Added firmware

firmware/.gitignore

@@ -0,0 +1 @@
+magic.hex

firmware/Makefile

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+all:
+	make build
+	make program
+
+build:
+	avr-gcc main.c -o magic.hex -mmcu=attiny84a -Wall -Wextra -Os
+
+program:
+	avrdude -p attiny84 -c usbtiny -U flash:w:magic.hex
+
+.PHONY: all build program

firmware/main.c

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+#include <avr/io.h>
+#include <avr/sleep.h>
+
+#include <stdint.h>
+
+int main()
+{
+	uint8_t input;
+	uint8_t temp;
+	uint8_t count;
+	uint8_t i;
+
+	// Setup
+	PORTA = (1 << PORTA7) | (1 << PORTA6) | (1 << PORTA5) | (1 << PORTA4);
+	DDRA = (1 << DDA7) | (1 << DDA6) | (1 << DDA5) | (1 << DDA4);
+	PORTB = (1 << PORTB2);
+	DDRB = (1 << DDB2);
+	PRR = (1 << PRTIM1) | (1 << PRTIM0) | (1 << PRUSI) | (1 << PRADC);
+
+	// Read Inputs
+	input = PINA;
+	temp = input;
+	count = 0;
+
+	// How many inputs are set
+	for (i = 0; i < 4; ++i)
+	{
+		if (temp & 0x01)
+		{
+			++count;
+		}
+		temp >>= 1;
+	}
+
+	// If none or more than 1 are providing power - do nothing until reboot
+	if (count != 1)
+	{
+		goto end;
+	}
+
+	// Set Upstream Facing Port
+	input = ~input;
+	input <<= 4;
+	PORTA = input;
+	PORTB &= ~(1 << PORTB2);
+
+end:
+	// Power Down Mode
+	MCUCR = (1 << SM1) | (1 << SE);
+	while (1)
+	{
+		sleep_cpu();
+	}
+}