Accelerometers and temperature sensors fight SIDS

A simple microcontroller-based design may be a weapon in the fight against a dreaded problem.

Manish Shakya, Emmanuel Tuazon, Mohammed Bhatti, and Subra Ganesan, Oakland University -- EDN, February 17, 2011

SIDS (sudden-infant-death syndrome), or crib death, is the sudden and unexplained death of infants from causes that forensic and death-scene investigation cannot explain (Reference 1). It is one of the leading causes of death during infancy, with an estimated 2500 SIDS-related deaths annually in the United States and thousands more worldwide. Although these rates, in the United States, are at an all-time low and have fallen by about 50% since 1983, the number of infants dying from SIDS remains a cause for concern. Globally, especially in developing nations, where access to quality medical care and accurate information is far lower than in the United States, SIDS-related deaths remain high.

Although there is no agreement on a single cause for SIDS, factors linked to the phenomenon include babies’ sleeping on their stomach; overheating from excessive sleepwear and bedding; tobacco-smoke exposure following birth; maternal smoking, drinking, or drug use during pregnancy; poor prenatal care; prematurity or low birth weight; and maternal age of less than 20 years. Some of the suggested causes behind SIDS are related to choices that parents make—smoking, early pregnancies, and poor obstetrical care—and can be addressed through better education about the impact of lifestyle choices. Other suggested causes relate to the environment in which an infant sleeps; the parents can address these causes by monitoring the infant and intervening when necessary. Current research suggests that a variety of preventive measures, such as ensuring that infants sleep on their backs rather than their stomachs and removing from the crib blankets, pillows, or other objects that might cause the infant to suffocate, are the best means of reducing the potential of a SIDS-related death.

With advances in computing technology and the plummeting prices of components, other available products can supplement physician-recommended preventive practices. A microprocessor-based baby-monitoring system fulfills demand from parents looking for peace of mind. Using this system allows parents to better monitor their infants and act more quickly to pre-empt some of the suggested causes of SIDS. The system can monitor both babies sleeping on their stomachs and those who are overheating.

Monitor Design


The baby-monitor unit comprises a temperature sensor, an accelerometer, and a wireless transmitter/receiver in a microcontroller. An LCD allows users to monitor both the data the monitor receives from the sensors and the status of the system. Status includes whether communication with the control unit exists and whether an alarm is present. Using this system, the parent attaches the unit to the infant by bringing the sensors into contact with the baby.

The position sensor connects to the analog input on the microcontroller, and the temperature sensor is the built-in sensor available on the microcontroller. A serial interface transfers this data to the wireless HRTF (head-related- transfer-function) module. The HRTF module then transmits or receives data using FSK (frequency-shift-keying) technology to an identical HRTF module that attaches to the control unit.


Software displays various parameters on a menu on the LCD that attaches to the microcontroller. The parent uses the keypad to browse through the menu, access various options, and enter input. The alarm activates when the values of certain monitored parameters exit a predetermined safe zone. The accelerometer resets the LCD to the default view when the user shakes the device and so offers an easy way to exit the various menu options a user might be adjusting or viewing.

Implementation

The system uses two Hope Microelectronic HCS12 Mini-Dragon-plus2 development boards employing the Compact MC9S12DG256 board with a solderless breadboard, two RS-232 ports, one CAN (controller-area-network) port, two H bridges, and four servo connectors and headers (Reference 2). The board offers both an LCD interface and a keypad interface, which allow for easy integration of those peripherals. The MC9S12DG256 offers a 16-bit CPU; 256 kbytes of flash memory; 12 kbytes of RAM; 4 kbytes of EEPROM; and SCI (serial-communications-interface), SPI (serial-peripheral-interface), and CAN 2.0 ports.


The HRTF module has a working voltage of 5V. If the Config pin is high at power-on, the module enters the configure mode to allow a user to set up work parameters. This system uses the default parameters. If the Config pin is low at power-on, then the module enters normal mode for data transmission. The Enable pin serves primarily as a means of regulating power consumption. When you set the Enable pin, the wireless module immediately enters sleep mode. This circuit does not use the Enable pin.




LCD and keyboard


The system uses one hexadecimal keypad for input and menu selection (Reference 4). The keypad is on the control unit and connects to Port A on the microcontroller. Ports 0 to 3 are input ports, and Ports 4 to 7 are output ports (references 5 and 6). The design uses internal pullup resistors from the microcontroller rather than external resistors.

The basic principle behind the operation of the keypad is as follows: A 16-character array stores the keypad codes. Firmware reads this array through a loop and assigns various codes to Port A. The microcontroller reads the codes after a few milliseconds and compares them with those in the previous values. If the comparison indicates identical values, then a key is pressed.

If the comparison is not equal, the key is not pressed. The keypad performs several functions in the system. It acts as the primary input peripheral for the user to set and reset the password, set the alarm to snooze mode, reset the alarm if it goes off, and select various tones for the alarm. It also resets the data-transmission counter on the LCD and can find use in debugging. Key A on the keyboard enables the keyboard to snooze for 10 seconds; Key B, 30 seconds; and Key C, 60 seconds. Key D stops the alarm.

Software implementation


In memory-constrained systems, such as deeply embedded systems, traditional multithreading may have too large of a memory overhead. In traditional multithreading, each thread requires its own stack, and each is typically overprovisioned. These stacks may use large parts of the available memory. In contrast, the main advantage of protothreads over ordinary threads is that protothreads are lightweight: A protothread does not require its own stack. Rather, all protothreads run on the same stack, and the system performs a context switch by stack rewinding.

This feature is advantageous in memory-constrained systems, in which a stack per thread might use a large part of the available memory. A protothread requires only 2 bytes of memory per protothread. Moreover, protothreads are implemented in pure C and require no machine-specific assembler code. For a description of the format for transmitting accelerometer data from the monitor unit, see sidebar “Transmission of data.”

Interrupts can be external or internal. External interrupts occur when the external circuitry sends an interrupt signal to the CPU. Internal interrupts come from the hardware circuitry inside the chip or from software errors. The system uses various interrupts to coordinate I/O activities as well as for periodic data acquisition. Both the monitoring unit and the control unit use three interrupts each: interrupts 7, 13, and 20. Interrupt 7 is a real-time interrupt to deal with the timing issues of the system.

Upon every real-time interrupt, the system increments a Tick variable, from which all system-timing information is derived. Interrupt 13 uses enhanced capture timer Channel 5 for tone generation, generating various frequencies by appropriate reloading values. Interrupt 20 is the SCI at Port 0 for wireless communication.


Adding a wireless sensor network allows you to monitor any number of babies. From a marketability standpoint, the prototype is too bulky (Figure 7). Size issues arise primarily from the size of the microcontroller-prototype boards (references 9, 10, and 11).

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Acknowledgment

The authors would like to thank Professor Richard Haskell of Oakland University for his support during this project.

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References

American SIDS Institute “HCS12: MiniDRAGON-Plus2 Development Board,” EVBplus. Hope Microelectronics www.kionix.com “LTC2990 Temperature, Voltage and Current Monitor, Linear Technology.” EVB Shopping Accessories, EVBplus. Dunkels, Adam, “Protothreads: Lightweight, Stackless Threads in C,” 2008. “Protothreads,” Wikipedia. Oakland University Shakya, Manish; Emmanuel Tuazon; and Mohammed Bhatti, “Anti-SIDS Demo,” YouTube. Haskell, Richard, and Darrin Hanna, Learning by Example Using C.

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