diff --git a/libraries/CRC/CRC12.cpp b/libraries/CRC/CRC12.cpp
index 64a689e4..09645c69 100644
--- a/libraries/CRC/CRC12.cpp
+++ b/libraries/CRC/CRC12.cpp
@@ -14,6 +14,20 @@ CRC12::CRC12()
 }
 
 
+CRC12::CRC12(uint16_t polynome, uint16_t XORstart, uint16_t XORend, bool reverseIn, bool reverseOut)
+{
+  _polynome   = polynome;
+  _startMask  = XORstart;
+  _endMask    = XORend;
+  _reverseIn  = reverseIn;
+  _reverseOut = reverseOut;
+  _crc        = 0;
+  _started    = false;
+  _count      = 0;
+  _canYield   = true;
+}
+
+
 void CRC12::reset()
 {
   _polynome   = CRC12_DEFAULT_POLYNOME;
diff --git a/libraries/CRC/CRC12.h b/libraries/CRC/CRC12.h
index 8235afe3..07a19fd7 100644
--- a/libraries/CRC/CRC12.h
+++ b/libraries/CRC/CRC12.h
@@ -1,6 +1,4 @@
 #pragma once
-#ifndef CRC12_H
-#define CRC12_H
 //
 //    FILE: CRC12.h
 //  AUTHOR: Rob Tillaart
@@ -18,6 +16,7 @@ class CRC12
 {
 public:
   CRC12();
+  CRC12(uint16_t polynome, uint16_t XORstart, uint16_t XORend, bool reverseIn, bool reverseOut);
 
   // set parameters to default
   void     reset();       // set all to constructor defaults
@@ -40,12 +39,12 @@ public:
   void     add(uint8_t value);
   void     add(const uint8_t * array, uint16_t length);
 
-  uint16_t getCRC();  // returns CRC
-  uint32_t count()    { return _count; };
+  uint16_t getCRC();       // returns CRC
+  uint32_t count()         { return _count; };
 
   // POWER USER ONLY
-  void     enableYield()  { _canYield = true; };
-  void     disableYield() { _canYield = false; };
+  void     enableYield()   { _canYield = true; };
+  void     disableYield()  { _canYield = false; };
 
 private:
   uint16_t _reverse(uint16_t value);
@@ -65,4 +64,4 @@ private:
 
 
 // -- END OF FILE --
-#endif
+
diff --git a/libraries/CRC/CRC16.cpp b/libraries/CRC/CRC16.cpp
index f05a297f..cb465ee3 100644
--- a/libraries/CRC/CRC16.cpp
+++ b/libraries/CRC/CRC16.cpp
@@ -14,6 +14,20 @@ CRC16::CRC16()
 }
 
 
+CRC16::CRC16(uint16_t polynome, uint16_t XORstart, uint16_t XORend, bool reverseIn, bool reverseOut)
+{
+  _polynome   = polynome;
+  _startMask  = XORstart;
+  _endMask    = XORend;
+  _reverseIn  = reverseIn;
+  _reverseOut = reverseOut;
+  _crc        = 0;
+  _started    = false;
+  _count      = 0;
+  _canYield   = true;
+}
+ 
+
 void CRC16::reset()
 {
   _polynome   = CRC16_DEFAULT_POLYNOME;
diff --git a/libraries/CRC/CRC16.h b/libraries/CRC/CRC16.h
index 97361c2e..9796b62d 100644
--- a/libraries/CRC/CRC16.h
+++ b/libraries/CRC/CRC16.h
@@ -1,6 +1,4 @@
 #pragma once
-#ifndef CRC16_H
-#define CRC16_H
 //
 //    FILE: CRC16.h
 //  AUTHOR: Rob Tillaart
@@ -17,6 +15,7 @@ class CRC16
 {
 public:
   CRC16();
+  CRC16(uint16_t polynome, uint16_t XORstart, uint16_t XORend, bool reverseIn, bool reverseOut);
 
   // set parameters to default
   void     reset();       // set all to constructor defaults
@@ -39,12 +38,12 @@ public:
   void     add(uint8_t value);
   void     add(const uint8_t * array, uint16_t length);
 
-  uint16_t getCRC();  // returns CRC
-  uint32_t count()    { return _count; };
+  uint16_t getCRC();       // returns CRC
+  uint32_t count()         { return _count; };
 
