The joy of bit-banging, part 1, Serial Transmit
Originally posted June 4, 2009. Updated May 11, 2019.
Need a good bit-bang? Sometimes you need one more UART than what your microcontroller has. In that case you need to bit-bang it out. "Bit-banging" sounds much more vulgar than it actually is - it just means that instead of using the built-in shift register, you are manually turning on/off the bits. For this example I implement it using a method that uses blocking waits, between bits. A better implementation would be to use a timer ISR to generate the bit timings. You can bit bang most serial protocols: SPI, I2C, or plain old asynchronous serial.
The most common asynchronous serial data format is 8N1: 8 data bits, no parity bits, one stop bit. This only tells part of the story. Actually there are a total of 10 bits: (see wikipedia)
One start bit (a "1")
Eight data bits
One stop bit (a "0")
For 9600 baud, bit spacing is 104uSec. The calculation of this is left as an exercise for the reader. To perfect the bit spacing I first just twiddled a bit on and off with the delay loop between bits until I got exactly 104uSec.
On the hardware side, I ran the output of this through a simple level converter and then into an RS-232 port on a PC to verify that everything was working ok. One level shifter I like is available from sparkfun.com here. Surprisingly, this part is still available in 2019. It will be more difficult to find a PC with a serial port though. However, we usually use the FTDI USB to serial converter.
Notes on the code: nowadays we exclusively use the C fixed width integer data types (uint16_t, etc.) and not integers with unspecified size.
/**
* Bit-Bang UART transmit
* Transmits one byte out the specified pin
* Baud rate is 9600 (104uSec bit timing)
* 8-N-1: 8 data bits, no parity, 1 stop bit
*
* Line is nominally at '1' (high)
* 1 Start bit = '0'
* Data, MSB first, LSB last. '1' = line high, '0' = line low
* 1 Stop bit = '1' (idle, or high)
*
* PRECONDITION: LINE IS high (idle) and port/bit configured for output!
*/
#define BIT_BANG_TX_PORT P4OUT
#define BIT_BANG_TX_BIT BIT6
#define BIT_LENGTH_9600 160
void bitBangOutput(unsigned char byte)
{
//start bit - pull line down
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT;
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
//LSB
if (byte & BIT0)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
if (byte & BIT1)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
if (byte & BIT2)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
if (byte & BIT3)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
if (byte & BIT4)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
if (byte & BIT5)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
if (byte & BIT6)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
//MSB
if (byte & BIT7)
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //'1' = line high
else
BIT_BANG_TX_PORT &= ~BIT_BANG_TX_BIT; //'0' = line low
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
//Stop bit
BIT_BANG_TX_PORT |= BIT_BANG_TX_BIT; //let line go high
for (unsigned int i = 0; i < BIT_LENGTH_9600; i++) ; //104uSec delay
}