In designing embedded devices for the variety of medical devices, selecting the right components to meet design specifications, keep costs down, maximize power efficiency, and manage the physical size of the device are only some of the factors to be considered.
If these weren't sufficient, developers must also guarantee device reliability while ensuring that the components used adhere to FDA rules.
One such FDA rule is that the components that comprise the medical device have to be in production for the next five years. Given these constraints, many developers are turning to System-on-Chip architectures to shorten design cycle time, reduce component count, and reduce product cost in medical applications.
|Figure 1: Blood glucose monitor|
|Figure 2: Blood pressure monitor|
Present approach to design
Shown in Figure 1 and Figure 2 above are the block diagrams of two popular handheld medical devices presently on the market. The first is the block diagram of a blood glucose monitor while the second is a block diagram of a blood pressure monitor. Shown in Figure 3 below is a power management unit.
|Figure 3: Power Management Unit|
Both of these devices are reasonably similar to each other in their basic construction. The primary difference is the transducer with which each interfaces.
The components that typically comprise such devices include:
* Microcontroller: Performs all data processing and system level management tasks
* Transducer: Invasive/Non-invasive bio sensor in the case of a glucose monitor and a pressure transducer in the case of the blood pressure monitor
* ADC: Converts the analog signal from the transducer into a digital form
* Filter: Filters out noise in input signal
* LCD controller and LCD glass: Displays relevant information to the user
* USB interface: To sync with a PC to monitor and store data for later analysis
* Non-volatile Flash memory: For code and data store
* Power Management System: Includes a boost converter and linear regulator.
The peripheral components are external to the microcontroller and interface to the microcontroller through either GPIO or dedicated pins. Such a design approach introduces several limitations:
* With the use of so many external components, component count increases, as does required PCB space
* More external components increase the chance of noise being induced on PCB traces
* Individual external components may have to be FDA approved
* Increased development time, both in terms of bringing up hardware and software development effort
* Changes cannot be made or introduced easily.
SoC-based design approach
System-on-Chip (SOC) architectures provide an alternative design approach where the majority of peripheral components can be integrated or emulated on the chip that also houses the micro-controller.
ASICs are often more cost-effective if a device is being manufactured in large numbers, but SOCs make more sense when programmability and flexibility is desirable. The block diagram for such a design is shown in Figure 4 below.
|Figure 4: SOC based design|
As can be seen, there is an immediate reduction in the component count on the board, with most of the peripheral components now being integrated into the SOC used. Apart from this, the new design approach also has the following advantages:
* Reduction in component count could significantly reduce design cycle time
* Since hardware is being emulated inside the chip using software, changes are easier to make to the design as and when required
* Reduction in external peripherals reduces the noise induced into the board from different sources
* Power management is simplified since certain features of the SOC can be disabled when they are not required.
The above advantages apply for any embedded device design, but are of particular advantage to designers of medical devices. For example, medical device application code is complex and sometimes very difficult to write for an engineer.
Above all, creating an architecture which allows all components to work together seamlessly is the most difficult task, addressing issues such as what interface each component will use, whether there are enough I/Os, and how to multiplex the different interfaces to allow more than one component to work.
With an SOC, managing interfaces through SOC reconfigurability becomes less a manual task of creating device drivers, APIs, and code and more selecting the best configuration settings and writing minimal code with respect to having components work together.
This approach also enables developers to release different versions of a product meeting various price points by disabling or enabling features by changing the SOC configuration.
Now the product can be sold at a lower price, or vice versa, features can be added to reach a higher price point. This approach, using reconfigurability, cuts down development time, saves on cost, and above all allows a company to sell their product in different segments.
Such a design approach also reduces validation and test time for devices as well. In addition, adopting an SOC-based approach helps meet FDA regulations as it is easier to ensure that a particular SOC will remain in production for the next 5 years than it is to ensure the same for a long list of external components.
An example of a medical device using a reconfigurable architecture, in this case a Programmable System on Chip (PSoC) device with programmable digital and analog blocks, is shown in Figure 5 below.
|Figure 5: PSoC based design|
Programmable SOC architectures also give designer the flexibility to change functionality at run time, provide better power management and better noise immunity, depending upon the application and its particular operating environment.
One potential limitation to this approach is that for certain designs, depending upon application volume and component availability, is that a SOC may prove to be more expensive than using a low cost microcontroller with external peripheral components.
However, given the advantages of using a single device in the place of many " including ease of design and added reliability and the total cost of ownership savings these bring " an SOC-based design approach is a compelling alternative for many medical applications.
Amit Nanda has worked in the mass storage/semiconductor industry for 7 years. He graduated from Cal Poly Pomona in 2000 and has an MBA from National University in Business Management. His hobbies are tennis, cricket, and re-modeling. At Cypress Semiconductor Corp., he is responsible for USB Mass Storage Products and Wireless Products. He can be reached at Amit.Nanda@cypress.com.
Viren Ranjan is an Applications Engineer at Cypress, involved with the development and testing of Programmable System-on-Chip (PSoC)-based embedded designs. He received his Master's Degree in Robotics from The University of Southern California. He can be reached at firstname.lastname@example.org