Datasheet

115
2586N–AVR–04/11
ATtiny25/45/85
...
SlaveSPITransfer:
out USIDR,r16
ldi r16,(1<<USIOIF)
out USISR,r16
SlaveSPITransfer_loop:
in r16, USISR
sbrs r16, USIOIF
rjmp SlaveSPITransfer_loop
in r16,USIDR
ret
The code is size optimized using only eight instructions (plus return). The code example
assumes that the DO and USCK pins have been enabled as outputs in DDRB. The value stored
in register r16 prior to the function is called is transferred to the master device, and when the
transfer is completed the data received from the master is stored back into the register r16.
Note that the first two instructions is for initialization, only, and need only be executed once.
These instructions set three-wire mode and positive edge clock. The loop is repeated until the
USI Counter Overflow Flag is set.
15.3.4 Two-wire Mode
The USI two-wire mode is compliant to the Inter IC (TWI) bus protocol, but without slew rate lim-
iting on outputs and without input noise filtering. Pin names used in this mode are SCL and SDA.
Figure 15-4. Two-wire Mode Operation, Simplified Diagram
MASTER
SLAVE
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
SDA
SCL
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
Two-wire Clock
Control Unit
HOLD
SCL
PORTxn
SDA
SCL
VCC