Boost portable medical design of embedded processor

The particularity of portable medical equipment they should be user friendly, must work in a sterile environment, and small footprint, low energy consumption.

At the same time, portable medical devices also need enough computing power for processing medical data, to be able to connect to a wireless or wired interface to record and send data.

From a designer point of view, the demand for low-power microcontroller (MCU) and digital signal controller (DSC Controller, Digital Signal).

It is precisely because of the embedded processors, designers can design can be used in a variety of medical applications of compact and energy saving devices, including Implantable devices, portable devices, household equipment and security equipment, etc.

Application examples from their own over the counter buy sphygmomanometer, until you can take the patient's gut internal micro wireless camera. This article will describe how designers in portable medical applications using the latest MCU and DSC.

Portable medical electronic equipment design issues

In the design of the compact portable medical electronic devices, system designers have to face many challenges, such as the signal used to regulate medical resources on medical devices are critical to the final cost.

In high-end applications, electrocardiogram (ECG) Analyzer is a complex and expensive instruments, to stimulate the heart of two rooms and two rooms measuring electrical signals of the heart. First of all, near the heart of the skin probe to detect electrical signals and signal is processed, and graphics. Because the probe is placed in the patient's skin, prone to attenuate the signal. Therefore will place multiple probes in the heart of the skin near the overcome attenuation, in order to get a better signal. Then, by using digital filtering based on firmware technology, that is, embedded FFT (Fast Fourier Transform, FFT) library of spectral analysis software that can signal attenuation estimation. ECG on probe number depends on the control of the ECG's DSC on the available number of ADC channels. For example, the latest of Microchip dsPIC DSC with 6 ~ 32 ADC channels and integrated handles medical signal other useful peripherals, using these DSC allows ECG cost substantive reduction.

Chip designers have devices integrate several power-saving features (Figure 1), medical equipment designers to control power consumption.

To adjust power consumption, such as idle, hibernation and standby mode and other energy-saving features for designers provide considerable flexibility. When used in medical applications, these devices to the system controller in most of the time in a low-power state, just periodically been wake-up timer interrupt in order to run the program code.

Lower power consumption method

Designers should be aware that in a portable system the only way to save energy is to allow single-chip control power consumption of the internal and external peripherals.

The system is running, you should try to follow the specific energy consumption design respectively. In the design of a portable medical equipment, determine the desired operational state, turn off the circuit is not required. If the medical device on a peripheral device consumes most of the power consumption, reduced MCU power does not have a lot of total system power consumption.

When you do not need peripherals, you should close it.

For example, in the battery-powered applications, you do not need undervoltage reset function (Reset, BOR Brown-out). On the other hand, you can also level to Tarnawa Microchip PIC18F series monolithic integrated circuits that use free instruction (Figure 2) close the CPU and peripherals to continue running.

In energy conservation and even further measures.

By entering the sleep state, power consumption can be reduced by nearly 96%. In a sensor, a memory and a battery of portable medical systems, storage and supply of sensor may have been, this is the energy consumption in consuming system. In the microcomputer control, use the microcontroller I/O connection close these peripherals; only when needed, the memory and the sensor power supply.

Introducing a controllable clock

For low power applications, the oscillator startup time on-chip power consumption plays a vital role.

During startup, has been stabilized to the oscillator, MCU even idle will continue to consume energy. Oscillator startup time is affected by many factors, including the crystal oscillator, load capacitance, the system environment and oscillator mode and so on. In the lower clock rates, although the low-frequency oscillator run time consuming less power, but it requires a startup time will be longer, so you will have a significant impact on the system power consumption.

Choose Start with oscillator SCM timer, which will help to ensure a normal startup, ensure that there is enough time to build.

Oscillator timer to execute code SCM precisely to ensure steady vibration of oscillator. But there is also a shortage of oscillator timer because each wake cycle time is extended accordingly. The problem for oscillator startup time of the solution is to use the two-speed oscillator startup, so you can start to switch to a faster internal oscillator frequency. Use advanced microcomputer in two internal clock frequencies, i.e. normal run-time selection software configurable 8 MHz oscillator, while maintaining low power 31 kHz oscillator is selected. Use these single-chip can at any time switch clock frequency (external clock and to convert between internal oscillator), while no code execution latency, thus saving valuable medical devices "on time" (Up).

The I/O thread is configured as analog or digital cable

Portable device of each signal line will consume energy, this requires creative handling single-chip I/O pins on a bi-directional, because some of the I/O pins can process analog inputs.

Through careful observation of the signal on the PIN, the designer ensures minimal power consumption. Due to the analog input provides high-impedance state, their current consumption is very small. In particular, when applied voltage at VDD and VSS intermediateThe analog input than digital input uses less electricity. As long as possible, through forced digital input into a low-power state, the reuse of digital/analog pin configuration to analog input in order to save power consumption. In addition to trying to drive through digital output, in addition to the external circuit to the external circuits power supply current, digital output pins do not incur additional overhead for current consumption.

The unused port pins configured to output pins (output for high or low average), or configure them to input pin and then connect an external pull-up resistor to VDD or VSS, you can save power consumption.

When configured as input, through the PIN out of mere pin input leakage current, and if the PIN directly connected to VDD or VSS, you will have the same electric current to flow. Use this method, the flexibility to put the PIN as input or output, and not for too much hardware changes.

Energy saving through power budget

In portable applications, designers can use power budget this technique to calculate the current consumption and the estimated battery life.

It is first necessary to understand the operation of the system, it is envisaged that all of the features of the device. Therefore, if the portable medical applications simply as data access, system operation mode is clearly: Hibernate, acquire data, processing and storage of data.

Now, through a careful study of the control program can be estimated for each mode of time consumption.

Refer to the manufacturer's data sheet, with access to the system the current consumption of the devices. Then, put each State total current multiplied by the per-loop cycle duration of the State, to calculate the charge for each State of consumption. According to the power budget, you can easily calculate the meet the application requirements of the current size, use the formula is: average current = total charges/total time. If designers notes in a State consume too much power, you can work around the lower the power consumption of the State.

The design of a portable medical security

In medical applications, security is a high priority, system designers must prevent device because the blackout or program out there for emergencies.

For example, the loss of the clock source may trigger erroneously execute product control program. Single-chip suppliers offers several effective mechanisms to ensure that the system safety operation of MCU and predictable operation. In some MCU, designers can use fault protection clock monitor function to detect the loss of the clock source. When it detects that the clock is lost, MCU internal oscillator clock signal will be available to help the system to shut down, or even if you do not want to shut down the system, you can enter "standby (Stay-Alive)" mode.

Summary

By using the latest single-chip, designers can implement power management technology, design and cost effectiveness of a portable medical equipment.

In medical devices, the less power consumption, you can use a smaller battery. In addition, through the use of low-cost controller, design after using certain number implementand medical appliances have become possible.