gistfile1.c

#define F_CPU 8000000L
#include <avr/interrupt.h>
#include <util/delay.h>
#include <avr/io.h>

/*

MCU: ATTINY25

FUSE SETTINGS:

lfuse=0xe2 = 1110 0010
hfuse=0xdf
efuse=0xff

CONNECTOR 1

1: GND
2: 7-20VDC

CONNECTOR 2

1: GND
2: 5VDC
3: PWM output

I/O PORTS

1: PB5: (i) reset
2: PB3: (i) Pot (frequency) (ADC3)
3: PB4: (o) PWM output
4: GND
5: PB0: (i) mosi
6: PB1: (o) miso / LED
7: PB2: (i) Pot (pulse ratio) (ADC1)
8: VCC

*/

#define LED_PORT PORTB
#define LED_PIN 1
#define INPUT_FREQ_CHANNEL 3
#define INPUT_RATIO_CHANNEL 1

uint16_t adc_read(uint8_t channel)
{
	// Vcc disconnected from aref
	ADMUX = 0 | channel;
	_delay_us(500); // Allow the voltage to stabilize inside ADC
	ADCSRA |= (1<<ADSC);
	while(!(ADCSRA & (1<<ADIF)));
	return ADC;
}

int main(void)
{
	// I/O ports

	DDRB = (1<<4) | (1<<1);
	PORTB = 0x00;

	// Timer1

	//      CK/64
	TCCR1 = (1<<CS12) | (1<<CS11) | (1<<CS10);
	// Enable PWM output
	GTCCR |= (1<<PWM1B) | (1<<COM1B1);
	OCR1B = 128;

	// ADC

	// Enable; clk/64
	ADCSRA = (1<<ADEN) | (1<<ADPS2) | (1<<ADPS1);

	// Enable interrupts

	sei();

	// Misc. initialization

	LED_PORT |= (1<<LED_PIN);

	TCNT1 = 128;

	// Main loop
	
	for(;;){
		uint16_t input_freq = adc_read(INPUT_FREQ_CHANNEL);
		uint8_t v = ((1023 - input_freq) / 64) & 0x0f;
		if(v == 0)
			v = 1;
		TCCR1 = v;
		uint32_t input_freq_limit_low = (v-1) * 64;
		uint32_t input_freq_limit_high = (v-0) * 64;
		uint32_t v_between = input_freq - input_freq_limit_low;
		uint32_t v_space = 64;
		OCR1C = 255 - (uint16_t)128 * v_between / v_space;
		if(OCR1C < 10)
			OCR1C = 10;
		uint16_t input_ratio = adc_read(INPUT_RATIO_CHANNEL);
		OCR1B = (int16_t)(input_ratio >> 2) * (OCR1C+3) / 255;
	}

	return 0;
}