The blood-pressure monitor is one of the more common medical devices found in homes. Although this type of equipment has been around for decades, designers keep changing it. Monitors now come in many flavors. Those that focus on low cost deliver only the bare minimum of features. Others take into account the physical state of the patient. Some battery-powered monitors simply take one blood-pressure measurement, report it to the user, then shut down. Others with many features allow for multiple measurements, store data, and interface to other equipment.

Even the most inexpensive electronic blood-pressure monitors are getting advanced features, and features on line-powered monitors are showing up in battery-powered versions. Upgraded devices now can store data, measure more accurately, and use less power. And these new blood-pressure monitors are often available in ever smaller form factors. Incorporating these new features is due in large part to advances in the microcontrollers implemented in the monitors. The QE128 family of microcontrollers from Freescale Semiconductor contain up to 128K bytes of non-volatile flash memory and 8k of RAM to satisfy the needs of data and program storage. They come in packages as small as 7 mm², have multiple 12-bit A/D converters, and support multiple power-down modes putting the device in deep sleep when not used. A clock-gating mechanism allows shutting down the clocks to individual peripherals as needed to conserve even more power.

As an automated extension of the classic art of measuring blood (or, more accurately, pulse) pressure, most blood-pressure-measuring devices today are “down ramp” devices. That is, they mimic the way nurses gather blood-pressure information using an inflatable arm cuff, a squeezable inflation bulb, a mercury manometer, and a stethoscope. Although the electronic devices do not measure pulse pressure directly, they derive it by processing pressure measurements taken in a data-gathering phase of the measurement cycle. Because there is a difference in the manual and electronic methods of blood pressure measurement, technology also allows a second “up ramp” method in which the measurement is made while the arm cuff is being inflated (as compared to the traditional method, which determines the blood pressure information while the arm cuff is being deflated).

Although down and up-ramp systems share most components, their implementation varies significantly. The greatest difference is in the implementation of the processing algorithms that determine results from the measured data. An automated down-ramp system which inflates an arm (or wrist) cuff acquires and processes its data something like this: An air pump inflates the cuff to a preset value above the “normal” pressure. The pump stops and lets the cuff deflate at a predetermined rate through a controlled leak while measuring pressure in the cuff. While it deflates, the device “looks” for the first pressure pulses and notes the pressure at which these are measured. The cuff continues deflating and the device determines the pressure at which the pulses are no longer measured, and the device records this pressure value.

The down and up-ramp methods share the same principles of searching for the pressure values at which the high and low pressure pulses are found. However, the up-ramp system becomes a bit more complicated because pressure measurements are made while the cuff is inflated. This requires the microcontroller in the device to extract the pressure pulses from a “noisy” pressure system because the motor is constantly changing the pressure in the system as the cuff inflates. In some cases, more than one sensor measures pressure at locations in the system. Also, the up-ramp motor control is usually more complex than in a down-ramp system, where motor control can be as simple as turning on a motor driven pump and letting it run without further control until it reaches the required pressure. In a down-ramp system some patients feel uncomfortable as the cuff pressure reaches its preset value and then slowly ramps down to get diastolic pressure. Up-ramp systems minimize this discomfort because the cuff pressure is immediately released after determining the patient's systolic pressure (or after a preset maximum pressure).

A low cost microcontroller, such as the MC9S08QE128 from Freescale, with built-in peripherals and low power consumption might be sufficient for a simple down-ramp blood-pressure monitor with an automated cuff inflation and push button user interface. Even when the monitor includes features such as automated cuff inflation, data logging, and serial interfaces to other machines for data retrieval, the aforementioned microcontroller would likely still suffice. The capabilities of an eight-bit-processing core are typically outpaced by implementing an up-ramp system with more features on the same printed circuit board with additional data processing and algorithm requirements to filter the pulse from the noisy inflating pump. Freescale offers an eight bit and a 32 bit microcontroller with identical peripheral sets in identical physical packages, letting printed circuit board designers keep the physical footprint for the microcontroller the same regardless of processing core. This frees designers to implement numerous options for even a simple blood-pressure monitor on a single printed circuit board, and provides for significant processing headroom.