nonblocking Software Serial Maple CortexM3

// Non-blocking simple software serial send example
// written for maple rev5, using hardware timer.
// Author: J.Heinrich 09-11-2011, www.heisystec.com
//... some times 3 ARM CortexM3 USART's arn't enough... ;-)
// This code is provided as is under the GNU public license.

uint8 LCDtransmitPin= 14;   //5V tolerant on Maple rev5
HardwareTimer timerLCD(2);  //initializing hardware timer instance

volatile int   LCDbitcnt  =  9; //starts with -1 on the stop bit
volatile int   LCDwritePt =  -1;//writing index on output buffer
volatile int   LCDreadPt  =  -1;//reading index of output buffer
volatile char  LCDoutbuf[128];  //Software serial output buffer

void setup(){
    LCDbegin(38600);
}

void loop(){
    char s[] = "Hello World";
    LCDprint(s);
    delay(1000); //Take care not to overfill the output buffer,
    //do not send more than %buffersize% chars than can be send with
    //current baud rate by time.
    //NONBLOCKING (fire and forget...):
    //There are NO delay() function calls in the sending routine.
    //After putting chars into the output buffer by LCDprint(),
    //the maple hardware timer will do the rest.
    //At a baud rate 38600 this yields about 1800 processor cycles more each bit(!)
    //available for other stuff - compared with a software serial timing with
    //delay(BitPeriod) for each bit. MAPLE 74MHz ROCKS!
    //Means: each byte, you win about 15.000 proc cycles.
}


void LCDbegin(int baud){
    pinMode(LCDtransmitPin, OUTPUT);//NOTE: pin must be pulled up by 10kOhm to +3,3V externally
    //in order to avoid noise output during boot-up or flashing the maple
    LCDbaudRate = baud;
    LCDbitPeriod = 1000000 / LCDbaudRate; //[µs] Binary modulation, bit rate = baud rate
    digitalWrite(LCDtransmitPin, HIGH); //prepare start bit...

    //preparing the maple hardware timer======:
    timerLCD.pause();
    timerLCD.setChannel1Mode(TIMER_OUTPUT_COMPARE);
    timerLCD.setCompare(TIMER_CH1, 1);  // Interrupt 1 count after each update
    timerLCD.setPeriod(LCDbitPeriod);   // in microseconds
    timerLCD.attachCompare1Interrupt(LCDnextBit); //interrupt handler attached
    timerLCD.refresh();//resetting the timer counter
    timerLCD.resume(); //runs endlessly, waiting for bytes in the send buffer
}

//Print a string i.e. array of char =============:
void LCDprint(const char *s){
  while (*s)
    LCDprint(*s++);
}

//some basic print routine for separate char/byte =============:
void LCDprint(uint8 b){
   if (LCDwritePt==127){LCDwritePt=0;} //increment write pointer
   else {LCDwritePt++;}
   LCDoutbuf[LCDwritePt] = b; //new byte to send-buffer,
   //send-timer runs indefinitely, fetches new bytes, and sends them
}

//timer interrupt routine, beeing called infinitely each bit period =:
//Concerning communication speed, we loose one bit period each byte,
//but we need one more bit cycle to fetch the next byte,
//case: LCDbitcnt=9.
//This gives the receiver about 26 µsec @38600 baud to do something else...
void LCDnextBit(){
    switch(LCDbitcnt){
        case -1:
            digitalWrite(LCDtransmitPin, LOW); //send start bit
            LCDbitcnt++;
        break;
        case 0: //do for all 8 data bits the same stuff:
        case 1:
        case 2:
        case 3:
        case 4:
        case 5:
        case 6:
        case 7:
            if (bitRead(LCDoutbuf[LCDreadPt],LCDbitcnt)==1){
                digitalWrite(LCDtransmitPin,HIGH); // send 1
            }
            else{
                digitalWrite(LCDtransmitPin,LOW); // send 0
            }
            LCDbitcnt++; //increment bit counter
        break;
        case 8:
            digitalWrite(LCDtransmitPin, HIGH); //send stop bit
            LCDbitcnt++; //increment bit counter
        break;
        case 9:
            if (LCDreadPt!=LCDwritePt){//something new in output buffer
                if (LCDreadPt==127){LCDreadPt=0;}
                else {LCDreadPt++;}         //if 127, reset to zero
                LCDbitcnt  = -1;  //send start bit on next timer
            }
        break;
    }
}