Add a program that demonstrates PWM and interrupts

flake.nix

@@ -30,21 +30,24 @@
           pkgs = import nixpkgs { system = "x86_64-linux"; };
           avr = pkgs.pkgsCross.avr.buildPackages;
         in pkgs.stdenv.mkDerivation rec {
-          name = "hello";
+          name = "pwm";
           src = ./.;
 
+          buildInputs = [ pkgs.simavr ];
+
           MCU = "attiny85";
           CHIP_SELECT = "AVR_ATtiny85";
           F_CPU = "8000000";
           CFLAGS = ''-O -finline-functions -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -Wall -Wstrict-prototypes -DF_CPU=${F_CPU} -std=gnu99 -D__${CHIP_SELECT}__=1 -mmcu=${MCU}'';
 
           buildPhase = ''
-            set -x
-            ${avr.gcc}/bin/avr-gcc ${CFLAGS} -I${src}/base/include/ -o main.elf ${src}/prototype/src/main.c
+            ${avr.gcc}/bin/avr-gcc ${CFLAGS} -I${src}/base/include/ -E -o main.post-cc ${src}/pwm/src/main.c
+            ${avr.gcc}/bin/avr-gcc ${CFLAGS} -I${src}/base/include/ -o main.elf ${src}/pwm/src/main.c
             $OBJCOPY -O ihex main.elf main.hex
             '';
           installPhase = ''
             mkdir $out
+            cp main.post-cc $out
             cp main.elf main.hex $out
             '';
         };
@@ -56,6 +59,7 @@
             binutils
             gcc
             avrdude
+            simavr
           ];
         in
         pkgs.mkShell {

pwm/src/main.c

@@ -0,0 +1,74 @@
+#include <avr/io.h>
+#include <util/delay.h>
+#include <avr/interrupt.h>
+
+#include <base.h>
+
+#define FIVE_SECOND_TICK (8000000 * 10) / 256
+
+volatile uint32_t iteration = 0;
+
+void set_up_pwm(void);
+void start_pwm(void);
+
+ISR(TIMER0_OVF_vect) {
+    if (iteration >= FIVE_SECOND_TICK) {
+        GTCCR |= _BV(TSM);
+        /*
+        PORTB &= ~(_BV(2));
+        PORTB |= _BV(1);
+        */
+        PORTB = _BV(1);
+    } else {
+        iteration += 1;
+    }
+}
+
+void set_up_pwm(void) {
+    // Stop the clock
+    GTCCR = _BV(TSM) | _BV(PSR0);
+    
+    TCCR0B = _BV(CS00);
+
+    // Set normal mode
+    TCCR0A = _BV(COM0A1) | _BV(WGM01) | _BV(WGM00);
+
+    // Enable the overflow interrupt
+    TIMSK = _BV(TOIE0);
+}
+
+void start_pwm(void) {
+    GTCCR &= ~(_BV(TSM));
+}
+
+int main (void) {
+    int8_t step = 1;
+    uint8_t target = 0;
+
+    set_up_pwm();
+    OCR0A = 0;
+
+    PORTB = 0;
+    DDRB = _BV(2) | _BV(1) | _BV(0);
+    _delay_ms(50);
+    PORTB |= _BV(2);
+
+    sei();
+    start_pwm();
+
+    while (iteration != FIVE_SECOND_TICK) {
+        OCR0A = target;
+        target += step;
+
+        if (target == 255) {
+            step = -1;
+        } else if (target == 0) {
+            step = 1;
+        }
+
+        _delay_ms(10);
+    }
+
+    while (1) {}
+}
+