Rainbow Electronics ATtiny43U - Manual (Page 104)

104
8048B–AVR–03/09
ATtiny43U
14.3.3 SPI Slave Operation Example
The following code demonstrates how to use the USI module as a SPI Slave:
init:
ldi r16,(1<<USIWM0)|(1<<USICS1)
out USICR,r16
...
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 DDRA. 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 are 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.
14.3.4 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 14-4 shows two USI units operating in two-wire mode, one as master and one as slave. It
is only the physical layer that is shown since the system operation is highly dependent of the
communication scheme used. The main differences between the master and slave operation at
this level is the serial clock generation which is always done by the master. Only the slave uses
the clock control unit.
Clock generation must be implemented in software, but the shift operation is done automatically
in both devices. Note that clocking only on negative edges for shifting data is of practical use in
this mode. The slave can insert wait states at start or end of transfer by forcing the SCL clock
low. This means that the master must always check if the SCL line was actually released after it
has generated a positive edge.
Since the clock also increments the counter, a counter overflow can be used to indicate that the
transfer is completed. The clock is generated by the master by toggling the USCK pin via the
PORTA register.