├── README.md
├── LICENSE.txt
└── main.c


/README.md:
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 1 | Software ("bit-bang") UART Transmitter (  8 data bits, 1 stop bit, no parity ) for Attiny24A
 2 | this can easily be ported to other AVR8 µC's or might even work without changes on many of them
 3 | 
 4 | the Baud rate is calculated as follows:
 5 | BAUD = F_CPU / ( TIMER0_PRESCALER * (OCR0A + 1)
 6 | so we can modify the prescaler and/or the OCR0A value to achieve a certain baud rate.
 7 | 
 8 | In this example I am using the internal 8MHz oscillator as clock source, so F_CPU=8000000
 9 | and a baud rate of 9600.
10 | 
11 | The program can be compiled with avr-gcc and the avr-libc libraries.


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/LICENSE.txt:
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 1 | MIT License
 2 | 
 3 | Copyright (c) 2018 Marcel Meyer-Garcia
 4 | 
 5 | Permission is hereby granted, free of charge, to any person obtaining a copy
 6 | of this software and associated documentation files (the "Software"), to deal
 7 | in the Software without restriction, including without limitation the rights
 8 | to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 9 | copies of the Software, and to permit persons to whom the Software is
10 | furnished to do so, subject to the following conditions:
11 | 
12 | The above copyright notice and this permission notice shall be included in all
13 | copies or substantial portions of the Software.
14 | 
15 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 | FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
18 | AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
20 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
21 | SOFTWARE.
22 | 


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/main.c:
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  1 | /*  Software ("bit-bang") UART Transmitter (8 data bits, 1 stop bit, no parity)
  2 |     for Attiny24A/44A/84A using the internal 8MHz oscillator as clock source
  3 | 
  4 |     (c) 2018 Marcel Meyer-Garcia
  5 |     see LICENCE.txt
  6 |  */
  7 | 
  8 |  /* NOTE: since the internal 8MHz oscillator is not very accurate, the value for OCR0A can be tuned
  9 |     to achieve the desired baud rate (nominal value is 103)
 10 |  */
 11 | 
 12 | #include <avr/io.h>
 13 | #include <avr/interrupt.h>
 14 | #include <util/delay.h>
 15 | 
 16 | // change these to use another pin
 17 | #define TX_PORT PORTB
 18 | #define TX_PIN  PB0
 19 | #define TX_DDR  DDRB
 20 | #define TX_DDR_PIN DDB0
 21 | 
 22 | volatile uint16_t tx_shift_reg = 0;
 23 | 
 24 | 
 25 | void UART_tx(char character)
 26 | {
 27 |    uint16_t local_tx_shift_reg = tx_shift_reg;
 28 |    //if sending the previous character is not yet finished, return
 29 |    //transmission is finished when tx_shift_reg == 0
 30 |    if(local_tx_shift_reg){return;}
 31 |    //fill the TX shift register witch the character to be sent and the start & stop bits (start bit (1<<0) is already 0)
 32 |    local_tx_shift_reg = (character<<1) | (1<<9); //stop bit (1<<9)
 33 |    tx_shift_reg = local_tx_shift_reg;
 34 |    //start timer0 with a prescaler of 8
 35 |    TCCR0B = (1<<CS01);
 36 | }
 37 | 
 38 | void UART_tx_str(char* string){
 39 |     while( *string ){
 40 |         UART_tx( *string++ );
 41 |         //wait until transmission is finished
 42 |         while(tx_shift_reg);
 43 |     }
 44 | }
 45 | 
 46 | void UART_init(){
 47 |    //set TX pin as output
 48 |    TX_DDR |= (1<<TX_DDR_PIN);
 49 |    TX_PORT |= (1<<TX_PIN);
 50 |    //set timer0 to CTC mode
 51 |    TCCR0A = (1<<WGM01);
 52 |    //enable output compare 0 A interrupt
 53 |    TIMSK0 |= (1<<OCF0A);
 54 |    //set compare value to 103 to achieve a 9600 baud rate (i.e. 104µs)
 55 |    //together with the 8MHz/8=1MHz timer0 clock
 56 |    /*NOTE: since the internal 8MHz oscillator is not very accurate, this value can be tuned
 57 |      to achieve the desired baud rate, so if it doesn't work with the nominal value (103), try
 58 |      increasing or decreasing the value by 1 or 2 */
 59 |    OCR0A = 103;
 60 |    //enable interrupts
 61 |    sei();
 62 | }
 63 | 
 64 | int main(void)
 65 | {
 66 |    UART_init();
 67 | 
 68 |    while(1)
 69 |    {
 70 |     UART_tx_str("Hello world!\n");
 71 |     _delay_ms(100);
 72 |    }
 73 | 
 74 |    return 0;
 75 | }
 76 | 
 77 | //timer0 compare A match interrupt
 78 | ISR(TIM0_COMPA_vect )
 79 | {
 80 |    uint16_t local_tx_shift_reg = tx_shift_reg;
 81 |    //output LSB of the TX shift register at the TX pin
 82 |    if( local_tx_shift_reg & 0x01 )
 83 |    {
 84 |       TX_PORT |= (1<<TX_PIN);
 85 |    }
 86 |    else
 87 |    {
 88 |       TX_PORT &=~ (1<<TX_PIN);
 89 |    }
 90 |    //shift the TX shift register one bit to the right
 91 |    local_tx_shift_reg >>= 1;
 92 |    tx_shift_reg = local_tx_shift_reg;
 93 |    //if the stop bit has been sent, the shift register will be 0
 94 |    //and the transmission is completed, so we can stop & reset timer0
 95 |    if(!local_tx_shift_reg)
 96 |    {
 97 |       TCCR0B = 0;
 98 |       TCNT0 = 0;
 99 |    }
100 | }
101 | 


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