nvSRAMs Help Bring the Hospital Home
Non-volatile memory protects patient-critical data in portable medical equipment. Choosing the right kind of non-volatile memory is critical.
CHRIS GILBERT, SIMTEK
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Over the past decade, there has been a growing trend in the health care field to send hospital patients home as quickly as possible. Because of this, there is often a need for these patients to “bring the hospital home” with them. This has precipitated a rapid rise in the need for portable and highly reliable medical treatment, monitoring and data logging equipment.
Because this medical equipment is being installed and operated in patients’ homes, key requirements include compact size, very low power consumption and extreme dependability. Critical to meeting these criteria is highly reliable, non-volatile semiconductor memory to ensure that neither physician-prescribed equipment settings, nor stored patient information, such as vital signs, are accidentally lost.
From Hospital to Home
One piece of medical equipment that frequently moves with the patient from the hospital to the home is the portable infusion pump. Infusion pumps are used to introduce fluids into the patient’s circulatory system for hydration, pain management, chemotherapy and total parenteral nutrition (TPN), a treatment where all of the body’s nutrients are provided intravenously, bypassing the normal process of eating and digestion.
In addition to the mechanical components such as the pump and its DC motor, a portable fusion pump is a complex embedded system with a CPU at its core (Figure 1). The user, who is often the patient, controls operation of the system via a keypad mounted on the front of the device. Alternately, the infusion pump can be remotely programmed and operated by health-care providers using a telephone line and built-in modem. The system has a digital LCD display that shows the unit’s status, including programmed medication quantities and infusion times. In addition to the requisite beeps and buzzes, some modern systems actually include pre-recorded spoken messages that remind the patient of important tasks to perform. Operation of the mechanical components, including the pump and valves, are performed by a digital IC controlled by the CPU. Likewise, another IC interfaces to sensors and provides conditioning of signals throughout the system.
Last but not least is the system’s memory and real-time clock. In addition to routine scratchpad RAM, a portion of the memory space is dedicated to non-volatile memory. This non-volatile memory is critical to the operation of the infusion pump since it retains the calibration parameters, dynamic algorithm coefficients, stepper motor control data tables, intravenous therapy schedule, system state information and the data log, which is a detailed history of the patient’s physical parameters, including blood pressure, temperature and respiration rate. The real-time clock provides an accurate time and calendar source to administer the intravenous therapy schedule and time-stamp the data log.
As is common in many systems, but absolutely crucial to infusion pumps, the system must recover from any unexpected loss of power. If power is lost during the infusion of fluid, a battery pack takes over and ensures that normal operation continues. However, in the event that the battery fails, even for a very brief time, the unit not only must be able to resume the infusion once power is restored, but it must also have access to all programming data, including dates and times of medication delivery, as well as to the data log. Needless to say, loss of programming data can result in potentially fatal consequences, since the details of the patient’s medication delivery would be unavailable, resulting in possible under- or overdoses of medications being administered upon system restart. Similarly, losing the data log can result in important patient data being lost—data that may be critical to the physician learning of medical conditions previously not exhibited by the patient. In short, the contents of the infusion pump’s memory are often a matter of life-and-death.
In the event of a power-related disturbance of an infusion pump, the unit must detect the loss of power, then store vital system state information and the data log to non-volatile memory prior to the complete loss of power. Once power is restored to the system, this state information is used as a recovery point, and the system resumes operation at the exact point it was prior to the power loss.
The portable infusion pump therefore places a number of requirements on the non-volatile memory. First, because the unit is portable, low power consumption and a small form factor are important. Second, memory endurance is an essential aspect as well, since portions of the memory are tasked with repeated reads and writes, and the underlying memory technology must be immune from any wearing out of memory cells.
Third, the memory solution must be robust to ensure that vital system configuration and programming data, as well as the patient’s data log, is not lost. Fourth, the memory must be sufficiently fast to handle the real-time demands of medical applications. Other considerations that are not hard requirements but simplify the design are the physical interface of the device, the power conditioning requirements of the memory device, and whether the memory is byte or block writeable.
Which Non-Volatile Memory Technology?
A number of non-volatile memory technologies exist in the market today, and include flash, battery-backed SRAM (BBSRAM), electrically erased programmable read-only memory (EEPROM) and nvSRAM. Table 1 provides a summary of the key features of these memory types.
EEPROM is frequently used to store small amounts of non-volatile configuration data. However, compared with other non-volatile technologies, EEPROM is much lower in density and it has much slower read and write speeds, making it unacceptable as a single memory technology for many medical applications.
