Bit banging

Bit banging is a technique for serial communications using software instead of dedicated hardware. Software directly sets and samples the state of pins on the microcontroller, and is responsible for all parameters of the signal: timing, levels, synchronization, etc. In contrast to bit banging, dedicated hardware (such as a modem, UART, or shift register) handles these parameters and provides a (buffered) data interface in other systems, so software is not required to perform signal demodulation. Bit banging can be implemented at very low cost, and is used in, for example, embedded systems.

Although it is often considered to be something of a hack, bit banging does allow the same device to use different protocols with minimal or no hardware changes required.

C code example

Sending a byte on an SPI bus.

// transmit byte serially, MSB first
void send_8bit_serial_data(unsigned char data)
{
   int i;

   // select device
   output_high(SD_CS);

   // send bits 7..0
   for (i = 0; i < 8; i++)
   {
       // consider leftmost bit
       // set line high if bit is 1, low if bit is 0
       if (data & 0x80)
           output_high(SD_DI);
       else
           output_low(SD_DI);

       // pulse clock to indicate that bit value should be read
       output_low(SD_CLK);
       output_high(SD_CLK);

       // shift byte left so next bit will be leftmost
       data <<= 1;
   }

   // deselect device
   output_low(SD_CS);
}

Problems

There are some problems with bit banging. The software emulation process consumes more processing power than does supporting dedicated hardware. The microcontroller spends much of its time reading or sending samples to and from the pin, at the expense of other tasks. The signal produced usually has more jitter or glitches, especially if the processor is also executing other tasks while communicating. However, if the bit-banging software is interrupt-driven by the signal, this may be of minor importance, especially if control signals such as RTS, CTS, or DCD are available.

See also

• Bit manipulation
• Bit twiddler (disambiguation)

External links

Asynchronous serial (RS-232)

• Notes on bit-banging async serial
• Bit banging for Async Serial Communication
• Bit banging for RS-232

I²C bus

• I2C on AVR using bit banging

SPI bus

• Efficient bit-banged SPI for 8051 microcontroller