Communications using Microchips or FPGA?

Question

Good Evening,

We are currently doing a project in my school. Our idea is to build a device which is worn on hand like a watch and could measure a heart beat of a particular patient. Whenever the heart beat reach the "critical heartbeat" which we had set in the programming area, it is suppose to send a signal through Wireless to the main control centre to alert the staff. Since the device is to worn on a hand, it must be small.

My question is I am not sure which model of Microchip or FPGA should i use is most suitable for this project? Since my aim for the chip is to be able to send the signal at a very low power to the control centre, how much power is need?

Thanks.

Calvin

nickgray · Answer

It is not easy to answer your question as to how much power is needed because the answer depends upon so many things. For example, the more interference there is from other signals, the more power you must transmit to limit the effect of the interfering signals, but the power must not be so great that it interfers with other electronic systems. Another factor is how far the signal must be transmitted and how sensitive is the receiver. The sensitivity of the receiver is a measure of how small a signal the receiver will accurately accept. The more sensitive is the receiver, the more susceptible it is to interference, so you do not want it to be too sensitive. Also, there are so many processors that could do this function that it is a very large task to determine which would use the lowest power in this application . I suggest than an FPGA would probably use more power than a processor, although I am not sure because all of the FPGAs I have used were extremely fast, so used a lot of power. I have never looked for a slow, low power FPGA. Electronic systems have more than one function. Your system has the functions of counting, transmitting, comparing beats with a maximum, decision, and alarm sounding. System partitioning is the process of deciding where each which function will be done. For the sake of power consumption, I suggest that you have as little circuitry in the wrist unit as practical and only transmmit when the heart rate gets above the threshold. It is probably more practical to look at the time between heart beats and determine heart rate from that. For example, one second between heart beats works out to be 60 beats per minute: 60 seconds in a minute divided by 1 beat per second is mathematically shown as (60 sec/min) / (1 sec/beat) The seconds cancel out and we have (60/min) / (1/beat) Mathematically this is the same as 60 beat / 1 min, or 60 beats per minute Of course, you can change the 1 second to another time period and find the resulting heart rate. An interval between heart beats that is 0.8 seconds (8 tenths of a second) comes out to (60 / 0.8) = 75 beats per minute. I would partition the circuitry so that the wrist unit transmitted a pulse every time there is a heart beat and the receiver circuit would determine the heart rate and when an alarm should be sounded. The heart beat would produce a small signal that is amplified and this signal would drive a micropower voltage comparator. The output of the comparator would go low for a few milliseconds when there is a hearat beat. I would use a low power oscillator circuit, such as the one at http://www.discovercircuits.com/DJ-Circuits/ultralowpwrxtlosc1.htm , If you use this circuit above, I suggest a small caopacitor of 4.7pF be added from the the transistor collector to ground. You will need a power circuit to drive your antenna. Connect the "ground" of the power amplifier to the collector of the voltage comparator. When the comparator output goes low, the circuit will now transmit a pulse, which is at each heart beat. The receiver will receive the pulses and determine the heart rate. Since the receiver can be powered from a wall plug, there is not as much concern for power consumption in the receiver. I suggested a micropower oscillator and you should have no problem finding a micropower voltage comparator. The power stage is pulsed on and off and, since it is off most of the time, its overall average power consumption will be pretty low. I would start with a transmit power of 50 to 100 mW and see how it works. Once you have it working, reduce the power by half and see how it works. Keep reducing the power by half until it does not work well, then go back to the previous power level and stay there. Remember that every time you want to double the range you must provide 4 times the transmit power to get there. You will note that the wrist unit is all analog and the receiver has both analog and digital circuitry. I believe this is the most efficient way to do such a design. Analog circuits generally are easier to get low power consumption than are digital circuits. Good luck with your project!

Former Member · Answer

Calvin, Have a look at this http://focus.ti.com/docs/toolsw/folders/print/ez430-chronos.html?DCMP=Chronos&HQS=Other+OT+chronos and see how they do it. R