diff --git a/src/components/heartrate/Biquad.cpp b/src/components/heartrate/Biquad.cpp
index 37b3ba67..6a4b8181 100644
--- a/src/components/heartrate/Biquad.cpp
+++ b/src/components/heartrate/Biquad.cpp
@@ -1,7 +1,14 @@
+/*
+  SPDX-License-Identifier: LGPL-3.0-or-later
+  Original work Copyright (C) 2020 Daniel Thompson
+  C++ port Copyright (C) 2021 Jean-François Milants
+*/
+
 #include "Biquad.h"
 
 using namespace Pinetime::Controllers;
 
+/** Original implementation from wasp-os : https://github.com/daniel-thompson/wasp-os/blob/master/wasp/ppg.py */
 Biquad::Biquad(float b0, float b1, float b2, float a1, float a2) : b0{b0}, b1{b1}, b2{b2}, a1{a1}, a2{a2} {
 
 }
diff --git a/src/components/heartrate/Ppg.cpp b/src/components/heartrate/Ppg.cpp
index 27617c15..233c3003 100644
--- a/src/components/heartrate/Ppg.cpp
+++ b/src/components/heartrate/Ppg.cpp
@@ -1,10 +1,16 @@
+/*
+  SPDX-License-Identifier: LGPL-3.0-or-later
+  Original work Copyright (C) 2020 Daniel Thompson
+  C++ port Copyright (C) 2021 Jean-François Milants
+*/
+
 #include <vector>
 #include <nrf_log.h>
 #include "Ppg.h"
 using namespace Pinetime::Controllers;
 
+/** Original implementation from wasp-os : https://github.com/daniel-thompson/wasp-os/blob/master/wasp/ppg.py */
 namespace {
-  // TODO no vector!
   int Compare(int* d1, int* d2, size_t count) {
     int e = 0;
     for(int i = 0; i < count; i++) {
diff --git a/src/components/heartrate/Ptagc.cpp b/src/components/heartrate/Ptagc.cpp
index 9302af97..dd7c4411 100644
--- a/src/components/heartrate/Ptagc.cpp
+++ b/src/components/heartrate/Ptagc.cpp
@@ -1,8 +1,15 @@
+/*
+  SPDX-License-Identifier: LGPL-3.0-or-later
+  Original work Copyright (C) 2020 Daniel Thompson
+  C++ port Copyright (C) 2021 Jean-François Milants
+*/
+
 #include <cmath>
 #include "Ptagc.h"
 
 using namespace Pinetime::Controllers;
 
+/** Original implementation from wasp-os : https://github.com/daniel-thompson/wasp-os/blob/master/wasp/ppg.py */
 Ptagc::Ptagc(float start, float decay, float threshold) : peak{start}, decay{decay}, boost{1.0f/decay}, threshold{threshold} {
 
 }
diff --git a/src/drivers/Hrs3300.cpp b/src/drivers/Hrs3300.cpp
index 31956575..2aded7d3 100644
--- a/src/drivers/Hrs3300.cpp
+++ b/src/drivers/Hrs3300.cpp
@@ -1,3 +1,9 @@
+/*
+  SPDX-License-Identifier: LGPL-3.0-or-later
+  Original work Copyright (C) 2020 Daniel Thompson
+  C++ port Copyright (C) 2021 Jean-François Milants
+*/
+
 #include <algorithm>
 #include <nrf_gpio.h>
 #include "Hrs3300.h"
@@ -7,7 +13,9 @@
 #include <nrf_log.h>
 
 using namespace Pinetime::Drivers;
-
+/** Driver for the HRS3300 heart rate sensor.
+ * Original implementation from wasp-os : https://github.com/daniel-thompson/wasp-os/blob/master/wasp/drivers/hrs3300.py
+ */
 Hrs3300::Hrs3300(TwiMaster &twiMaster, uint8_t twiAddress) : twiMaster{twiMaster}, twiAddress{twiAddress} {
 
 }