diff --git a/ACS712.cpp b/ACS712.cpp
index 4d1023a..fb8dba7 100644
--- a/ACS712.cpp
+++ b/ACS712.cpp
@@ -1,7 +1,7 @@
 //
 //    FILE: ACS712.cpp
 //  AUTHOR: Rob Tillaart, Pete Thompson
-// VERSION: 0.3.8
+// VERSION: 0.3.9
 //    DATE: 2020-08-02
 // PURPOSE: ACS712 library - current measurement
 //     URL: https://github.com/RobTillaart/ACS712
@@ -81,8 +81,8 @@ float ACS712::mA_AC(float frequency, uint16_t cycles)
     uint16_t samples = 0;
     uint16_t zeros   = 0;
 
-    int _min, _max;
-    _min = _max = _analogRead(_pin);
+    int minimum, maximum;
+    minimum = maximum = _analogRead(_pin);
 
     //  find minimum and maximum and count the zero-level "percentage"
     uint32_t start = micros();
@@ -95,12 +95,12 @@ float ACS712::mA_AC(float frequency, uint16_t cycles)
         value = (value + _analogRead(_pin))/2;
       }
       //  determine extremes
-      if (value < _min) _min = value;
-      else if (value > _max) _max = value;
+      if (value < minimum) minimum = value;
+      else if (value > maximum) maximum = value;
       //  count zeros
       if (abs(value - _midPoint) <= zeroLevel ) zeros++;
     }
-    int peak2peak = _max - _min;
+    int peak2peak = maximum - minimum;
 
     //  automatic determine _formFactor / crest factor
     float D = 0;
@@ -179,6 +179,8 @@ float ACS712::mA_DC(uint16_t cycles)
     {
       value = (value + _analogRead(_pin))/2;
     }
+    //  for RTOS
+    if (i % 2 == 1) yield();
     sum += (value - _midPoint);
   }
   float mA = sum * _mAPerStep;
diff --git a/ACS712.h b/ACS712.h
index d343715..272b043 100644
--- a/ACS712.h
+++ b/ACS712.h
@@ -2,7 +2,7 @@
 //
 //    FILE: ACS712.h
 //  AUTHOR: Rob Tillaart, Pete Thompson
-// VERSION: 0.3.8
+// VERSION: 0.3.9
 //    DATE: 2020-08-02
 // PURPOSE: ACS712 library - current measurement
 //     URL: https://github.com/RobTillaart/ACS712
@@ -14,7 +14,7 @@
 #include "Arduino.h"
 
 
-#define ACS712_LIB_VERSION        (F("0.3.8"))
+#define ACS712_LIB_VERSION        (F("0.3.9"))
 
 
 //  ACS712_FF_SINUS == 1.0/sqrt(2) == 0.5 * sqrt(2)
@@ -42,23 +42,31 @@ class ACS712
     //  30A        66.0
     ACS712(uint8_t analogPin, float volts = 5.0, uint16_t maxADC = 1023, float mVperAmpere = 100);
 
-   //   returns mA peak2peak current.
+    //  returns mA peak2peak current.
+    //  blocks ~20-21 ms per cycle to sample a whole 50 or 60 Hz period.
+    //  does NOT call yield() as that would disrupt measurement
    float mA_peak2peak(float frequency = ACS712_DEFAULT_FREQ, uint16_t cycles = 1);
 
+
     //  returns mA
-    //  blocks 20-21 ms to sample a whole 50 or 60 Hz period.
+    //  blocks ~20-21 ms per cycle to sample a whole 50 or 60 Hz period.
     //  works with peak2peak level and (crest) Form Factor.
     //  lower frequencies block longer.
+    //  does NOT call yield() as that would disrupt measurement
     float    mA_AC(float frequency = ACS712_DEFAULT_FREQ, uint16_t cycles = 1);
 
