diff --git a/CMakeLists.txt b/CMakeLists.txt
index ee55d68..63758b3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -125,7 +125,7 @@ endif()
 
 if(CMAKE_CROSSCOMPILING)
   # mcu related settings
-  set(MCU_FLAGS -mmcu=atmega32u4)
+  set(MCU_FLAGS -mmcu=atmega32u4 -DF_CPU=16000000L)
   add_compile_options(${MCU_FLAGS})
   add_link_options(${MCU_FLAGS})
 
@@ -170,6 +170,8 @@ objcopy(firmware "ihex" ".hex")
 
 add_custom_command(TARGET firmware POST_BUILD COMMAND avr-objdump ARGS -CSd firmware > firmware.txt)
 
+add_custom_command(TARGET firmware POST_BUILD COMMAND avr-size ARGS -C --mcu=atmega32u4 firmware)
+
 # generate linker map file
 target_link_options(firmware PUBLIC -Wl,-Map=firmware.map)
 
diff --git a/src/hal/_rules.txt b/src/hal/_rules.txt
index 65c89c5..9dae508 100644
--- a/src/hal/_rules.txt
+++ b/src/hal/_rules.txt
@@ -2,3 +2,6 @@ Use a class whenever you need to store some context data along with the function
 A typical scenario is the UART which uses some RX and TX buffers.
 
 Use a simple C-style otherwise, but it is advised to wrap the interface into a namespace as proposed in existing header files.
+
+`__attribute__((always_inline)) inline` was necessary for most functions because the generated code wasn't efficient enough otherwise.
+It will be interesting when the STM32 hal will have to be included as well. Hopefully it won't be a nightmare :P
diff --git a/src/hal/gpio.h b/src/hal/gpio.h
index 80f3545..4d473bf 100644
--- a/src/hal/gpio.h
+++ b/src/hal/gpio.h
@@ -1,3 +1,4 @@
+#pragma once
 #include <inttypes.h>
 #include <avr/io.h>
 
@@ -30,9 +31,6 @@ namespace gpio {
         Mode mode;
         Pull pull;
         Level level;
-        inline GPIO_InitTypeDef()
-            : mode(Mode::input)
-            , pull(Pull::none) {};
         inline GPIO_InitTypeDef(Mode mode, Pull pull)
             : mode(mode)
             , pull(pull) {};
@@ -41,36 +39,50 @@ namespace gpio {
             , level(level) {};
     };
 
-    inline void WritePin(GPIO_TypeDef *const port, const uint8_t pin, Level level) {
+    struct GPIO_pin {
+        GPIO_TypeDef *const port;
+        const uint8_t pin;
+        inline GPIO_pin(GPIO_TypeDef *const port, const uint8_t pin)
+            : port(port)
+            , pin(pin) {};
+    };
+
+    __attribute__((always_inline)) inline void WritePin(const GPIO_pin portPin, Level level) {
         if (level == Level::high)
-            port->PORTx |= (1 << pin);
+            portPin.port->PORTx |= (1 << portPin.pin);
         else
-            port->PORTx &= ~(1 << pin);
+            portPin.port->PORTx &= ~(1 << portPin.pin);
     }
 
-    inline Level ReadPin(GPIO_TypeDef *const port, const uint8_t pin) {
-        return (Level)(port->PINx & (1 << pin));
+    __attribute__((always_inline)) inline Level ReadPin(const GPIO_pin portPin) {
+        return (Level)(portPin.port->PINx & (1 << portPin.pin));
     }
 
-    inline void TogglePin(GPIO_TypeDef *const port, const uint8_t pin) {
-        port->PINx |= (1 << pin);
+    __attribute__((always_inline)) inline void TogglePin(const GPIO_pin portPin) {
+        portPin.port->PINx |= (1 << portPin.pin);
     }
 
-    inline void Init(GPIO_TypeDef *const port, const uint8_t pin, GPIO_InitTypeDef GPIO_Init) {
+    __attribute__((always_inline)) inline void Init(const GPIO_pin portPin, GPIO_InitTypeDef GPIO_Init) {
         if (GPIO_Init.mode == Mode::output) {
-            WritePin(port, pin, GPIO_Init.level);
-            port->DDRx |= (1 << pin);
+            WritePin(portPin, GPIO_Init.level);
+            portPin.port->DDRx |= (1 << portPin.pin);
         } else {
-            port->DDRx &= ~(1 << pin);
-            WritePin(port, pin, (Level)GPIO_Init.pull);
+            portPin.port->DDRx &= ~(1 << portPin.pin);
+            WritePin(portPin, (Level)GPIO_Init.pull);
         }
     }
 
