Atmega644 spi configuration – Pololu Orangutan X2 User Manual
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2. ATmega644 SPI Configuration
The mega644 SPI module should be configured as follows:
• SPI enabled
• MSB first
• master mode
• clock polarity 0 (clock line low when idle)
• clock phase 0 (sample on leading ege)
• Maximum frequency: 2.5 MHz (clock/8)
If you have the latest
installed (version 20070525 at the time this was
written), the following C code will set up the SPI module, assuming you have your device set as atmega644. If
you are using an older version of WinAVR, you will need to add zeroes to the ends of all of the SPI register/bit
names (e.g. change SPCR to SPCR0, SPSR to SPSR0, SPE to SPE0, etc).
#include <avr/io.h>
// global flag used to help us track when an SPI transmission is in progress
unsigned char SPITransmitting;
void SPIInit()
{
// make the MOSI, SCK, and SS pins outputs
DDRB |= ( 1 << PB5 ) | ( 1 << PB7 ) | ( 1 << PB4 );
// make sure the MISO pin is input
DDRB &= ~( 1 << PB6 );
// set up the SPI module: SPI enabled, MSB first, master mode,
// clock polarity and phase = 0, F_osc/8
SPCR = ( 1 << SPE ) | ( 1 << MSTR ) | ( 1 << SPR0 );
SPSR = 1; // set double SPI speed for F_osc/8
// the previous settings clear the SPIF bit of SPCR, so we use our global
// flag to indicate that this does not mean we are currently transmitting
SPITransmitting = 0;
}
Most commands require data flow in only one direction: from the mega644 to the mega168. This can be achieved
with an SPI transmit command.
void SPITransmit( unsigned char data )
{
if ( SPITransmitting ) // if we really are transmitting
while ( ! ( SPSR & ( 1 << SPIF ))) // wait for completion of
; // previous transmission
SPDR = data; // begin transmission
SPITransmitting = 1; // flag transmission in progress
}
Reading data back from the mega168 is only slightly more complicated since we need to give the mega168 time
(~3us) to prepare the data byte we’re requesting. Once it’s ready we then need to transmit an extra byte since
every SPI transaction has data flow in both directions. As our byte is being sent to the mega168, the data we’re
interested is being sent to us. At the end of the transmission, the value from the mega168 will be in the mega644’s
SPI data register, SPDR. We need an extra function to perform our 3 microsecond delay, so we’ll include an
example here:
static inline void delay_us(unsigned int microseconds) __attribute__((always_inline));
void delay_us(unsigned int microseconds)
Orangutan X2 Command Documentation v1.01
© 2001–2010 Pololu Corporation
2. ATmega644 SPI Configuration
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