The goal of this assignment is implementing a heart rate monitor, which is made of two components:

• a Linux character-based driver (cDD) used to access a “virtual” Photopletismography (PPG) sensor
• a Linux user application (APP)

The APP performs an endless loop, where it samples the PPG sensor, and every 2048 acquired samples, it performs a 2048-points FFT, computes the Power Spectral Density (PSD), identifies the base frequency of the signal (the frequency where the PSD is maximum), which is the heart rate in Hz, and it prints the heart rate in beat-per-minutes, according to the following specifications:

• sampling frequency: 50 Hz
• number of samples to be acquired: 2048

The cDD provides access to the virtual PPG sensor. Each time the read function of the cDD is invoked, a pre-defined value is provided to the user application.

Assuming you already built and setup a pocky Linux distribution on your machine for Raspberry PI 4 (or any other version) , you can either

• use a bash script I wrote to automate the whole process
• do the whole setup by hand as instructed in the following sections

In both cases, make sure that

• your conf/bblayers.conf looks like this

  #POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf 
  # changes incompatibly
  POKY_BBLAYERS_CONF_VERSION = "2"
  
  BBPATH = "${TOPDIR}" 
  BBFILES ?= ""
  
  BBLAYERS ?= " \
    /home/elle/poky/meta \
    /home/elle/poky/meta-poky \
    /home/elle/poky/meta-yocto-bsp \
    /home/elle/poky/meta-openembedded/meta-oe \
    /home/elle/poky/meta-openembedded/meta-multimedia \
    /home/elle/poky/meta-openembedded/meta-networking \
    /home/elle/poky/meta-openembedded/meta-python \
    /home/elle/poky/meta-raspberrypi \
    "

• your conf/local.conf file contains the following lines

  MACHINE ?= "raspberrypi4"
  ENABLE_UART = "1"
  
  EXTRA_IMAGE_FEATURES += "debug-tweaks tools-debug eclipse-debug ssh-server-openssh"
  IMAGE_INSTALL_append = " linux-firmware-rpidistro-bcm43455"
  IMAGE_FSTYPES = "tar.xz ext3 rpi-sdimg"
  

Assuming your poky directory is located at ~/ , create a bash file called setup.sh

touch ~/setup.sh

and copy the following code in it

WARNING: Make sure it fits your system before using. In particular, make sure your output device is actually called /dev/sdb otherwise modify last line with the appropriate one (check using sudo fdisk -l) . Also, if your build directory has a different name, just replace build_rpi4 with the correct one.

#!/bin/bash

cd poky
git clone https://github.com/PronElle/meta-heartmonitor

shopt -s expand_aliases
source oe-init-build-env build_rpi4

bitbake-layers add-layer ../meta-heartmonitor

echo "IMAGE_INSTALL_append = \" heartbeat\"" >> conf/local.conf
echo "IMAGE_INSTALL_append = \" ppgmod\"" >> conf/local.conf
echo "KERNEL_MODULE_AUTOLOAD += \"ppgmod\"" >> conf/local.conf

bitbake core-image-full-cmdline
sudo dd if=tmp/deploy/images/raspberrypi4/core-image-full-cmdline-raspberrypi4.rpi-sdimg of=/dev/sdb bs=1M

Then, make it executable

sudo chmod u+x setup.sh

Plug your SD card reader and run the script

./setup.sh

Eventually, connect via ssh to your rpi (or use a serial to USB cable or HDMI cable plus external monitor) and type heartbeat to test the application.

Alternatively, do the setup by hand as described below.

Reach your poky root directory

cd poky

then, clone this repository as follows

git clone https://github.com/PronElle/meta-heartmonitor

and initialize the environment

source oe-init-build-env build_rpi4

now the layer needs to be added to the configuration

bitbake-layers add-layer ../meta-heartmonitor

At this point, application and the kernel module need to be added to the Linux distro configurations. Edit the following file with your favorite text editor, e.g. vi

vi conf/local.conf

and add the following lines at the end of the file

IMAGE_INSTALL_append = " heartbeat"
IMAGE_INSTALL_append = " ppgmod"
KERNEL_MODULE_AUTOLOAD += "ppgmod"

and build the new image

bitbake core-image-full-cmdline

Plug the SD card reader in your computer and burn the image to the SD card.

WARNING: Make sure your output device is actually called /dev/sdb otherwise modify of=<outdevname> properly (check with sudo fdisk -l).

sudo dd if=tmp/deploy/images/raspberrypi4/core-image-full-cmdline-raspberrypi4.rpi-sdimg of=/dev/sdb bs=1M

Eventually, login to your machine (either via ssh, serial port or using an HDMI cable and an external monitor) and test the application running heartbeat from the command user interface.

A sampling frequency of 50 Hz is equal to 20 ms sampling period. To ensure the proper timing, an alarm-based approach relying on SIGALARM and setitimer function was employed.

Timer values are defined by the following structures

struct itimerval {
    struct timeval it_interval; /* next value */
    struct timeval it_value;    /* current value */
};

struct timeval {
    time_t      tv_sec;         /* seconds */
    suseconds_t tv_usec;        /* microseconds */
};

The relative error obtained on Raspberry PI 4 is around 0.1%.

• it embeds a FIFO mechanism, which proves useful as samples keep getting acquired and accumulated even during bpm computation, thus no data get lost;
• it doesn't require any synchronization mechanism (no semaphores, locks or mutexs) because pipes are blocking, so that a read is performed only when the pipe is not empty;
• using another thread to read and a semaphore to force it to wait while computing the bpm doesn't allow you to read the sensor while this action is performed.
• it's simple, efficient and low in terms of memory occupation.

In case you own no real target, use the following script to deploy your application on qemuarm.

#!/bin/bash

cd poky
git clone https://github.com/PronElle/meta-heartmonitor

shopt -s expand_aliases
source oe-init-build-env build_qemuarm

bitbake-layers add-layer ../meta-heartmonitor

echo "IMAGE_INSTALL_append = \" heartbeat\"" >> conf/local.conf
echo "IMAGE_INSTALL_append = \" ppgmod\"" >> conf/local.conf
echo "KERNEL_MODULE_AUTOLOAD += \"ppgmod\"" >> conf/local.conf

bitbake core-image-minimal
runqemu qemuarm

NOTICE: qemuarm is far from precise in terms of timing. Do not rely on it if you want to test the application from that point of view.