Hacked together serial-controlled pin power pulser (AVR)

avr_serial_pin_pulse.c

#include <avr/interrupt.h>
#include <avr/io.h>
#include <avr/wdt.h>

#include <stdlib.h>
#include <string.h>

void init_usart(void)
{
    cli();

    // Set baud rate to 9600 (with 16MHz clock)
    UBRR0H = (uint8_t)(103 >> 8);
    UBRR0L = (uint8_t)(103);

    // Enable receiver and transmitter
    UCSR0B = _BV(RXEN0) | _BV(TXEN0);

    // Set frame format: 8 data, 1 stop bit
    UCSR0C = (3<<UCSZ00);
    UCSR0C &= ~(1<<USBS0);

    sei();
}

void usart_transmit(uint8_t data)
{
    // Wait for empty transmit buffer
    while ( !( UCSR0A & (1<<UDRE0)) );

    // Put data into buffer, sends the data
    UDR0 = data;
}

uint8_t usart_receive(void)
{
    // Wait for data to be received
    while ( !(UCSR0A & (1<<RXC0)) );

    // Get and return received data from buffer
    return UDR0;
}

void print(char* string)
{
    uint8_t i = 0;
    while (string[i] != '\0') {
        usart_transmit(string[i]);
        ++i;
    }
}

void println(char* string)
{
    print(string);
    usart_transmit('\n');
}

#define BUF_SIZE 10
char buffer[BUF_SIZE] = {} ;

int main(void)
{
    // Recover from any watchdog timer reset mishaps
    wdt_reset();
    wdt_disable();

    init_usart();

    println("Microswitch Timer v0.1\r");
    println("Copyright 1989, Some Guy");

    println("\r\nEnter a value in tenths of milliseconds (100uS) or press enter to repeat the last value.\r\n");

    int delay = 1;

    for (;;) {
        print("> ");

        memset(buffer, NULL, BUF_SIZE);

        for (int8_t i = 0; i < 9; ++i) {
            char byte = usart_receive();

            // Move back in buffer on backspace
            if (byte =='\b') {
                if (i > 1) i -= 2;
                continue;
            }

            // Exit early on user submit
            if (byte == '\f' || byte == '\n' || byte == '\r') {
                print("\r\n");
                break;
            }

            buffer[i] = byte;
        }

        // Update delay if a value was entered
        if (buffer[0] != NULL) {
            delay = atoi(buffer);
        }

        char outBuf[10];
        itoa(delay, outBuf, 10);

        print("Powering for ");
        print(outBuf);
        print("00 uS...\r\n");

        if (delay < 1) {
            println("(nothing to do...)\r\n");
            continue;
        }

        // Stop timer (prescaler = 0)
        TCCR1B &= ~(_BV(CS10) | _BV(CS11) | _BV(CS12));

        // There are 68.717 ticks per millisecond with a 256 prescaler at 16MHz
        //
        //   ((16000000/256)/(2^16-1)) * 1000  =>  953.6888
        //   (2^16-1)/953.6888  =>  68.717
        //
        uint16_t counterTarget = 7 * delay;

        // Load value to count to
        OCR1A = counterTarget;

        // Reset timer to zero
        TCNT1H = 0;
        TCNT1L = 0;

        // Clear compare flag
        TIFR |= _BV(OCF1A);

        // Enable port
        DDRA = 0xFF;
        PORTA = 0xFF;

        // Start timer with 256 prescaler
        TCCR1B |= _BV(CS12);

        // Wait for timer to roll hit target
        while ( (TIFR & _BV(OCF1A)) == 0 ) continue;

        // Disable port
        PORTA = 0x0;
    }

    return 0;
}