Wow; thanks for all the rapid responses.
Reduced to a minimum here's what works and what does not.
Using digitalWrite to a DIFFERENT PORT pin does not cause any problem.
So it looks like DigitalWrites outside should not be used in ISRs as well. Doing port writes don't seem to cause any problems.
// V0.1B Data streaming
// Platform; Arduino Mini 16Mhz
// IDE Arduino 0017
// V0.1A 27 Jan 2010 Set up 250usec interrupt and toogle pin 13
// V0.1B 28 Jan 2010 Biphase data streaming on pin 12; sync pulses on pin 13
//------------------------------------------------------------------------------
// #define dataPin 12
#define led 13
volatile byte temp1;
volatile byte toggle;
volatile byte data[9];
//#define led 13
#define dataPin 12
volatile byte i=0;
volatile byte j=0;
volatile byte buffer;
volatile byte mask;
volatile boolean dataBit=0;
//-------------------------------- set up ---------------------------------------
void setup(){
//set up timer 2 cotrol registers here
TCCR2A = B00000000; //select Normal Mode,freerunning counter, no connection to hardware pins
TCCR2B = B00000011; //normale, prescaler equals clock/32. With 16Mhz clock this give 2uSec clock to the TCNT2
OCR2A = 125; //initialise compare register for first interrupt
TIMSK2 = B00000010; //generate interrupt on output compare match A
pinMode(led,OUTPUT); // set DDR
pinMode(dataPin,OUTPUT);
pinMode(11,OUTPUT);
pinMode(7,OUTPUT);
data[1]=B00000000; // dummy data
data[0]=B00011001; // dummy data
buffer=data[0];
mask=B00000001;
}
//-------------------------------- interrupt service routine ---------------------
ISR (TIMER2_COMPA_vect){
OCR2A += 125; //add 250uSec increment to timer2
toggle = !toggle;
//digitalWrite(led,toggle); // nice stable 2Khz square wave here just for initial testing
digitalWrite(led,1); // start pulse for synching 'scope
digitalWrite(dataPin,dataBit); //write nice stable biphase data to output
if (i!=1) { // shift data only every second interrupt
i=1;
dataBit=!dataBit; // data bit must be toggledat every data read
temp1 =buffer & mask; //put zero or non zero depending on the state of the bit under the mask
mask=mask<<1; // shift the mask left 1 bit
if (mask==0) {
mask=B00000001; // reset the mask bit
j=j+1; // increment data pointer
if (j>1) {
j=0;
}
buffer=data[j]; // get the next data byte
}
}
else{
i=0;
if (temp1!=0) { // if bit to be sent is a 1 then toggle dataBit again on the next interrupt
dataBit=!dataBit;
}
}
digitalWrite(led,0); //end sync pulse. THe pulse length indicates the fraction used of the time available
// So far this code uses about 10uSec out of the 250uSec period. There is still
// space for a fair bit of code.
}
//--------------------------------------- main loop ------------------------------
// this does notcause any problems with waveform made in ISR
void loop(){
digitalWrite(7,HIGH);
digitalWrite(7,LOW);
//this affects ISR
//digitalWrite(11,HIGH);
//digitalWrite(11,LOW);
}