Electronic – Issues related to Bit-banging transceiver modules

Bit banging is creating the whole series of pulses in software, instead of relying on a piece of hardware inside the microcontroller.

Many microcontrollers have a hardware SPI, and then all you have to do is write a byte to the output register, and the SPI controller will shift the data out, and at the same time receive data from the slave. You can get an interrupt when the transfer is complete, and then read the received data.

But some microcontrollers don't have the SPI hardware on board and then you have to simulate it by doing everything manually. SPI has a number of different modes, I'll use this pulse diagram as an example:

So while a dedicated SPI controller takes care of all the pulses, data shifting and timing, when bit-banging you have to take every action yourself:

Make Slave Select low  
Short delay
Do 8 times
  Make the SCK (Serial Clock) pin low 
  Make the MOSI (Master-Out-Slave-In) pin high or low depending on bit 7 of the data  
  Add brief delay  
  Make the SCK output high
  Read MISO (Master-In-Slave-Out) pin
  Shift received data left, and shift the bit just read in as bit 0   
  Add brief delay  
  Shift the data byte 1 bit left
Make Slave Select high again  

Bit-banging SPI is relatively simple, the code for bit-banging I2C for instance will be more complex, and you'll need a timer somehow if you want to bit-bang the UART protocol.

While your calculation of 104uS is correct for 9600BPS your loop and the various operations it performs will be adding an additional delay. There are a few ways you could go about tuning the timing:

• Subtract a constant from your uS delay until it starts working. It'd probably be best to determine the minimum / maximum number it works with and pick the middle value.

• Do something similar using a scope to check the final timing if you have one available.

• Look at the assembler output from the compiler and determine how many cycles the loop takes.

I also see you're using the RC clock. I normally like to keep my serial timing within 2% for reliable operation so also check to make sure the part has that much stability when using the RC clock for reliable operation.