+
     //  returns mA
-    //  blocks 20-21 ms to sample a whole 50 or 60 Hz period.
+    //  blocks 20-21 ms per cycle to sample a whole 50 or 60 Hz period.
     //  works with sampling.
     //  lower frequencies block longer.
+    //  does NOT call yield() as that would disrupt measurement
     float    mA_AC_sampling(float frequency = ACS712_DEFAULT_FREQ, uint16_t cycles = 1);
 
+
     //  returns mA
-    //  blocks < 1 ms
+    //  blocks < 1 ms (depending on # samples and ADC used)
+    //  does call yield() for RTOS.
     float    mA_DC(uint16_t samples = 1);
 
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c5a938a..9f16961 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,11 +6,16 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 
+## [0.3.9] - 2024-01-11
+- add yield() in multiple reads to better support RTOS
+- update readme.md (empty compatibility table)
+- minor edits
+
+
 ## [0.3.8] - 2023-09-19
 - add badges to readme.md
 - minor edits
 
-
 ## [0.3.7] - 2023-05-20
 - add example **estimateMidPointAC.ino** #37
 - update readme.md (sampling trick #38).
diff --git a/LICENSE b/LICENSE
index 42604f3..18dfaaf 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2020-2023 Rob Tillaart
+Copyright (c) 2020-2024 Rob Tillaart
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/README.md b/README.md
index 3c1f8aa..ffc3d8a 100644
--- a/README.md
+++ b/README.md
@@ -71,6 +71,16 @@ could be obtained with such an ADC. It triggered the experimental supporting
 of external ADC's with this library.
 
 
+#### Calibration and accuracy
+
+The library has no means to calibrate the output or use an offset.
+However sort of calibrating can relatively easy be done by using 
+the MultiMap library.
+MultiMap approaches a non-linear mapping by multiple linear mappings.
+
+See https://github.com/RobTillaart/MultiMap.
+
+
 #### Tests
 
 The library is at least confirmed to work with the following boards:
@@ -91,12 +101,25 @@ Please let me know of other working platforms / processors (and failing ones!).
 Robodyn has a breakout for the ACS758 - 50 A. - See resolution below.
 This sensor has versions up to 200 Amps, so use with care!
 
-Allegromicro offers a lot of different current sensors that might be compatible.
+AllegroMicro offers a lot of different current sensors that might be compatible.
 These include bidirectional and unidirectional ones.
 The unidirectional seem to be for DC only.
 
 https://www.allegromicro.com/en/products/sense/current-sensor-ics/current-sensors-innovations
 
+
+Devices that could be compatible:
+
+|        | ACS720 | ACS724 | ACS725 | ACS732 | ACS733|  ACS758 | ACS772 | ACS773 | ACS780 | ACS781 |
+|:------:|:------:|:------:|:------:|:------:|:------:|:------:|:------:|:------:|:------:|:------:|
+| tested |        |        |   #44  |        |        |        |        |        |        |        |
+
+
+|        | ACS37002 | ACS37003 | ACS71240 | ACS3761X | ACS37800 | ACS72981 |
+|:------:|:--------:|:--------:|:--------:|:--------:|:--------:|:--------:|
+| tested |          |          |          |          |          |          |
+
+
 If you have tested a compatible sensor, please share your experiences.
 (can be done by opening an issue to update documentation)
 
@@ -151,6 +174,7 @@ This function is intended for signals with unknown Form Factor.
 - **float mA_DC(uint16_t samples = 1)** blocks < 1 ms (Arduino UNO) as it calls **analogRead()** twice.
 A negative value indicates the current flows in the opposite direction.
   - 0.2.8 the parameter samples allow to average over a number of samples.
+  - 0.3.9 calls yield() every 3 analogRead's to support RTOS (a bit).
 