   // POWER USER ONLY
-  void     enableYield()  { _canYield = true; };
-  void     disableYield() { _canYield = false; };
+  void     enableYield()   { _canYield = true; };
+  void     disableYield()  { _canYield = false; };
 
 private:
   uint16_t _reverse(uint16_t value);
@@ -64,4 +63,4 @@ private:
 
 
 // -- END OF FILE --
-#endif
+
diff --git a/libraries/CRC/CRC32.cpp b/libraries/CRC/CRC32.cpp
index b3814b46..4b4a031d 100644
--- a/libraries/CRC/CRC32.cpp
+++ b/libraries/CRC/CRC32.cpp
@@ -14,6 +14,20 @@ CRC32::CRC32()
 }
 
 
+CRC32::CRC32(uint32_t polynome, uint32_t XORstart, uint32_t XORend, bool reverseIn, bool reverseOut)
+{
+  _polynome   = polynome;
+  _startMask  = XORstart;
+  _endMask    = XORend;
+  _reverseIn  = reverseIn;
+  _reverseOut = reverseOut;
+  _crc        = 0;
+  _started    = false;
+  _count      = 0;
+  _canYield   = true;
+}
+
+
 void CRC32::reset()
 {
   _polynome   = CRC32_DEFAULT_POLYNOME;
diff --git a/libraries/CRC/CRC32.h b/libraries/CRC/CRC32.h
index 101fc497..c5170755 100644
--- a/libraries/CRC/CRC32.h
+++ b/libraries/CRC/CRC32.h
@@ -1,6 +1,4 @@
 #pragma once
-#ifndef CRC32_H
-#define CRC32_H
 //
 //    FILE: CRC32.h
 //  AUTHOR: Rob Tillaart
@@ -17,6 +15,7 @@ class CRC32
 {
 public:
   CRC32();
+  CRC32(uint32_t polynome, uint32_t XORstart, uint32_t XORend, bool reverseIn, bool reverseOut);
 
   // set parameters to default
   void     reset();       // set all to constructor defaults
@@ -39,12 +38,12 @@ public:
   void     add(uint8_t value);
   void     add(const uint8_t * array, uint16_t length);
 
-  uint32_t getCRC();  // returns CRC
-  uint32_t count()    { return _count; };
+  uint32_t getCRC();       // returns CRC
+  uint32_t count()         { return _count; };
 
   // POWER USER ONLY
-  void     enableYield()  { _canYield = true; };
-  void     disableYield() { _canYield = false; };
+  void     enableYield()   { _canYield = true; };
+  void     disableYield()  { _canYield = false; };
 
 private:
   uint32_t _reverse(uint32_t value);
@@ -64,4 +63,4 @@ private:
 
 
 // -- END OF FILE --
-#endif
+
diff --git a/libraries/CRC/CRC64.cpp b/libraries/CRC/CRC64.cpp
index 74a4c3d2..a7d85584 100644
--- a/libraries/CRC/CRC64.cpp
+++ b/libraries/CRC/CRC64.cpp
@@ -14,6 +14,20 @@ CRC64::CRC64()
 }
 
 
+CRC64::CRC64(uint64_t polynome, uint64_t XORstart, uint64_t XORend, bool reverseIn, bool reverseOut)
+{
+  _polynome   = polynome;
+  _startMask  = XORstart;
+  _endMask    = XORend;
+  _reverseIn  = reverseIn;
+  _reverseOut = reverseOut;
+  _crc        = 0;
+  _started    = false;
+  _count      = 0;
+  _canYield   = true;
+}
+
+
 void CRC64::reset()
 {
   _polynome   = CRC64_DEFAULT_POLYNOME;
diff --git a/libraries/CRC/CRC64.h b/libraries/CRC/CRC64.h
index ea14037a..68628c3b 100644
--- a/libraries/CRC/CRC64.h
+++ b/libraries/CRC/CRC64.h
@@ -1,6 +1,4 @@
 #pragma once
-#ifndef CRC64_H
-#define CRC64_H
 //
 //    FILE: CRC64.h
 //  AUTHOR: Rob Tillaart
@@ -17,6 +15,7 @@ class CRC64
 {
 public:
   CRC64();
+  CRC64(uint64_t polynome, uint64_t XORstart, uint64_t XORend, bool reverseIn, bool reverseOut);
 
   // set parameters to default
   void     reset();       // set all to constructor defaults
@@ -39,12 +38,12 @@ public:
   void     add(uint8_t value);
   void     add(const uint8_t * array, uint16_t length);
 
-  uint64_t getCRC();  // returns CRC
-  uint64_t count()    { return _count; };
+  uint64_t getCRC();       // returns CRC
+  uint64_t count()         { return _count; };
 
