Was heisst "bleibt stehen" ?
   "error opening datalog.txt"  ?

oder gar nix ?

Undefiniertes Verhalten ist of ein Speicherproblem.
Ich fürchte, SD.open() braucht 512 byte freien RAM am Stück. ( Evtl auch je nach blocksize mehr ?)

Auch ein 2560 hat nur begrenzt RAM.

Aber schon merkwürdig, dass es mit "0" ewig gut geht, mit anderen Werten aber nicht.

dataString +=  ... ; ist ziemlich aufwändig !
Und wenn dann die temporären Teil-Strings unterschiedlich lang sind, kann es evtl. zu mehr  Problemen führen, als wenn es sich immer um  String(",") ound String("0") handelt ...

Ich würde ein statisches char array[80] nehmen, und das mit sprintf füllen ( evtl. auch hübsch in Spalten ausgerichtet).

Im playground gibt es mem_check und ähnliche ...

Your code nearly does it already:
 what's the purpose of int x  ( the one inside getFloatFromSerialMonitor : you might use that as index into your float array to return. )

Questions remaining :
How do you distinguish between 2 numbers ? ( A comma , semicolon, space ?)
Do you know in advance how many floats you get ?

modify float getFloatFromSerialMonitor() to
float* get3FloatsFromSerialMonitor()

Finding nice code is great, but latest to enhance it, you need to understand it.

Another hint
Your unused String str; shows that you are not afraid of memory problems. Perhaps dynamically creating the float array might work for you, too.
It's better however, if the calling loop() provides the float array, and if you remove that String str;

Deiner Konstruktion

Code:

String cppString;
char cstring[] = "  3.45\000xy";  // so sollte  das Ergebnis von dtostrf(val,6,2, target); aussehen: 7 der 10 byte definiert
cppString = cstring;

traue ich nicht über den Weg, aber ...

 ... ausnahmsweise ist der böse String wohl nicht schuld, da :

>>Was passiert denn, wenn Du direkt den StrBuffer per Serial.println() ausgibst?
>    das A ist immer noch dran.
^{( Du meinst sicher " ist schon dran " )}

[avrdude: programmer operation not supported]

Solution: the chips were fused to require an external crystal.

I had to take my only UNO from its project ( I don't have other crystals, as I think 8MHz is fast enough ),
replace the chip with the other one, attach usbtinyISP to the UNO and ran my read signature cmd:

Works perfect.  No need for -b 115200 option (which I don't understand, btw), no need for -B250

lfuse was FF : changed fuses to recommendation by http://www.engbedded.com fuse calculator successfully

Moved modified chip to my minimal breadboard, attached usbtinyISP: works !

--> avrdude: programmer operation not supported   is just for fun, this message persists,
but avrdude: initialization failed, rc=-1 is serious, the -F option suggestion is rather useless then.

_{(added this follow up just in case other newbies have the patience to read this thread to the end)}

I guess, once OP has learned how to enter a float number via Serial into a float variable,
he could also set it to NaN, -INF, +INF ,  if he really wants to.

I'm not sure what the word "support" means in that scenario, but these bit patterns are possible, the ATMEGA 328 controller itself does not care.

What is a "less black hole" ?  _{-- just to keep this thread off topic} 

Ich habe aktuell etwa 20 unterschiedliche Funktionen, die vom 84'er ausgeführt werden sollen

OK
Zählt da die Funktion "nix tun" dazu ?
Sind das Dauerfunktionen ?
Wie schnell soll der Wechsel zwischen Funktionen sein ?

pulseIn(pin, HIGH) wartet übrigens auf LOW und dann wieder auf HIGH
Damit pulseIn sauber funktioniert, müssen also immer Pulse kommen!

Ein Vorschlag:
    0 -  1999 µs = Grundzustand    delay(1)

 2000 -  4000 µs = Funktion A      delay(3)
 4000 -  6000 µs = Funktion B
 ...
52000 - 54000 µs = Funktion Z      delay(53)
54000 - ...      =    Fehler  ???

Zum Dekodieren:

Code:

byte funktion = (byte)(pulseIn(pin, HIGH)/2000)*2 + 1;
 switch (funktion) {
   case 1: break;
   case 3: Funktion_A(); break;
   ...
 }

*2 + 1 ist nur dafür da, um auf Sender- (delay ) und Empfängerseite (case ) die gleichen Werte zu haben, ...

