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+Serial peripheral interface
+---------------------------
+
+## Functions
+### spi_init
+```C
+static inline void spi_init(enum spiMode mode)
+```
+Initializes SPI interface.
+Parameters:
+    mode - Specify mode of SPI interface
+
+NOTE: Global interrupt must be enabled for right function of SPI.
+ { SREG |= _BV(7) }
+
+### spi_busy
+```C
+static inline int8_t spi_busy(void)
+```
+Returns NULL when device is not busy.
+When device is busy return values in non-zero.
+
+### spi_join    
+```C
+static inline void spi_join(void)
+```
+Blocks processor until device is not busy.
+
+### spi_send
+```C
+static inline uint8_t spi_send(uint8_t data)
+```
+Swap bytes with slave over SPI.
+This function blocks execution until device isn't busy (transfer completed).
+WARNING: Invoke this only when interface is initialized in MASTER mode.
+
+### spi_transfer
+```C
+static inline void spi_transfer(uint8_t data)
+```
+Transfer byte to slave over SPI.
+This function isn't blocking execution until transfer is complete.
+Always call spi_join before this function when called outside of spi_receive().
+WARNING: Invoke this only when interface is initialized in MASTER mode.
+
+### spi_expose
+```C
+static inline void spi_expose(uint8_t data)
+```
+Expose data for next master request.
+Please don't use this when device is busy.
+Best place to call this is spi_receive().
+WARNING: Invoke this only when interface is initialized in SLAVE mode.
+
+## Function pointer spi_receive
+```C
+extern void (*spi_receive)(uint8_t data)
+```
+This function is called every time transfer is finished.
+And until return from this function interrupts are disabled.