   // POWER USER ONLY
-  void     enableYield()  { _canYield = true; };
-  void     disableYield() { _canYield = false; };
+  void     enableYield()   { _canYield = true; };
+  void     disableYield()  { _canYield = false; };
 
 private:
   uint64_t _reverse(uint64_t value);
@@ -64,4 +63,3 @@ private:
 
 
 // -- END OF FILE --
-#endif
diff --git a/libraries/CRC/CRC8.cpp b/libraries/CRC/CRC8.cpp
index f8f09d19..5c28e4f0 100644
--- a/libraries/CRC/CRC8.cpp
+++ b/libraries/CRC/CRC8.cpp
@@ -14,6 +14,20 @@ CRC8::CRC8()
 }
 
 
+CRC8::CRC8(uint8_t polynome, uint8_t XORstart, uint8_t XORend, bool reverseIn, bool reverseOut)
+{
+  _polynome   = polynome;
+  _startMask  = XORstart;
+  _endMask    = XORend;
+  _reverseIn  = reverseIn;
+  _reverseOut = reverseOut;
+  _crc        = 0;
+  _started    = false;
+  _count      = 0;
+  _canYield   = true;
+}
+
+
 void CRC8::reset()
 {
   _polynome   = CRC8_DEFAULT_POLYNOME;
diff --git a/libraries/CRC/CRC8.h b/libraries/CRC/CRC8.h
index bd6db2b5..c7b9e1ef 100644
--- a/libraries/CRC/CRC8.h
+++ b/libraries/CRC/CRC8.h
@@ -1,6 +1,4 @@
 #pragma once
-#ifndef CRC8_H
-#define CRC8_H
 //
 //    FILE: CRC8.h
 //  AUTHOR: Rob Tillaart
@@ -17,6 +15,7 @@ class CRC8
 {
 public:
   CRC8();
+  CRC8(uint8_t polynome, uint8_t XORstart, uint8_t XORend, bool reverseIn, bool reverseOut);
 
   // set parameters to default
   void     reset();       // set all to constructor defaults
@@ -39,12 +38,12 @@ public:
   void     add(uint8_t value);
   void     add(const uint8_t * array, uint16_t length);
 
-  uint8_t  getCRC();  // returns CRC
-  uint32_t count()    { return _count; };
+  uint8_t  getCRC();       // returns CRC
+  uint32_t count()         { return _count; };
 
   // POWER USER ONLY
-  void     enableYield()  { _canYield = true; };
-  void     disableYield() { _canYield = false; };
+  void     enableYield()   { _canYield = true; };
+  void     disableYield()  { _canYield = false; };
 
 private:
   uint8_t  _reverse(uint8_t value);
@@ -63,4 +62,4 @@ private:
 
 
 // -- END OF FILE --
-#endif
+
diff --git a/libraries/CRC/README.md b/libraries/CRC/README.md
index 3756a6a3..3d3d1e8b 100644
--- a/libraries/CRC/README.md
+++ b/libraries/CRC/README.md
@@ -16,30 +16,31 @@ Arduino library with CRC8, CRC12, CRC16, CRC32 and CRC64 functions.
 Goal of this library is to have a flexible and portable set of generic 
 CRC functions and classes.
 
-The CRCx classes have a number of added values. Most important is that 
-they allow one to verify intermediate CRC values. This is useful if one 
-sends a "train of packets" which include a CRC so far. This detects both 
-errors in one packet but also missing packets, or injected packages.
+The CRCx classes have a number of added values. 
+Most important is that they allow one to verify intermediate CRC values. 
+This is useful if one sends a "train of packets" which include a CRC so far. 
+This detects both errors in one single packet but also optional missing packets, 
+or even injected packets.
 
-Another trick one can do is change the polynome or the reverse flag during 
-the process. This makes it harder to imitate.
+Another trick one can do with the class CRCx is to change the polynome or 
+the reverse flag runtime during the process. This makes it harder to imitate.
 
 Furthermore the class allows to add values in single steps and continue too.
 
-Finally the class version gives more readable code (imho) as the parameters 
+Finally the class version gives more readable code (IMHO) as the parameters 
 are explicitly set.
 
 
 **Note** the classes have same names as the static functions, except the class
-is UPPER case. So CRC8 is a class and **crc8()** is the function. 
+is UPPER case. So **CRC8** is a class and **crc8()** is the function. 
 