 }
 }
 
+#define GPIOA ((hal::gpio::GPIO_TypeDef *)&PINA)
 #define GPIOB ((hal::gpio::GPIO_TypeDef *)&PINB)
 #define GPIOC ((hal::gpio::GPIO_TypeDef *)&PINC)
 #define GPIOD ((hal::gpio::GPIO_TypeDef *)&PIND)
 #define GPIOE ((hal::gpio::GPIO_TypeDef *)&PINE)
 #define GPIOF ((hal::gpio::GPIO_TypeDef *)&PINF)
+#define GPIOG ((hal::gpio::GPIO_TypeDef *)&PING)
+#define GPIOH ((hal::gpio::GPIO_TypeDef *)&PINH)
+#define GPIOJ ((hal::gpio::GPIO_TypeDef *)&PINJ)
+#define GPIOK ((hal::gpio::GPIO_TypeDef *)&PINK)
+#define GPIOL ((hal::gpio::GPIO_TypeDef *)&PINL)
diff --git a/src/hal/pins.h b/src/hal/pins.h
deleted file mode 100644
index 8745ad5..0000000
--- a/src/hal/pins.h
+++ /dev/null
@@ -1,11 +0,0 @@
-#pragma once
-
-/// Hardware Abstraction Layer for the CPU's features and peripherals
-
-namespace hal {
-namespace pins {
-
-    /// pin definitions
-
-} // namespace pins
-} // namespace hal
diff --git a/src/hal/spi.h b/src/hal/spi.h
new file mode 100644
index 0000000..5e067d3
--- /dev/null
+++ b/src/hal/spi.h
@@ -0,0 +1,48 @@
+#pragma once
+#include <inttypes.h>
+#include "gpio.h"
+
+/// SPI interface
+
+namespace hal {
+namespace spi {
+    struct SPI_TypeDef {
+        volatile uint8_t SPCRx;
+        volatile uint8_t SPSRx;
+        volatile uint8_t SPDRx;
+    };
+
+    struct SPI_InitTypeDef {
+        hal::gpio::GPIO_pin miso_pin;
+        hal::gpio::GPIO_pin mosi_pin;
+        hal::gpio::GPIO_pin sck_pin;
+        hal::gpio::GPIO_pin ss_pin;
+        uint8_t prescaler;
+        uint8_t cpha;
+        uint8_t cpol;
+    };
+
+    __attribute__((always_inline)) inline void Init(SPI_TypeDef *const hspi, SPI_InitTypeDef *const conf) {
+        using namespace hal;
+        gpio::Init(conf->miso_pin, gpio::GPIO_InitTypeDef(gpio::Mode::input, gpio::Pull::none));
+        gpio::Init(conf->mosi_pin, gpio::GPIO_InitTypeDef(gpio::Mode::output, gpio::Level::low));
+        gpio::Init(conf->sck_pin, gpio::GPIO_InitTypeDef(gpio::Mode::output, gpio::Level::low));
+        gpio::Init(conf->ss_pin, gpio::GPIO_InitTypeDef(gpio::Mode::output, gpio::Level::high)); //the AVR requires this pin to be an output for SPI master mode to work properly.
+
+        const uint8_t spi2x = (conf->prescaler == 7) ? 0 : (conf->prescaler & 0x01);
+        const uint8_t spr = ((conf->prescaler - 1) >> 1) & 0x03;
+
+        hspi->SPCRx = (0 << SPIE) | (1 << SPE) | (0 << DORD) | (1 << MSTR) | ((conf->cpol & 0x01) << CPOL) | ((conf->cpha & 0x01) << CPHA) | (spr << SPR0);
+        hspi->SPSRx = (spi2x << SPI2X);
+    }
+
+    __attribute__((always_inline)) inline uint8_t TxRx(SPI_TypeDef *const hspi, uint8_t val) {
+        hspi->SPDRx = val;
+        while (!(hspi->SPSRx & (1 << SPIF)))
+            ;
+        return hspi->SPDRx;
+    }
+}
+}
+
+#define SPI0 ((hal::spi::SPI_TypeDef *)&SPCR)
diff --git a/src/main.cpp b/src/main.cpp
index f26c6b1..679406f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1,12 +1,42 @@
 #include "logic/mm_control.h"
 #include "hal/gpio.h"
+#include "hal/spi.h"
+#include "pins.h"
 
