Stay on top of heartbeats with SFH 7072 and STM32F031K6

Heart Rate 5 Click consists of an analog front end and the optical front end. The main task of the analog front-end IC is to drive LEDs and condition the signal received by the photodiode (PD) by eliminating the background noise and ambient light influence on the measurement. Besides that, it also provides conversion of the measurement into digital information, which the MCU can use. For the conversion to be accurate, the analog front-end device must not introduce any artifacts into the readings. Heart Rate 5 click employs AFE4404, an integrated analog front-end (AFE) device used for optical heart-rate monitoring and bio-sensing from Texas Instruments to achieve accurate measurements. This IC supports up to three switching LEDs and a single PD element. The current from the PD element is converted to a linear voltage by means of the integrated trans-impedance amplifier section (TIA) with a programmable gain so that it can be sampled by the AD section, which features a 22-bit ADC converter. The signal chain is kept fully differential throughout the receiver channel to achieve good rejection of common-mode noise and the noise from the power supply. The AFE IC uses the I2C communication, with its pins routed to the corresponding mikroBUS™ I2C pins. As the optical front end, Heart Rate 5 click uses the top-of-the-class integrated BIOFY® SFH 7072 sensor from OSRAM, which features two green LEDs, one red LED, one infrared LED, and two PDs, of which one is a broadband PD used for the HRM, while the other is the IR band cut PD, used for the SpO2 readings. These LEDs are specially designed for the HRM and SpO2 measurement applications, offering a set of calibrated wavelengths for both light-emitting diodes (LED) and photo-sensing

diodes (PD). Since the SF7072 sensor offers more elements than the ALS can support, the choice is made by flipping the onboard SMD switch labeled as MODE SEL: the choice can be made between the broadband PD for the HRM readings and the IR band PD for SnO2 readings. Both poles of the switch SW1 need to stay in the same position (both to the left or both to the right), as they are routed to each end of the respective PD element. The analog front end works with periodically repeated operations (a pulse repetition frequency or PRF). There are four sampling phases per cycle. The four different readings are stored in separate 24-bit output registers. There are also four filters on the TIA output, which allow pulses from the PD to pass through the ADC, isolating the time when the emitting LEDs are ON and switching to the different filters in every sampling phase. The sampling phases are determined by the LED modes: two LED modes or three LED modes. This affects which LEDs are pulsed during the corresponding sampling cycles – LED1 and LED2, or LED 1, LED 2, and LED 3. The AFE4404 IC also incorporates a DAC to cancel the DC offset from the PD. When the TIA gain is set to a high value, it will amplify the DC component of the PD signal. This component needs to be removed from the signal path to allow ADC conversion, so the DAC with the opposite current direction is introduced at the input stage based on the existing DC offset. This allows for higher amplification of the signal from the PD and, thus, more useful AC signal detection sensitivity. The LED drivers allow 6 bits of LED current control for each channel individually. This allows 63 steps between 0 and 50mA. This range can be doubled to 100mA. The LED driver voltage can be set by the onboard SMD jumper,

labeled as the LED SUP. It offers a selection between 3.3V and 5V. The ADC_RDY pin provides an interrupt to the host MCU, saving it from having to poll the sensor for data constantly. This pin is set to a HIGH logic level when the PRF cycle ends,allowing four output data register to be read. The PRF can vary between 10 to 1000 samples per second. This pin is routed to the INT pin of the mikroBUS™. The AFE4404 IC can be clocked both internally and externally. It is advised to drive the Heart Rate 5 Click by the same clock as the host MCU for precise and synchronized measurement. The input clock can go up to 60MHz, but the internal divider of the IC has to be set so that the clock stays within the range from 4MHz to 6MHz. When driven by the internal clock, the device runs at 4MHz. By default, the external clock input is selected. The clock signal can be introduced via the PWM pin of the mikroBUS™. After the power-on, the AFE IC requires a reset. The RESETZ pin of this IC is routed to the RST pin of the mikroBUS™, allowing it to be reset by the host MCU. Pulling this signal to a LOW logic level for about 25 µs to 50 µs will cause a device reset. Pulling this pin for more than 200 µs will put the device into the POWER DOWN mode. The device can also be reset by setting a bit in the appropriate register via the I2C. The onboard pull-up resistor pulls This pin to a HIGH logic level. More information about the registers and how to set them can be found in the AFE4404 IC datasheet. However, included library contains functions that allow easy configuration and use of the Heart Rate 5 click. The included exemplary (demo) application demonstrates its functionality and can be used as a reference for a custom design.

Nucleo 32 with STM32F031K6 MCU board provides an affordable and flexible platform for experimenting with STM32 microcontrollers in 32-pin packages. Featuring Arduino™ Nano connectivity, it allows easy expansion with specialized shields, while being mbed-enabled for seamless integration with online resources. The

board includes an on-board ST-LINK/V2-1 debugger/programmer, supporting USB reenumeration with three interfaces: Virtual Com port, mass storage, and debug port. It offers a flexible power supply through either USB VBUS or an external source. Additionally, it includes three LEDs (LD1 for USB communication, LD2 for power,

and LD3 as a user LED) and a reset push button. The STM32 Nucleo-32 board is supported by various Integrated Development Environments (IDEs) such as IAR™, Keil®, and GCC-based IDEs like AC6 SW4STM32, making it a versatile tool for developers.