-Deeper tech info -https://en.wikipedia.org/wiki/Cyclic_redundancy_check
+Deeper tech info - https://en.wikipedia.org/wiki/Cyclic_redundancy_check
 and many other websites.
 
 
 ## Interface CRC classes
 
-These interfaces are very similar for CRC8, CRC12, CRC16, CRC32 and CRC64 class.
+The interfaces are very similar for CRC8, CRC12, CRC16, CRC32 and CRC64 class.
 The only difference is the data type for polynome, start- and end-mask, 
 and the returned CRC.
 
@@ -48,8 +49,9 @@ and the returned CRC.
 
 Use **\#include "CRC8.h"**
 
-- **CRC8()** Constructor.
-- **void reset()** set all internals to constructor defaults.
+- **CRC8()** default - parameterless - constructor.
+- **CRC8(polynome, XORstart, XORend, reverseIn, reverseOut)** Constructor to set all parameters at once.
+- **void reset()** set all internals to defaults of the **CRC8()** parameterless constructor.
 - **void restart()** reset internal CRC and count only;
 reuse values for other e.g polynome, XOR masks and reverse flags.
 - **void add(value)** add a single value to CRC calculation.
@@ -62,7 +64,7 @@ a really large stream at intermediate moments, e.g. to link multiple packets.
 
 #### parameters
 
-These parameters do not have defaults so the user must set them explicitly.
+The parameters do not have defaults so the user must set them explicitly.
 
 - **void setPolynome(polynome)** set polynome, note reset sets a default polynome.
 - **void setStartXOR(start)** set start-mask, default 0.
@@ -92,6 +94,9 @@ to handle interrupts etc. So use at own risk.
 _Note: the static functions in this library also call **yield()** but this 
 cannot be disabled (for now)._
 
+_Note: a parameter could be a future option to set the number of adds before 
+**yield()** is called. **setYield(0)** would be disable it._
+
 
 ### Example snippet
 
@@ -163,17 +168,20 @@ See examples.
 - https://en.wikipedia.org/wiki/Cyclic_redundancy_check - generic background.
 - http://zorc.breitbandkatze.de/crc.html - online CRC calculator (any base up to 64 is supported.)
 - https://crccalc.com/ - online CRC calculator to verify.
+- https://www.lddgo.net/en/encrypt/crc - online CRC calculator
 
 
 ## Future
 
 - extend examples.
   - example showing multiple packages of data linked by their CRC.
-- table versions for performance?  (performance - memory discussion)
+- table versions for performance?  (performance - memory discussion).
 - stream version - 4 classes class?
 - **setCRC(value)** to be able to pick up where one left ?
   - can be done with **setStartXOR()**
-  - needs **getRawCRC()**  without reverse and endmask
+  - needs **getRawCRC()**  without reverse and end mask
+- Think about default parameters for constructor **CRC8(polynome, XORstart, XORend, reverseIn, reverseOut)** 
+  - same as reset so constructors merge? Note the CRC-functions do have defaults too.
 
 
 #### Exotic CRC's ?
@@ -184,6 +192,8 @@ See examples.
 - One CRC() with #bits as parameter?
   - up to 64 bit for all missing ones.?
   - performance penalty
+- One CRC() template class?
+
 
 #### Won't
 
diff --git a/libraries/CRC/library.json b/libraries/CRC/library.json
index f15c0515..e010dcd6 100644
--- a/libraries/CRC/library.json
+++ b/libraries/CRC/library.json
@@ -15,7 +15,7 @@
     "type": "git",
     "url": "https://github.com/RobTillaart/CRC"
   },
-  "version": "0.3.0",
+  "version": "0.3.1",
   "license": "MIT",
   "frameworks": "arduino",
   "platforms": "*",
diff --git a/libraries/CRC/library.properties b/libraries/CRC/library.properties
index a91cf59c..eec88e67 100644
--- a/libraries/CRC/library.properties
+++ b/libraries/CRC/library.properties
@@ -1,5 +1,5 @@
 name=CRC
-version=0.3.0
+version=0.3.1
 author=Rob Tillaart <rob.tillaart@gmail.com>
 maintainer=Rob Tillaart <rob.tillaart@gmail.com>
 sentence=Library for CRC for Arduino
diff --git a/libraries/CRC/releaseNotes.md b/libraries/CRC/releaseNotes.md
index 4d708dfd..addd4fb0 100644
--- a/libraries/CRC/releaseNotes.md
+++ b/libraries/CRC/releaseNotes.md
@@ -1,6 +1,16 @@
 
