Smoke detectors are typically housed in a disk-shaped plastic enclosure about 150 millimetres in diameter and 25 millimetres thick, but the shape can vary by manufacturer or product line. Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), while others use both detection methods to increase sensitivity to smoke. Sensitive alarms can be used to detect, and thus deter, smoking in areas where it is banned such as toilets and schools. Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. However, in many single family detached and smaller multiple family housings, a smoke alarm is often powered only by a single disposable battery.
IMPLEMENTATION/ DESIGN
Smoke Detectors are designed for 3 basic types of system implementations; Standalone, Wireless Connectivity, and Fixed/Wired Connectivity, the first two being battery powered implementations. Smoke Detectors have 3 main functional blocks; The Sensor and Signal Chain, Processor, and the Communication Interface. Where the processor and communications interface typically are not needed in your very basic standalone detectors.
Power consumption is a very critical aspect in wireless smoke detector because they need to be able to run for very long periods of time on a battery. This makes microcontrollers like the MSP430 ideal for the application; their high level of system integration also simplifies the design and reduces system cost. An infrared (IR) diode and IR receiver are used inside a smoke chamber to detect the presence of smoke. The IR diode is pulsed periodically, and the IR receiver signal is examined to determine if smoke is present in the chamber. An operational amplifier is used to magnify the IR receiver current as a trans-impedance amplifier, so it can be sampled by the ADC in the MSP430. Between sampling periods, the operational amplifier and IR circuitry are shut down, and the microcontroller is in a standby mode, consuming less then 1-mA current.
When selecting an external operational amplifier for the application, it is important to balance cost vs. settling-time performance, while minimizing current consumption. Settling time is important in allowing the detector to provide multiple reports of a smoke event in a short period of time, so as to minimize any false alarms. To further reduce current consumption of external components, some may be powered directly from an MSP430 port pin, even though it has a shutdown feature. This will take the current consumption of the amplifier to zero, when the MSP430 is in a standby state, significantly increasing the battery run time of this application.
Communication Interfaces
RF Transceiver : Range, network configuration and power consumption are important factors when selecting a Low Power Wireless (LPW) solution. Range is affected by output power, sensitivity and selectivity, which in turn impact the jamming of other signal sources and the ability to distinguish the desired signal from local interference. Selectivity is also important in RF design, especially when designing products in the 2.4GHz band, where interference from other equipment is likely. When making the RF radio selection, it is also important to understand the network configuration is which the smoke detector will be use: Point to Point, Star or Mesh Network, as it may impacted the radio, processor, memory and power requirements of the system.
Line Powered : Power over Ethernet (IEEE 802.3af) integrates data and power over standard LAN connection. It provides uninterrupted 15W max (13W load), 48V nominal supply to the devices connected to the system. The power requirement for smoke detectors is well below the ~12.5W limit for powered devices and can be easily powered from PoE. This sort of implementation removes the need to run AC power to sensor locations, and reduces the cost of the power supply in the detector by requiring only DC/DC power conversion.
2. CHOICE OF MICROPROCESSOR
MSP430 Microcontrollers (MCUs) from Texas Instruments (TI) are 16-bit, RISC-based, mixed-signal processors designed specifically for ultra-low-power (ULP). MSP430 MCUs have the right mix of intelligent peripherals, ease of use, low cost and lowest power consumption for thousands of applications - including yours. TI offers robust design support for the MSP430 MCU platform along with technical documents, training, tools and software to help designers develop products and release them to market faster.
FEATURES
Ultra-Low Power - The MSP430 MCU is designed specifically for ultra-low-power applications.Its flexible clocking system, multiple low-power modes, instant wakeup and intelligent autonomous peripherals enable true ultra-low-power optimization, dramatically extending battery life.
Flexible Clocking System - The MSP430 MCU clock system has the ability to enable and disable various clocks and oscillators which allow the device to enter various low-power modes (LPMs). The flexible clocking system optimizes overall current consumption by only enabling the required clocks when appropriate.
Instant Wakeup - The MSP430 MCU can wake-up instantly from LPMs. This ultra-fast wake-up is enabled by the MSP430 MCU's internal digitally controlled oscillator (DCO), which can source up to 25 MHz and be active and stable in 1μs. Instant wake-up functionality is important in ultra-low power applications since it allows the microcontroller to use the CPU in very efficient bursts and spend more time in LPMs.
Zero-Power Brown-Out Reset (BOR) - The MSP430 MCU's BOR is always enabled and active in all modes of operation. This ensures the most reliable performance possible while maintaining ultra-lowpower consumption. The BOR circuit detects low supply voltages and resets the device when power is applied or removed. This functionality is especially critical in battery-powered applications.
3. CHOICE OF MEMORY
Two types of memory can be usede for this application.
FLASH RAM - Flash memory is a non-volatile computer storage technology that can be electrically erased and reprogrammed. It is primarily used in memory cards, USB flash drives, and solid-state drives for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM (electrically erasable programmable read-only memory) that is erased and programmed in large blocks; in early flash the entire chip had to be erased at once. Flash memory costs far less than byte-programmable EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid state storage is needed. Example applications include PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones. It has also gained popularity in console video game hardware, where it is often used instead of EEPROMs or battery-powered static RAM (SRAM) for game save data.
Since flash memory is non-volatile, no power is needed to maintain the information stored in the chip. In addition, flash memory offers fast read access times (although not as fast as volatile DRAM memory used for main memory in PCs) and better kinetic shock resistance than hard disks. These characteristics explain the popularity of flash memory in portable devices. Another feature of flash memory is that when packaged in a "memory card," it is extremely durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water.
