ATmega325P datasheet(193/364 Pages) ATMEL

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8023F–AVR–07/09

ATmega325P/3250P

Figure 19-3. Three-wire Mode, Timing Diagram

The Three-wire mode timing is shown in Figure 19-3. At the top of the figure is a USCK cycle ref-

erence. One bit is shifted into the USI Shift 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 (Data Register is shifted by one) at negative

edges. External Clock mode 1 (USICS0 = 1) uses the opposite edges versus mode 0, i.e., sam-

ples data at negative and changes the output at positive edges. The USI clock modes

corresponds to the SPI data mode 0 and 1.

Referring to the timing diagram (Figure 19-3), a bus transfer involves the following steps:

The Slave device and Master device sets up its data output and, depending on the proto-

col used, enables its output driver (mark A and B). The output is set up by writing the

data to be transmitted to the Serial Data Register. Enabling of the output is done by set-

ting the corresponding bit in the port Data Direction Register. Note that point A and B

does not have any specific order, but both must be at least one half USCK cycle before

point C where the data is sampled. This must be done to ensure that the data setup

requirement is satisfied. The 4-bit counter is reset to zero.

The Master generates a clock pulse by software toggling the USCK line twice (C and D).

The bit value on the slave and master’s data input (DI) pin is 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.

Step 2. is repeated eight times for a complete register (byte) transfer.

After eight clock pulses (i.e., 16 clock edges) the counter will overflow and indicate that

the transfer is completed. The data bytes transferred must now be processed before a

new transfer can be initiated. The overflow interrupt will wake up the processor if it is set

to Idle mode. Depending of the protocol used the slave device can now set its output to

high impedance.

19.3.2

SPI Master Operation Example

The following code demonstrates how to use the USI module as a SPI Master:

SPITransfer:

sts

USIDR,r16

ldi

r16,(1<<USIOIF)

sts

USISR,r16

ldi

r16,(1<<USIWM0)|(1<<USICS1)|(1<<USICLK)|(1<<USITC)

SPITransfer_loop:

sts

USICR,r16

lds

r16, USISR

sbrs

r16, USIOIF

MSB

654321

LSB

654321

LSB

USCK

CYCLE ( Reference )

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