Add basic TWI support

To support basic TWI support some defines are created to keep track of
port, pins and address. Fundamental procedures such as:
- `twi_error()`
- `twi_start()`
- `twi_transmit()`
- `twi_receive()`
- `twi_stop()`
are created. Those procedures are used in `twi_read_register()` and
`twi_write_register()` which read and write data from and to registers
respectively. Lastly the TWI bit rate is set to 200kHz.

firmware/src/main.c

@@ -2,6 +2,7 @@
 #include <avr/io.h>
 #include <avr/interrupt.h>
 #include <util/delay.h>
+#include <util/twi.h>
 #include <assert.h>
 #include <stdbool.h>
 
@@ -17,6 +18,12 @@
 #define ENC_A (PINB & (1 << PB6))
 #define ENC_B (PINB & (1 << PB7))
 
+#define TWI_PORT PORTC
+#define TWI_SDA PC4
+#define TWI_SCL PC5
+
+#define SI5351_ADDRESS (0x60 << 1)
+
 #define SYM_ENTRY(SYM) { SYM, sizeof(SYM) / 2 }
 
 static uint8_t EEMEM eeprom_contrast = 8;
@@ -312,6 +319,83 @@ static void lcd_splash(void) {
     }
 }
 
+static void twi_error(void) {
+    lcd_fill(0xFF);
+
+    for (;;);
+}
+
+static void twi_start(void) {
+    TWCR = (1 << TWINT) | (1 << TWSTA) | (1 << TWEN); // Start transmission
+    while (!(TWCR & (1 << TWINT))); // Wait until start is transmitted
+
+    if (TW_STATUS != TW_START) // Check status
+        twi_error();
+}
+
+static uint8_t twi_transmit(const uint8_t data) {
+    TWDR = data;
+    TWCR = (1 << TWINT) | (1 << TWEN); // Transmit data
+    while (!(TWCR & (1 << TWINT))); // Wait until data is transmitted
+
+    return TW_STATUS;
+}
+
+static uint8_t twi_receive(void) {
+    TWCR = (1 << TWINT) | (1 << TWEN); // Receive data
+    while (!(TWCR & (1 << TWINT))); // Wait until data is received
+
+    return TW_STATUS;
+}
+
+static void twi_stop(void) {
+    TWCR = (1 << TWINT) | (1 << TWSTO) | (1 << TWEN);
+}
+
+static uint8_t twi_read_register(const uint8_t address,
+                                 const uint8_t reg) {
+    twi_start();
+
+    if (twi_transmit(address) != TW_MT_SLA_ACK)
+        twi_error();
+
+    if (twi_transmit(reg) != TW_MT_DATA_ACK)
+        twi_error();
+
+    twi_stop();
+
+    _delay_us(2);
+
+    twi_start();
+
+    if (twi_transmit(address + 1) != TW_MR_SLA_ACK)
+        twi_error();
+
+    if (twi_receive() != TW_MR_DATA_NACK)
+        twi_error();
+
+    twi_stop();
+
+    return TWDR;
+}
+
+static void twi_write_register(const uint8_t address,
+                               const uint8_t reg,
+                               const uint8_t data) {
+    twi_start();
+
+    if (twi_transmit(address) != TW_MT_SLA_ACK)
+        twi_error();
+
+    if (twi_transmit(reg) != TW_MT_DATA_ACK)
+        twi_error();
+
+    if (twi_transmit(data) != TW_MT_DATA_ACK)
+        twi_error();
+
+    twi_stop();
+}
+
 static void lcd_home(void) {
     lcd_fill(0x00);
 
@@ -682,6 +766,12 @@ int main(void) {
     // Enable interrupts
     sei();
 
+    // Set TWI bit rate to 200kHz
+    TWBR = 12;
+
+    (void) &twi_read_register;
+    (void) &twi_write_register;
+
     // Run...
     for (;;);
 }