EEPROM - EEPROM (also written E2PROM and pronounced "e-e-prom," "double-e prom" or simply "e-squared") stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration tables or device configuration.
When larger amounts of static data are to be stored (such as in USB flash drives) a specific type of EEPROM such as flash memory is more economical than traditional EEPROM devices. EEPROMs are realized as arrays of floating-gate transistors.
EEPROM is user-modifiable read-only memory (ROM) that can be erased and reprogrammed (written to) repeatedly through the application of higher than normal electrical voltage generated externally or internally in the case of modern EEPROMs. EPROM usually must be removed from the device for erasing and programming, whereas EEPROMs can be programmed and erased in circuit. Originally, EEPROMs were limited to single byte operations which made them slower, but modern EEPROMs allow multi-byte page operations. It also has a limited life - that is, the number of times it could be reprogrammed was limited to tens or hundreds of thousands of times. That limitation has been extended to a million write operations in modern EEPROMs. In an EEPROM that is frequently reprogrammed while the computer is in use, the life of the EEPROM can be an important design consideration. It is for this reason that EEPROMs were used for configuration information, rather than random access memory.
Although technically FLASH is a type of EEPROM, the term "EEPROM" is generally used to refer specifically to non-flash EEPROM which is erasable in small blocks, typically bytes. Because erase cycles are slow, the large block sizes used in flash memory erasing give it a significant speed advantage over old-style EEPROM when writing large amounts of data.
4. CHOICE OF ADC/DAC
An analog-to-digital converter (ADC) is a device that converts a continuous quantity to a discrete digital number. The reverse operation is performed by a digital-to-analog converter (DAC). Typically, an ADC is an electronic device that converts an input analog voltage (or current) to a digital number proportional to the magnitude of the voltage or current.
The ADC used for this application could have a resolution of either 12 bit or 16 bit. 12 bit ADCs are sufficient for this application but usually 16 bit ADCs are preferred since higher the resolution lower is the quantization error. Also, to more accurately replicate the analog signal, the resolution of the ADC must be high. But the 16 bit ADCs are significantly costly than the 12 bit ADCs. So, depending upon the cost requirements of the end user the resolution of the ADC is suitably selected.
The ADS7886 is a 12-bit, 1-MSPS analog-to-digital converter (ADC). The device includes a capacitor based SAR A/D converter with inherent sample and hold. The serial interface in each device is controlled by the CS and SCLK signals for glueless connections with microprocessors and DSPs. The input signal is sampled with the falling edge of CS, and SCLK is used for conversion and serial data output.
The device operates from a wide supply range from 2.35 V to 5.25 V. The low power consumption of the device makes it suitable for battery-powered applications. The device also includes a powerdown feature for power saving at lower conversion speeds. The high level of the digital input to the device is not limited to device VDD. This means the digital input can go as
high as 5.25 V when device supply is 2.35 V. This feature is useful when digital signals are coming from other circuit with different supply levels. Also this relaxes restriction on power up sequencing. The ADS7886 is available in 6-pin SOT23 and SC70 packages and is specified for operation from -40°C to 125°C.
Since the output alarm system and the LEDs are driven by the microprocessor in this application, DACs are not required at the output end.
5. SAMPLING RATES OF THE CONTROLLED LOOPS
The smoke detector samples the IR circuitry for the presence of smoke every eight seconds. The MSP430 has an internal RC oscillator called the VLO that is used on conjunction with Timer_A to generate an eight-second interrupt. This interrupt brings the MSP430 out of LPM3. The VLO is calibrated by using the on-chip calibrated DCO oscillator to determine how many VLO clock cycles are required for a one-second interval. This number is used as the rollover period for Timer_A, and Timer_A is clocked from the VLO. The input clock divider for Timer_A is set to 8, which gives an eight-second wake-interrupt to the MSP430. When the MSP430 comes out of LPM3, it turns on the operational amplifier, allows for a settling time, and then samples the IR receiver current with the IR diode off. Then it turns on the IR diode, allows for a settling time, and measures the IR receiver current again. The two measurements are compared to determine if smoke is present.
To prevent false alarms, smoke must be detected three times before sounding the alarm. After the first detection, the clock divider of Timer_A is set to ¸4, giving a four-second interval between the first indication of smoke and the next sampling. If smoke is determined to be present in the second sampling, the Timer_A clock divider is set to ¸1, giving only a one-second interval between the second detection of smoke and the next sampling. If smoke is detected the third time, the alarm is sounded, and the detector continues sampling for smoke at one-second intervals.
6. REAL-TIME SCHEDULING ALGORITHM
Sporadic scheduling
Sporadic scheduling is an important algorithm for real-time systems as it allows the system to deal with aperiodic events such as events that occur only randomly (a sporadic event may for example be a key-press or a smoke-detector sensing smoke).
A thread assigned the sporadic scheduler will have a few properties of special interest: Its normal priority pf, its background priority pb, its initial budget c and its replenishment period t. When a sporadic thread begins execution it is executed at priority pf for c amount of time unless it is preempted at time d in which case a replenishment operation is scheduled to take place at time t+d when the thread will be replenished with d units of time.
When a thread as exhausted its time c it is dropped to its lower priority pb and allowed to execute at that level unless it is pre-empted. After t+d amount of time has been consumed, the scheduler will replenish the threads normal-priority time c with d units and it is then rescheduled. This process is repeated sporadically with new replenishments of size dk for every k times it is forced to yield execution. If a process is able to run without being preempted, these intervals will occur every t units. Notice that it is possible to limit the amount of times a thread can be replenished.
7. FAILURE CONDITIONS