 
 #### mA_AC_sampling performance trick.
@@ -342,15 +366,24 @@ ACS.setADC(NULL, 5.0, 1023);
 - example ACS712_20_DC_external_ADC.ino
 - https://github.com/RobTillaart/ACS712/issues/31
 
+- example ACS712_ESP32_external_ADC.ino
+- https://github.com/RobTillaart/ACS712/issues/46
+
 
 Note that the use of an external ADC should meet certain performance requirements,
 especially for measuring **ma-AC()**.
-To 'catch' the peaks well enough one needs at least 2 samples per millisecond
-for a 60 Hz signal.
+
+To 'catch' the peaks well enough one needs at least 2 samples per millisecond (2000 sps)
+for a 60 Hz signal. That gives 34 samples for 360 degrees => 10.6 degrees, which 
+results in a max deviation of 5.3 degrees from peak => max 0.5% off.
+
+As a 50 Hz signal is a bit slower, 2000 sps would give 40 samples for => 9 degrees,
+which results in a max deviation of 4.5 degrees from peak => max 0.4% off.
+
 
 The 16 bit I2C **ADS1115** in continuous mode gives max 0.8 samples per millisecond.
 This will work perfect for high resolution **mA-DC()** but is not fast enough for
-doing **mA-AC()**.
+doing **mA-AC()**. It will get an accuracy around ~2%.
 
 The SPI based **MCP3202** ao can do up to 100 samples per millisecond at 12 bit.
 These ADC's are perfect both **mA-DC()** and **mA-AC()**.
@@ -419,6 +452,22 @@ Schema with PULL-UP.
 The library does not support this "extreme values" detection.
 
 
+## RTOS
+
+The library can be used in an RTOS environment, however a few functions of this
+library are blocking for relative long times.
+
+In version 0.3.9 the **mA_DC()** calls **yield()** between every three calls of analogRead.
+This is done both for the external and intern ADC to prevent blocking of other threads.
+
+For the **mA_AC()** and **mA_peak2peak()** a call to yield() is not desirable 
+as the samples are all needed to make a decent measurement.
+For the applications that need proper scheduling one should put the sampling of the 
+INA226 at least for **AC** in a separate thread.  
+
+There is no RTOS example. If you have and willing to share you are welcome.
+
+
 ## ESPhome
 
 For people who want to use this library for ESPhome, there exists a wrapper 
diff --git a/examples/ACS712_20_DC_external_ADC/ACS712_20_DC_external_ADC.ino b/examples/ACS712_20_DC_external_ADC/ACS712_20_DC_external_ADC.ino
index 55e4f4a..8c2c4b7 100644
--- a/examples/ACS712_20_DC_external_ADC/ACS712_20_DC_external_ADC.ino
+++ b/examples/ACS712_20_DC_external_ADC/ACS712_20_DC_external_ADC.ino
@@ -4,6 +4,8 @@
 // PURPOSE: demo to measure mA DC with external ADC
 //     URL: https://github.com/RobTillaart/ACS712
 
+//  see also ACS712_ESP32_external_ADC.ino
+
 //  use with Arduino Serial Plotter
 
