I am trying to create a stable communication channel without any mis-synchronization. In the SoftwareSerial library, the write function transmits one byte at a time. The mis-synchronization occurs when the receiver misjudges which bit is the start bit. Why is this a problem? Because the internal delay between the start bit and the first message bit is different from the internal delay between each message bit.
One of the really good suggestions I have heard is to use Manchester encoding on the Arduino. While I am testing out this option, I also wanted advice on altering the software serial library in the following way. If I modify the write function to transmit 128 bytes instead of 1, then this can avoid the problem I explained above, I assume.
Here is how I have altered the write function of SoftwareSerial. I haven't thought of any issues with this approach, but does anyone else see any problems? Here is the whole altered function. (I changed the parameter, and added 2 new loops - one to invert all the bytes if(inv) and another loop outside the loop that transmits each bit of a byte. I have attached the original function, further below.
size_t SoftwareSerial::write(byte* b)
{
if (_tx_delay == 0) {
setWriteError();
return 0;
}
// By declaring these as local variables, the compiler will put them
// in registers _before_ disabling interrupts and entering the
// critical timing sections below, which makes it a lot easier to
// verify the cycle timings
volatile uint8_t *reg = _transmitPortRegister;
uint8_t reg_mask = _transmitBitMask;
uint8_t inv_mask = ~_transmitBitMask;
uint8_t oldSREG = SREG;
bool inv = _inverse_logic;
uint16_t delay = _tx_delay;
//Doubtful of this loop ------------------
for(int i = 0; i < 128; i++)
{
if (inv)
b[i] = ~(b[i]);
}
cli(); // turn off interrupts for a clean txmit
// Write the start bit
if (inv)
*reg |= reg_mask;
else
*reg &= inv_mask;
tunedDelay(delay);
for(int j = 0; j < 128; j++)
{
// Write each of the 8 bits
for (long i = 8; i > 0; --i)
{
if (b[j] & 1) // choose bit
*reg |= reg_mask; // send 1
else
*reg &= inv_mask; // send 0
tunedDelay(delay);
b[j] >>= 1;
}
}
// restore pin to natural state
if (inv)
*reg &= inv_mask;
else
*reg |= reg_mask;
SREG = oldSREG; // turn interrupts back on
tunedDelay(_tx_delay);
return 1;
}
Here is the original function.
size_t SoftwareSerial::write(uint8_t b)
{
if (_tx_delay == 0) {
setWriteError();
return 0;
}
// By declaring these as local variables, the compiler will put them
// in registers _before_ disabling interrupts and entering the
// critical timing sections below, which makes it a lot easier to
// verify the cycle timings
volatile uint8_t *reg = _transmitPortRegister;
uint8_t reg_mask = _transmitBitMask;
uint8_t inv_mask = ~_transmitBitMask;
uint8_t oldSREG = SREG;
bool inv = _inverse_logic;
uint16_t delay = _tx_delay;
if (inv)
b = ~b;
cli(); // turn off interrupts for a clean txmit
// Write the start bit
if (inv)
*reg |= reg_mask;
else
*reg &= inv_mask;
tunedDelay(delay);
// Write each of the 8 bits
for (uint8_t i = 8; i > 0; --i)
{
if (b & 1) // choose bit
*reg |= reg_mask; // send 1
else
*reg &= inv_mask; // send 0
tunedDelay(delay);
b >>= 1;
}
// restore pin to natural state
if (inv)
*reg &= inv_mask;
else
*reg |= reg_mask;
SREG = oldSREG; // turn interrupts back on
tunedDelay(_tx_delay);
return 1;
}
The serial buffer can only hold 64 bytes at a time, but I am using the following code, courtesy of Nick Gammon to read the data from serial buffer without blocking. So it shouldn't be a problem on the receiving end either, I hope.
The mis-synchronization occurs when the receiver misjudges which bit is the start bit.- that would happen. You would need to allow a really big gap (greater than 1024 bits) to stop this happening. This gap would wipe out any savings by not just sending 8 bits at a time as is normal. I don't see what this technique achieves.