soft_uart_tx_only.c

//A SUPER SIMPLE soft UART TX for any AVR(ATmega/AT90/ATtiny).
// Tested on an ATmega16M1 @ 8mhz.
// 46 bytes of code space.
// 57600 baud at F_OSC 8mhz
// The baud rate will scale with CPU frequency so be careful.
//
// For basic debug only.
// No guarentee this will work.
//
// By: Chris Takahashi 2020
// Licence CC0 (public domain):
// To the extent possible under law, Chris Takahashi 
// has waived all copyright and related or neighboring 
// rights to this work (soft_uart_tx_only.c). 
// This work is published from: United States. 


#define SS_DDR_PORT DDRB
#define SS_PORT PORTB
#define SS_PIN 2


void init_suart(){
	SS_DDR_PORT |= 1<<SS_PIN;
}

void suart_tx(uint8_t c) { 
	//139 clocks at 8mhz is (close to) 57600 bps =17.3611us
	
	//START BIT
	SS_PORT &=~(1<<SS_PIN);
	__builtin_avr_delay_cycles(139-1);
	
	
	for(uint8_t x=8;x>0;x--) {
		if (c&1) {
			SS_PORT |=1<<SS_PIN;  //sbi = 1 cycle
		} else {
			SS_PORT &=~(1<<SS_PIN); //cbi = 1 cycle
			//it's shaky which branch the nops go.  
			// it depends on if the compiler uses SBIC or SBIS
			// check the listing or try sending 'U'(b'01010101) 
      // and check with a scope
			_NOP();
			_NOP(); 
			_NOP();
		}
		c = c>>1; // 1 inst
		//not going to lie,
		// I tweaked the delays with a scope until it was right.
		__builtin_avr_delay_cycles(139-6-4);
	}
	//stop bit
	SS_PORT |= 1<<SS_PIN;
	//You can delete the line below if you can guarentee
  // that you won't call this function again for another 140 cycles.
  __builtin_avr_delay_cycles(139 );
}