FPGA-based digital heart rate meter design implementation (1)

Heart rate meter is commonly used in medical examination equipment, real-time accurate heart rate measurement in patient monitoring, clinical treatment and sports competitions, and so has a wide range of applications.

Heart rate measurement including instantaneous heart rate measurement and average heart rate measurement. Instantaneous heart rate not only reflect the heart rate of speed. At the same time reflect the heart rate is the average heart rate constant Qi; although only reflect the heart rate of speed, but the record is convenient, so these two parameters in the measurement of time is necessary.

Heart rate measurement with analog and digital.

Simulation methods in a given time interval calculation R wave (or pulse wave) the number of pulses and pulse count multiplied by an appropriate constant heart rate measurement. The drawback of this method is the measurement error is large, component parameters, debug difficult, reliability is poor. Digital measurement method is the first adjacent time between the R wave, then the time is converted to a number of heartbeats per minute measuring heart rate. The advantages of this method is the measurement of high accuracy, reliability, and is capable of simultaneously measuring instantaneous heart rate and average heart rate. Heart rate measurement with digital methods of circuit is divided into two types: one is using an out-of-the-counter realization are Division circuits; the other is through automatic drop clock frequency measurement between adjacent R wave.

The heart rate meter in digital heart rate meter basis, using FPGA and VHDL, reduces the number of components used, improves measurement accuracy and reliability.

The circuit to real-time collection and measurement of human heartbeat of instantaneous and average heart rate, judgment and displays heart rate status (i.e. whether heartbeat is normal, too fast or too slow, is there a heart rate is not homogeneous phenomenon). If the heart rate is too fast or too slow, or uneven heart rate, then will use different color led for flashing alarm display.

1 measurement method and circuit

1.1 test method

As noted above, the use of digital measurement of the instantaneous heart rate (Heart Rate, IHR Intantaneous), the first measurement of two adjacent time between the R wave (i.e. heart rate cycle), the heart rate cycle converted to the number of heartbeats per minute.

As shown in Figure 1, the set heart rate cycle for T seconds, the instantaneous heart rate formula IHR = 60/T. If you use a frequency f0 of the clock pulse benchmarks for measuring time, in T seconds on the clock pulse meter FUNG, and design value is n, then t = N/f0 seconds, the instantaneous heart rate formula IHR = 60f0/N. When f0 = 1kHz, IHR = 60 x 1000/N = 60000/N.

Average heart rate (Average Heart Rate) measurement is a time of the transient inner heart rate averages.

Set the instantaneous heart rate measured as IHR1, IHR2, ... The average heart rate IHRn, is:

AHR=（IHR1+IHR2+…+IHRn）/n

1.2 circuit

System block diagram shown in Figure 2.

Pressing the start switch starts the measuring process, the sensor receives analog ECG (R-wave or pulse wave) has been enlarged to a comparer of a input, and the other end of the reference voltage for comparison, the ECG signal is converted to the same period of the square wave signal, and then enter the FPGA for heart rate measurement.

In FPGA, waveform transform circuit first the pulse width wide square-wave signal for differential, converted to the pulse width is equal to the clock signal (clk1) a cycle of a square wave signal, by cycle counters in heart rate cycle T hours count against the clock signal, then the front gives the instantaneous heart rate calculation formula to do a division operation to get instantaneous heart rate.

Instantaneous heart rate by decoding circuit is converted to a seven-segment display code to the FPGA external three LED display on the monitor. At the end of a measurement, measuring heart rate calculation module to the instantaneous heart rate, average heart rate, the average is converted to a seven-segment display code is sent to three LED display to display.

Alarm control module based on the instantaneous heart rate value in each one to determine whether the heart rate is normal, too fast or too slow, and on the basis of the adjacent two instantaneous heart rate values determine whether there is uneven heart rate, respectively, by the letter E (normal), F or S (too fast or too slow) and I (heart) of seven-segment display code send alarm to show circuit three LED monitor for display and three heart rate frequency to 8Hz to alarm display circuits colours of green, red, yellow three light-emitting diodes for flashing alarm display.

Press the stop switch to the end of the measurement process and average heart rate sent three LED display to display.

The system's main clock frequency is 32MHz, sent to the clock frequency in FPGA circuit 1kHz and 8Hz clock frequency, respectively, to use as the wave transformation, instantaneous heart rate heart rate calculation and display of the wave transformation module, heart rate calculation module and alarms control module.

System of digital circuit realization by FPGA chip, only a small amount of external analog device, including comparators, LED and led display, power circuits and crystal oscillator circuit and so on, thus the system of small, stable, high reliability.

2 FPGA Module circuit composition and working principle

2.1 wave converter circuit