EEPROM can be combined with inexpensive flash memory where configuration data is stored in the EEPROM, and the flash is used as primary data storage. However, this hybrid solution more than doubles the physical board space. In addition, this combination has unacceptably slow write times, and the flash memory has poor endurance characteristics, resulting in limited read/write cycles that can wear out the memory devices, ultimately causing loss of data.
While traditional SRAM is a volatile memory technology, there are variants that include an on-chip battery to maintain operation of the device in the event of an unexpected power loss. Known as BBSRAM, this type of memory requires a large physical size because the battery, power management circuitry and the SRAM are combined into a single package. BBSRAMs are generally optimized to conserve battery power since the small battery has limited power, so low-power SRAMs must be used. Unfortunately, an attribute of low-power SRAMs is slower access time, making them unsuitable for many medical applications like infusion pumps. Another shortcoming is the additional point of failure introduced by the battery due to its limited life span. Once the battery loses its ability to maintain a charge, the unit represents a significant risk because it is incapable of providing robust and secure backup of patient-critical data.
Because of the shortcomings encountered with these non-volatile memory types, an increasing number of manufacturers are using nvSRAMs. The nvSRAM is a memory technology that combines a high-speed SRAM with an equal amount of non-volatile EEPROM on the same chip. During normal system operation, the nvS¬RAM behaves exactly as standard fast SRAM and can be easily inter¬faced to existing microprocessors and microcontrollers. The nvSRAM constantly monitors its supplied power, and if it detects an event that may cause a loss of power, it automatically stores an exact copy of the SRAM’s contents to the on-chip EEPROM in a single, and extremely fast, parallel transaction. Even in the event that power is instantaneously removed from the nvSRAM, a small external capacitor provides sufficient power to the device to ensure that all of the SRAM’s contents are successfully copied to the EEPROM.
When power is restored to the system, the contents of the EEPROM are automatically transferred back to the SRAM, returning the system back to the same state as before the disruption of power. Since the vast majority of write operations take place with the on-chip SRAM, there is no possibility of memory degredation. With 200,000 cycles of endur¬ance, the NV cell can withstand more than 50 power interruptions per day over the typical 10-year lifetime of a semicon¬ductor device—a situation that would never exist with this kind of system in the real world.
nvSRAM devices combine the very best in high speed, low power and the ultimate in robust and safe data storage. Used for years in the military and avionics markets, the nvSRAM is now firmly entrenched in the medical marketplace and brings the same solid reliability to remote patient treatment and monitoring, ensuring that equipment settings and vital patient information can be stored and retrieved in the event of any power interruption.
Designing with nvSRAM
Designing with nvSRAMs is straightforward since the nvSRAM mimics the operation of an SRAM. An industry-standard address bus and a 16-bit or 8-bit data bus are accompanied by three control lines: write enable (/W), chip enable (/E) and output enable (/G). Consequently, the nvSRAM can be interfaced to virtually all standard microprocessors, FPGAs, or DSPs with little or no interface logic or memory wait states. Figure 2 depicts a typical interface connection between the nvSRAM and a microprocessor, FPGA, or DSP. Depending on the particular device, the interface may directly connect to the nvSRAM. In other instances, simple decode logic can be used to drive the nvSRAM control signals. The nvSRAM is well suited to supply the program or data memory space or act as the configuration device for an FPGA, making system design even easier.
Further integration and board space savings are possible by utilizing Simtek’s nvSRAM with on-chip real-time clock (RTC). A high-accuracy, full-featured RTC is combined with the nvSRAM in an ultra-reliable monolithic integrated circuit. The real-time clock function provides an accurate clock with leap year tracking and a programmable, high-accuracy oscillator. A programmable alarm function is available for one-time alarms or periodic minutes, hours, or days alarms. There is also a programma¬ble watchdog timer for processor control. Writing to the control registers or reading the time is accomplished by simply reading the RTC registers that are mapped to the highest 16 memory addresses of the SRAM. Reading the time is as simple as reading any other SRAM address.
Infusion pumps are just one of many medical applications that can benefit from the nvSRAM. In any medical equipment application where the safety of the patient is at risk, the system’s non-volatile memory is of paramount importance. The nvSRAM offers an attractive non-volatile memory alternative that provides fast access times, non-volatility, small form factor, low power, high MTBF and zero maintenance. Ease of design, and an optional integrated RTC, can provide additional savings in cost, physical space and increased system reliability.
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