 # Release Notes
 
+## 0.3.1  2022-05-20
+
+- added constructors with all parameters. 
+  CRC16(uint16_t polynome, uint16_t XORstart, uint16_t XORend, bool reverseIn, bool reverseOut);
+  All five parameters are mandatory, no defaults in this first release. 
+  Defaults would allow the constructors merge in the future.
+- remove #ifndef Header guards as these are not needed.
+- update documentation
+
+
 ## 0.3.0  2022-04-15
 
 - split CRC.h in CRC.h and CRC.cpp to fix #21 (again)
diff --git a/libraries/CRC/test/unit_test_crc12.cpp b/libraries/CRC/test/unit_test_crc12.cpp
index 7b88d539..629fd634 100644
--- a/libraries/CRC/test/unit_test_crc12.cpp
+++ b/libraries/CRC/test/unit_test_crc12.cpp
@@ -88,6 +88,15 @@ unittest(test_crc12)
 
 }
 
+unittest(test_crc12_param)
+{
+  fprintf(stderr, "TEST CRC12\n");
+
+  CRC12 crc(0x080D, 0, 0, false, false);
+  crc.add(data, 9);
+  assertEqual(0xEFB, crc.getCRC());
+}
+
 
 unittest_main()
 
diff --git a/libraries/CRC/test/unit_test_crc16.cpp b/libraries/CRC/test/unit_test_crc16.cpp
index 852d1c92..3e6e4dd0 100644
--- a/libraries/CRC/test/unit_test_crc16.cpp
+++ b/libraries/CRC/test/unit_test_crc16.cpp
@@ -304,6 +304,16 @@ unittest(test_crc16)
 }
 
 
+unittest(test_crc16_param)
+{
+  fprintf(stderr, "TEST CRC16 PARAM\n");
+
+  CRC16 crc(0x1021, 0xFFFF, 0, false, false);
+  crc.add(data, 9);
+  assertEqual(0x29B1, crc.getCRC());
+}
+
+
 unittest_main()
 
 // --------
diff --git a/libraries/CRC/test/unit_test_crc32.cpp b/libraries/CRC/test/unit_test_crc32.cpp
index e2b4bcb5..727a282c 100644
--- a/libraries/CRC/test/unit_test_crc32.cpp
+++ b/libraries/CRC/test/unit_test_crc32.cpp
@@ -180,6 +180,16 @@ unittest(test_crc32)
 }
 
 
+unittest(test_crc32_param)
+{
+  fprintf(stderr, "TEST CRC32 PARAM\n");
+
+  CRC32 crc(0x04C11DB7, 0xFFFFFFFF, 0xFFFFFFFF, true, true);
+  crc.add(data, 9);
+  assertEqual(0xCBF43926, crc.getCRC());
+}
+
+
 unittest_main()
 
 // --------
diff --git a/libraries/CRC/test/unit_test_crc64.cpp b/libraries/CRC/test/unit_test_crc64.cpp
index 14702702..25f2004b 100644
--- a/libraries/CRC/test/unit_test_crc64.cpp
+++ b/libraries/CRC/test/unit_test_crc64.cpp
@@ -84,6 +84,20 @@ unittest(test_crc64)
 }
 
 
+unittest(test_crc64_param)
+{
+  fprintf(stderr, "TEST CRC64 PARAM\n");
+
+  fprintf(stderr, "no reference yet\n");
+  assertEqual(1, 1);
+  
+  // just a dummy test
+  CRC64 crc(0x04C11DB704C11DB7, 0, 0, false, false);
+  crc.add(data, 9);
+  assertEqual(0xCE5CA2AD34A16112, crc.getCRC());  // 14869938934466568466
+}
+
+
 unittest_main()
 
 // --------
diff --git a/libraries/CRC/test/unit_test_crc8.cpp b/libraries/CRC/test/unit_test_crc8.cpp
index 3959d482..9d640d30 100644
--- a/libraries/CRC/test/unit_test_crc8.cpp
+++ b/libraries/CRC/test/unit_test_crc8.cpp
@@ -150,6 +150,16 @@ unittest(test_crc8)
 }
 
 
+unittest(test_crc8_param)
+{
+  fprintf(stderr, "TEST CRC8 PARAM\n");
+
+  CRC8 crc(0x07, 0, 0, false, false);
+  crc.add(data, 9);
+  assertEqual(0xF4, crc.getCRC());
+}
+
+
 unittest_main()
 
 // --------