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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.
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