 /// One-time setup of HW and SW components
 /// Called before entering the loop() function
-void setup(){
-    using namespace hal::gpio;
-    hal::gpio::Init(GPIOB, 1, GPIO_InitTypeDef(Mode::output, Level::low));
+void setup() {
+    using namespace hal;
 
+    spi::SPI_InitTypeDef spi_conf = {
+        .miso_pin = gpio::GPIO_pin(TMC2130_SPI_MISO_PIN),
+        .mosi_pin = gpio::GPIO_pin(TMC2130_SPI_MOSI_PIN),
+        .sck_pin = gpio::GPIO_pin(TMC2130_SPI_SCK_PIN),
+        .ss_pin = gpio::GPIO_pin(TMC2130_SPI_SS_PIN),
+        .prescaler = 2, //4mhz
+        .cpha = 1,
+        .cpol = 1,
+    };
+    spi::Init(SPI0, &spi_conf);
+
+    // SPI example
+    gpio::Init(gpio::GPIO_pin(GPIOC, 6), gpio::GPIO_InitTypeDef(gpio::Mode::output, gpio::Level::high));
+    uint8_t dat[5];
+    gpio::WritePin(gpio::GPIO_pin(GPIOC, 6), gpio::Level::low);
+    spi::TxRx(SPI0, 0x01);
+    spi::TxRx(SPI0, 0x00);
+    spi::TxRx(SPI0, 0x00);
+    spi::TxRx(SPI0, 0x00);
+    spi::TxRx(SPI0, 0x00);
+    gpio::WritePin(gpio::GPIO_pin(GPIOC, 6), gpio::Level::high);
+    gpio::WritePin(gpio::GPIO_pin(GPIOC, 6), gpio::Level::low);
+    dat[0] = spi::TxRx(SPI0, 0x00);
+    dat[1] = spi::TxRx(SPI0, 0x00);
+    dat[2] = spi::TxRx(SPI0, 0x00);
+    dat[3] = spi::TxRx(SPI0, 0x00);
+    dat[4] = spi::TxRx(SPI0, 0x00);
+    gpio::WritePin(gpio::GPIO_pin(GPIOC, 6), gpio::Level::high);
+    (void)dat;
 }
 
 /// Main loop of the firmware
@@ -23,17 +53,16 @@ void setup(){
 ///   StepWhateverElseNeedsStepping();
 /// The idea behind the Step* routines is to keep each automaton non-blocking allowing for some “concurrency”.
 /// Some FW components will leverage ISR to do their stuff (UART, motor stepping?, etc.)
-void loop(){
-
+void loop() {
 }
 
 int main() {
     setup();
-    for(;;){
+    for (;;) {
         using namespace hal::gpio;
-        WritePin(GPIOB, 5, Level::low);
-        TogglePin(GPIOB, 6);
-        if (hal::gpio::ReadPin(GPIOB, 7) == hal::gpio::Level::low)
+        WritePin(GPIO_pin(GPIOB, 5), Level::low);
+        TogglePin(GPIO_pin(GPIOB, 6));
+        if (hal::gpio::ReadPin(GPIO_pin(GPIOB, 7)) == hal::gpio::Level::low)
             break;
         loop();
     }
diff --git a/src/pins.h b/src/pins.h
new file mode 100644
index 0000000..154436f
--- /dev/null
+++ b/src/pins.h
@@ -0,0 +1,9 @@
+#pragma once
+#include "hal/gpio.h"
+
+/// pin definitions
+
+#define TMC2130_SPI_MISO_PIN GPIOB, 3
+#define TMC2130_SPI_MOSI_PIN GPIOB, 2
+#define TMC2130_SPI_SCK_PIN GPIOB, 1
+#define TMC2130_SPI_SS_PIN GPIOB, 0