Serial.read() reads a character and returns that ( or -1 while nothing is available)
This is just the starting point.
I recommend to study Nick's great tutorial on Serial http://gammon.com.au/serial to get started...

wie genau dieses Verfahren ist

Das war eher ein genereller Einwand.
Ganz abgesehen davon, dass pulseIn micros liefert, dein delay aber mit millis arbeitet.

Wenn du mehrere verschiedene diskrete ( = unterschiedliche, mit unterschiedlicher Bedeutung ) Werte übertragen willst, wäre eine seriell codierte Übertragung besser.
Wenn du aber ein quasi analoges Signal haben willst (PWM nur ohne elektrische Glättung), ist dein
if(duration == 20) {}
ein unpassendes Verfahren

Und was spricht gegen
Serial.print(var,2);
?

...
if (duration == 20){...}
...

Korrekt?

Fast. Was passiert, wenn du evtl. 19 oder 21 als duration gemessen hast?
_{Sei's drum dass ich es Pech nenn',}
^{man muss doch damit rechnen}

The variable named pointer is not a pointer. Lousy choice of names.

Agreed, but "hey, it's his code"

Strings are evil.  

Code:

char test[] =  "23456";

void setup()
{
   Serial.begin(9600);
}
void loop()
{
  int i=0;
  while (i < strlen(test) )
  {  
     char pointer = test[i++];
     Serial.println(pointer);
     delay(1000);
  }
}

Instead of boolean ( which are bytes ) you can work with single bits.

Instead of if (xyz == true) you'd use if (xy & Z_BIT)  or something similar.

You need to know where each bit sits, of course. In the pseudo code above the byte xy hosts a couple of single bits , Z_BIT being one of them.

If you're not familiar with c syntax on single bits, the arduino functions bitSet(), bitClear(), bitRead(), bitWrite() might be interesting for you.

It's definitely not 1024 millis in a second. ( Sometimes it does not take 1000 µs fom one millis() value to the next, to compensate for timer0 )

And, on my arduino UNO with 16MHz crystal, it's usually  (at 23 °C) rather 998 to 999 millis() in a second.

But 3600000UL is precise enough for your "requirement"  (+- 5 min/h)

I rechecked the wiring of my 10pin cable between usbtiny and atmega chip and I hope this is correct:
atmega       usbtiny         atmega
pin                          pins
17    MOSI    1   2    +5V   7, 20
              3
 1   RESET    5
19     SCK    7
18    MISO    9  10    GND   8, 22


Then I got a standalone WinAVR-20100110-install.exe and ran this avrdude cmd, to simply read the signature:

Code:

C:\WinAVR-20100110\bin>avrdude -p m328p -c usbtiny -v -n -F -U signature:r:mysig.txt:h

avrdude: Version 5.10, compiled on Jan 19 2010 at 10:45:23
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2009 Joerg Wunsch

         System wide configuration file is "C:\WinAVR-20100110\bin\avrdude.conf"

         Using Port                    : lpt1
         Using Programmer              : usbtiny
         AVR Part                      : ATMEGA328P
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :

                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom        65     5     4    0 no       1024    4      0  3600  3600 0xff 0xff
           flash         65     6   128    0 yes     32768  128    256  4500  4500 0xff 0xff
           lfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           lock           0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00

         Programmer Type : USBtiny
         Description     : USBtiny simple USB programmer, http://www.ladyada.net/make/usbtinyisp/
avrdude: programmer operation not supported

avrdude: Using SCK period of 10 usec
avrdude: initialization failed, rc=-1
avrdude: AVR device initialized and ready to accept instructions
avrdude: Device signature = 0x000000
avrdude: Yikes!  Invalid device signature.
avrdude: Expected signature for ATMEGA328P is 1E 95 0F

avrdude done.  Thank you.

programmer operation not supported Is this the root cause ? What does this mean ?
avrdude: initialization failed, rc=-1  This is certainly bad !
avrdude: AVR device initialized and ready to accept instructions I do not get this, however ???
avrdude: Device signature = 0x000000 Which is certainly wrong.


I'm new to programmers in general, usbtinyISP, barebone atmega µc, bootloader ...
The actual chip is new for me.  What else can I do with it to test it or get it alive ? Should I rather forget/return it ?
Or is it rather a problem between usbtinyISP and avrdude ?  ( programmer operation not supported )