What is Bit Banging and How to use it?

What is Bit Banging?

Bit banging is a software-based method of interfacing with external devices using standard input/output operations rather than specialized peripheral interfaces or hardware modules. The term “bit banging” comes from the fact that the software is directly “banging” (setting or clearing) bits to control an interface.

Imagine you’re working on an embedded project using an older or a very basic microcontroller that does not have a built-in hardware SPI module. You need to interface with an SPI-based EEPROM chip to store some configuration data.

The SPI protocol typically involves the following signals:

• 𝐌𝐎𝐒𝐈 (Master Out Slave In): Data line for sending data from the master to the slave.
• 𝐌𝐈𝐒𝐎 (Master In Slave Out): Data line for sending data from the slave to the master.
• 𝐒𝐂𝐊 (Serial Clock): Clock signal generated by the master to synchronize data transfer.
• 𝐂𝐒/𝐒𝐒 (Chip Select/Slave Select): Signal to activate a particular slave (in this case, the EEPROM).

To write a byte 𝟎𝐛𝟏𝟏𝟎𝟎𝟏𝟎𝟏𝟎 to the EEPROM, you’d need to generate the appropriate signals on these lines in sync with the SPI protocol.

Without a dedicated SPI hardware module, you’d use general-purpose I/O pins on your microcontroller to emulate the SPI signals. Here’s a simplified step-by-step process for sending the byte 0b11001010:

1. Set the CS/SS line low to select the EEPROM.
2. For each bit in the byte 0b11001010:
   • If the bit is 1, set MOSI high; if it’s 0, set MOSI low.
   • Toggle the SCK line (low to high to low) to indicate to the EEPROM to read the bit on the MOSI line.
3. Set the CS/SS line high to deselect the EEPROM and finish the operation.

For the byte 0b11001010:

1         High   Toggle SCK (L->H->L)
1         High   Toggle SCK (L->H->L)
0         Low    Toggle SCK (L->H->L)
0         Low    Toggle SCK (L->H->L)
1         High   Toggle SCK (L->H->L)
0         Low    Toggle SCK (L->H->L)
1         High   Toggle SCK (L->H->L)
0         Low    Toggle SCK (L->H->L)

This is just a simplified example. This manual control, bit-by-bit, of the interface is what constitutes “bit banging.”

How to observe bit-banging in the above?

1. Manually setting the CS/SS line low using a digital I/O operation.
2. Iterating over each bit of the byte 0b11001010:
   • Manually setting the MOSI line state based on each bit (high for ‘1’ and low for ‘0’) using a digital I/O operation.
   • Manually toggling the SCK line (low to high to low) using digital I/O operations.
3. Manually setting the CS/SS line high using a digital I/O operation.

Each of these steps involves software directly manipulating or “banging” the bits of the I/O pins to generate the desired signals, rather than relying on a specialized hardware module to do it. This direct control by the software is the essence of bit banging.

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