ATmega165 datasheet(179/336 Pages) ATMEL

179 / 336 page

179

ATmega165/V

2573G–AVR–07/09

Note that the first two instructions is for initialization only and needs only to be executed

once.These instructions sets Three-wire mode and positive edge Shift Register clock.

The loop is repeated until the USI Counter Overflow Flag is set.

Two-wire Mode

The USI Two-wire mode is compliant to the Inter IC (TWI) bus protocol, but without slew

rate limiting on outputs and input noise filtering. Pin names used by this mode are SCL

and SDA.

Figure 79. Two-wire Mode Operation, Simplified Diagram

Figure 79 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 Mas-

ter and Slave operation at this level, is the serial clock generation which is always done

by the Master, and only the Slave uses the clock control unit. Clock generation must be

implemented in software, but the shift operation is done automatically by both devices.

Note that only clocking on negative edge 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 PORT Register.

The data direction is not given by the physical layer. A protocol, like the one used by the

TWI-bus, must be implemented to control the data flow.

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

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