microwave ovens when operation is completed or goes into sleep mode when not in use/not set (by user), thus saving overall power.

MCUs in vacuum cleaners
MCUs in vacuum cleaners include the following blocks:

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Figure 7: Vacuum Cleaner Block Diagram using an MCU

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Figure 8: Vacuum Cleaner Block diagram using an integrated SoC

Ina vacuum cleaner, the MCU is used for driving and controlling athree-phase motor (BLDC) using PWM and comparators. It receives analoginputs from accelerometer, CT current sensor and suction strength sensorthrough an external ADC. Temperature sensing is done by an onboard RTDand thermistor, and an external EEPROM is used to store data. The MCUreceives filter inputs used for air filter monitoring.

MCUs in refrigerators
MCUs in refrigerators include the following blocks:

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Figure 9: Refrigerators Block Diagram using an MCU

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Figure 10: Refrigerators Block diagram using an integrated SoC

Inrefrigerators, the MCU drives and controls a three-phase motor (BLDC/PMSM) using a PWM and comparators for controlling fans as per userinput. It receives analog inputs from water level sensor, humiditysensor, cooling sensor, cool select zone sensor, door open sensor, andCT current sensor through external ADC. It receives other inputs such asfrost/moisture detect and water/ice dispenser. Temperature sensing isdone by internal and external temperature sensors (RTD and thermistorbased), and an external EEPROM is used to store old data, such ascustomized program, memory backup, child lock, and favorites. The MCUallows refrigerators to adjust to power cuts automatically.

TheMCU controls the motor and compressor based on the desired temperaturevalue set by the user. It also controls and regulates the heat flow byaltering the speed of the compressor efficiently.  Self-diagnosticsinclude water supply failure, ice cooling failure, auto restart withmemory backup, child lock, anti-virus protector, auto balance system,overflow protection, and door open.

The MCU uses a clock andtimers to set up a timer and calendar, implement sleep mode, and adddelays in operation. It uses a PWM-based buzzer to generate tones atdifferent frequencies and to send an alarm when the door is open forlonger than a specified time.

The MCU automatically switches offsome functions when the door is closed, thus saving overall power. Itdelays the rise in temperature by supplying cold air even when the poweris off (in sleep mode). User can set refrigerator in holiday mode sothe MCU (in sleep mode) runs at minimum energy mode. MCUs providedifferent internal and external interfaces to the refrigeratorsincluding FM radio, MP3 player, equalizer, mobile charger through USB,UART, etc.

Using MCU-based hardware programmable SoCs in home appliances
Homeappliances are currently implemented using MCU-based architectures.Today’s programmable system-on-chip (PSoC) architectures provide acombination of MCU and ASIC capabilities that increase ease-of-use inhome appliances as well as reduce both product development andmanufacturing costs.

Complementing and extending the softwareprogrammability of traditional MCUs, programmable SoC devices combine anMCU with hardware-programmable logic and high-performanceanalog-to-digital conversions and commonly used fixed-functionperipherals. This enables developers to integrate more functionality andcomponents into the MCU, resulting in lower component count, smallerPCBs, lower system cost, and better power efficiency. SoC devices alsohave integrated Flash, SRAM, and EEPROM.

Various digital andanalog components are available to support a variety of appliancefunctions. When an internal RTC is available, no externalclock/oscillator circuitry is required. Similarly, integratedinterfaces, such as USB and Secure Digital (SD) card interfaces, allowthe SoC to communicate without an external controller. Integrated DAC,ADC, PWM, and comparator resources further consolidate systems so that asingle SoC can provide the functionality required for an appliance.

Sensor vs sensorless motor control
Asensorless method of motor control does not requires hall sensors;instead it uses back Electromotive Force (Back EMF) zero crossingdetection technique to control the motor movement. When the motorrotates, each winding generates a voltage (Back EMF), which opposes themain voltage supplied to the windings. Back EMF polarity is in theopposite direction of the voltage used for winding excitation and isdirectly proportional to the motor speed.

Figure 11: Sensorless motor control using an integrated SoC

In Figure 11 ,back EMF signals from three phases terminate and the DC bus is scaledand routed to the programmable SoC. The SoC will switch the terminateinput to the comparator using the MUX, and then compare it with the DCbus voltage. Cascaded digital logic will filter out the PWM signal toget the real zero-crossing signal. The MCU will decide the commutationaccording to this information.

An optional current control willbe applied to the PWM output control to regulate the motor current. Thisinner loop is based on a comparator; the feedback bus current will becompared with the reference current value that is provided by a 12-bitDAC. Changing the DAC output will modify the output current value.

Bycomparison, the sensor-based brushless motor control uses Hall sensorinput to detect rotor position and thus control the motor movement. Itprovides Hall sensor inputs to the MCU and works as a closed loopsystem.

Touch-based appliance design
PSoCs are alsoadvantageous in applications where capacitive sensing technologyreplaces mechanical buttons with a touch-based keypad. This reducesfailures due to mechanical buttons and provides better productreliability.

Many SoCs offer production-ready libraries anddesign tools that auto-tune the sensitivity of buttons and slider, thuseliminating manual tuning from the design cycle. Capacitive-basedinterfaces will also need to be waterproof in home appliance solutions.They support proximity function for the front keypad; the front keypadwill be activated when a user places the hand near the keypad.
Implementationof touchscreen-based design on the front panel instead of an LCDdisplay and keypad will provide better user interface and flexibility.

Inexternal devices like iPod/iPhones, the SoC can communicate to theiPod/iPhone through the UART and USB protocol. Users can controliPod/iPhone devices and charge the devices in the home appliance.

Becauseof their software- and hardware-programmable nature, SoC devices mustsupport code security for home appliances. They may also include apersonal area network technology like Bluetooth so that users can createa network of automated home appliances.

Voice guiding technologyin home appliance like microwave ovens helps users at each step of thecooking process. To go through an auto-tuned menu, the user must pressthe button once and a voice will guide through the steps. This can beimplemented by an SoC as well.

Home appliance design challenges
Among the design challenges facing the system designer of home appliances using either approach are the following:

  • Protection against voltage fluctuations
  • Water proofing and water tolerance features
  • Implementing a universal power supply

Inaddition, power MOSFET selection with Low Ron and low gate capacitanceis required for driving three-phase automotive motors. Also, boardsusing high-power MOSFET driver circuitry must be designed to handle highon-board current from the power supply.

In appliances involvingelectro-mechanical construction it is necessary to design a compact andcost-effective electro-mechanical solution that can meet the EMI/EMCstandards certifications for safety.

Fault detection andrecovery mechanisms are required in home appliance applications. Powersupply design with battery protection, over-current, overheating, andstart-up fail condition are also required. Implementing self-diagnosisis another design challenge. Home appliances and associated devices areexpected to work continuously 24/7. Selection of reliable components isalso a design challenge for system developer.

Failure analysis and returned materials
Increasingthe number of internal and external interfaces on the board willincrease the number of ways that an intruder can create havoc on thesystem. This is one of the single largest limitations of home applianceembedded systems that can be solved by a single chip solution.

Ronak Desai  isa Staff Engineer at Cypress Semiconductor with nine years of industryexperience. He has a BE in Electronics and Communication from MumbaiUniversity, India. He is part of the Development Kits Group and is basedout of Bangalore, India. You can reach him at .

1 thought on “The role of MCUs and PSoCs in making home appliances smarter

  1. “Hi Ronak,nGood article but IMO, this article is great on comparison between PSOC and MCU but still not clear as why it is best suited for Home appliances:nnSome points in my view…n1. Coming to fast ADC-DAC, these are not special feature of PSOC.n2.

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