diff --git a/firmware/include/si5351.h b/firmware/include/si5351.h
new file mode 100644
index 0000000..54edb89
--- /dev/null
+++ b/firmware/include/si5351.h
@@ -0,0 +1,20 @@
+#ifndef __SI5351_H__
+#define __SI5351_H__ __SI5351_H__
+
+#include <stdint.h>
+
+#include "twi.h"
+
+const uint8_t SI5351_ADDRESS = 96;
+
+static inline uint8_t si5351_write8(uint8_t reg, uint8_t value) {
+    uint8_t data[3] = {reg, value};
+    return twi_write(SI5351_ADDRESS, data, 2);
+}
+
+static inline uint8_t si5351_write16(uint8_t reg, uint16_t value) {
+    uint8_t data[3] = {reg, value & 0xFF, (value & 0xFF00) >> 8};
+    return twi_write(SI5351_ADDRESS, data, 3);
+}
+
+#endif
diff --git a/firmware/main.c b/firmware/main.c
index a05c051..8d480fc 100644
--- a/firmware/main.c
+++ b/firmware/main.c
@@ -2,16 +2,19 @@
 #include <util/delay.h>
 
 #include "twi.h"
+#include "si5351.h"
 
 int main(void) {
 
     twi_init();
 
-    while(1) {
-        uint8_t data[] = {'T','E','S','T'};
-        twi_write(96, data, 4);
-        _delay_ms(10);
-    }
+    // Disable outputs
+    si5351_write8(3, 0xFF);
+
+    // Powerdown output drivers
+    si5351_write8(16, 0x80);
+    si5351_write8(17, 0x80);
+    si5351_write8(18, 0x80);
 
     while(1);
 }
diff --git a/firmware/twi.c b/firmware/twi.c
index ed4ca93..94b1e62 100644
--- a/firmware/twi.c
+++ b/firmware/twi.c
@@ -35,7 +35,7 @@ uint8_t twi_write(uint8_t address, uint8_t data[], uint8_t len) {
         return 1;
     }
 
-    TWDR = address | TW_WRITE;       // We want to write to the slave
+    TWDR = (address << 1) | TW_WRITE;       // We want to write to the slave
     TWCR = (1 << TWINT) | (1 << TWEN);  // Trigger transmission, keep TWI enabled
 
     // Wait for TWI unit to finish