 #include "ACS712.h"
@@ -43,7 +45,7 @@ void loop()
 }
 
 //  wrapper needed for external analogRead()
-//  as casting behavior is undefined between different function signatures.
+//  as casting behaviour is undefined between different function signatures.
 uint16_t testADC(uint8_t p)
 {
   //  simulation
diff --git a/examples/ACS712_ESP32_external_ADC/.arduino-ci.yml b/examples/ACS712_ESP32_external_ADC/.arduino-ci.yml
new file mode 100644
index 0000000..e1c447b
--- /dev/null
+++ b/examples/ACS712_ESP32_external_ADC/.arduino-ci.yml
@@ -0,0 +1,28 @@
+platforms:
+  rpipico:
+    board: rp2040:rp2040:rpipico
+    package: rp2040:rp2040
+    gcc:
+      features:
+      defines:
+        - ARDUINO_ARCH_RP2040
+      warnings:
+      flags:
+
+packages:
+  rp2040:rp2040:
+    url: https://github.com/earlephilhower/arduino-pico/releases/download/global/package_rp2040_index.json
+
+compile:
+  # Choosing to run compilation tests on 2 different Arduino platforms
+  platforms:
+    # - uno
+    # - due
+    # - zero
+    # - leonardo
+    # - m4
+    - esp32
+    # - esp8266
+    # - mega2560
+    # - rpipico
+
diff --git a/examples/ACS712_ESP32_external_ADC/ACS712_ESP32_external_ADC.ino b/examples/ACS712_ESP32_external_ADC/ACS712_ESP32_external_ADC.ino
new file mode 100644
index 0000000..b979c93
--- /dev/null
+++ b/examples/ACS712_ESP32_external_ADC/ACS712_ESP32_external_ADC.ino
@@ -0,0 +1,77 @@
+//    FILE: ACS712_ESP32_external_ADC.ino
+//  AUTHOR: Rob Tillaart
+// PURPOSE: demo to measure mA DC with external ADC
+//     URL: https://github.com/RobTillaart/ACS712
+
+//  see also ACS712_20_DC_external_ADC.ino
+
+#include <Arduino.h>
+#include <ACS712.h>
+#include <ADS1X15.h>
+
+
+//  I2C config
+#define ADS1015_ADDRESS         0x48
+#define ADS1015_SCL             22   //  default SCL ESP32
+#define ADS1015_SDA             21   //  default SDA ESP32
+
+//  ADS1x15 config
+#define SENSOR_ACS712_ADSPIN    1
+
+
+//  explicit params for demo
+ADS1015 ads1015(ADS1015_ADDRESS, &Wire);  //  ADS1015 == 12 bit
+
+
+//  SENSOR_ACS712_ADSPIN sets pin 1 of the ADS1015, 5.0 volt, 4095 = 12 bit, 100 = mVperAmpere
+ACS712 ACS(SENSOR_ACS712_ADSPIN, 5.0, 4095, 100);
+
+
+//  ACS712 ADC WRAPPER FOR ADS1015    Note: uses only one direction.
+uint16_t readADS1015(uint8_t pin)
+{
+  uint16_t ADS_raw = ads1015.readADC(pin);
+  Serial.print("ADS_raw: ");
+  Serial.println(ADS_raw);
+  return ADS_raw;
+}
+
+
+///////////////////////////////////////////////////////////////
+
+void setup() 
+{
+  Serial.begin(115200);
+  while (!Serial);
+  Serial.println(__FILE__);
+  Serial.print("ACS712_LIB_VERSION: ");
+  Serial.println(ACS712_LIB_VERSION);
+  Serial.println();
+
+
+  //  ESP32 set wire pins explicitly
+  Wire.begin(ADS1015_SDA, ADS1015_SCL);
+  Wire.setClock(400000);
+
+
+  //  initialize ADS1015, if fail => report
+  if (ads1015.begin() == false)
+  {
+    Serial.println("ADS1x15 not found. Check wires and pins. Reboot.");
+    while(1);
+  }
+
+  //  set up the external ADC for the ACS712
+  ACS.setADC(readADS1015, 5.0, 4095);
+}
+
+
+void loop()
+{
+  int mA = ACS.mA_DC();
+  Serial.println(mA);
+  delay(1000);
+}
+
+
+//  -- END OF FILE --
diff --git a/library.json b/library.json
index 2972f3f..6380823 100644
--- a/library.json
+++ b/library.json
@@ -21,7 +21,7 @@
     "type": "git",
     "url": "https://github.com/RobTillaart/ACS712.git"
   },
-  "version": "0.3.8",
+  "version": "0.3.9",
   "license": "MIT",
   "frameworks": "*",
   "platforms": "*",
diff --git a/library.properties b/library.properties
index 76178de..aebbd43 100644
--- a/library.properties
+++ b/library.properties
@@ -1,5 +1,5 @@
 name=ACS712
-version=0.3.8
+version=0.3.9
 author=Rob Tillaart <rob.tillaart@gmail.com>, Pete Thompson <pete.thompson@yahoo.com>
 maintainer=Rob Tillaart <rob.tillaart@gmail.com>
 sentence=ACS712 library for Arduino.