Rainbow Electronics ATmega128RFA1 - Manual (Page 332)

332
8266A-MCU Wireless-12/09
ATmega128RFA1
control logic will sample the incoming signal of the SCK pin. To ensure correct sampling
of the clock signal, the minimum low and high periods should be:
Low period: longer than 2 CPU clock cycles
High period: longer than 2 CPU clock cycles
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS
__
pins is
overridden according to Table 21-1. For more details on automatic port overrides, refer
to
"Alternate Port Functions" on page 191 .
Table 22-1. Pin Overrides
(1)
Pin Direction, Master SPI Direction, Slave SPI
MOSI User Defined Input
MISO Input User Defined
SCK User Defined Input
SS User Defined Input
Note: 1. See "Alternate Functions of Port B" on page 192 for a detailed description of how
to define the direction of the user defined SPI pins.
The following code examples show how to initialize the SPI as a Master and how to
perform a simple transmission. DDR_SPI in the examples must be replaced by the
actual Data Direction Register controlling the SPI pins. DD_MOSI, DD_MISO and
DD_SCK must be replaced by the actual data direction bits for these pins. E.g. if MOSI
is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with DDRB.
Assembly Code Example ( 1 )
SPI_MasterInit:
; Set MOSI and SCK output, all others input
ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)
out DDR_SPI,r17
; Enable SPI, Master, set clock rate fck/16
ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)
out SPCR,r17
ret
SPI_MasterTransmit:
; Start transmission of data (r16)
out SPDR,r16
Wait_Transmit:
; Wait for transmission complete
sbis SPSR,SPIF
rjmp Wait_Transmit
ret