2 spi master operation example, Attiny43u – Rainbow Electronics ATtiny43U User Manual
Page 102
102
8048B–AVR–03/09
ATtiny43U
Figure 14-3. Three-wire Mode, Timing Diagram
The three-wire mode timing is shown in
At the top of the figure is a USCK cycle ref-
erence. One bit is shifted into the USI Data Register (USIDR) for each of these cycles. The
USCK timing is shown for both external clock modes. In external clock mode 0 (USICS0 = 0), DI
is sampled at positive edges, and DO is changed (USI Data Register is shifted by one) at nega-
tive edges. In external clock mode 1 (USICS0 = 1) the opposite edges with respect to mode 0
are used. In other words, data is sampled at negative and output is changed at positive edges.
The USI clock modes corresponds to the SPI data mode 0 and 1.
Referring to the timing diagram (
), a bus transfer involves the following steps:
1.
The slave and master devices set up their data outputs and, depending on the protocol
used, enable their output drivers (mark A and B). The output is set up by writing the
data to be transmitted to the USI Data Register. The output is enabled by setting the
corresponding bit in the Data Direction Register of Port A. Note that there is not a pre-
ferred order of points A and B in the figure, but both must be at least one half USCK
cycle before point C, where the data is sampled. This is in order to ensure that the data
setup requirement is satisfied. The 4-bit counter is reset to zero.
2.
The master software generates a clock pulse by toggling the USCK line twice (C and
D). The bit values on the data input (DI) pins are sampled by the USI on the first edge
(C), and the data output is changed on the opposite edge (D). The 4-bit counter will
count both edges.
3.
Step 2. is repeated eight times for a complete register (byte) transfer.
4.
After eight clock pulses (i.e., 16 clock edges) the counter will overflow and indicate that
the transfer has been completed. If USI Buffer Registers are not used the data bytes
that have been transferred must now be processed before a new transfer can be initi-
ated. The overflow interrupt will wake up the processor if it is set to Idle mode.
Depending on the protocol used the slave device can now set its output to high
impedance.
14.3.2
SPI Master Operation Example
The following code demonstrates how to use the USI module as a SPI Master:
SPITransfer:
out
USIDR,r16
ldi
r16,(1<<USIOIF)
out
USISR,r16
ldi
r17,(1<<USIWM0)|(1<<USICS1)|(1<<USICLK)|(1<<USITC)
<continues>
MSB
MSB
6
5
4
3
2
1
LSB
1
2
3
4
5
6
7
8
6
5
4
3
2
1
LSB
USCK
USCK
DO
DI
D
C
B
A
E
CYCLE
( Reference )