ATMEGA649 datasheet(194/375 Pages) ATMEL

194 / 375 page

194

ATmega329/3290/649/6490

2552H–AVR–11/06

Start Condition Detector

The start condition detector is shown in Figure 82. The SDA line is delayed (in the range

of 50 to 300 ns) to ensure valid sampling of the SCL line. The start condition detector is

only enabled in Two-wire mode.

The start condition detector is working asynchronously and can therefore wake up the

processor from the Power-down sleep mode. However, the protocol used might have

restrictions on the SCL hold time. Therefore, when using this feature in this case the

Oscillator start-up time set by the CKSEL Fuses (see “Clock Systems and their Distribu-

tion” on page 25) must also be taken into the consideration. Refer to the USISIF bit

description on page 195 for further details.

Clock speed considerations.

Maximum frequency for SCL and SCK is f

CK /4. This is also the maximum data transmit

and receieve rate in both two- and three-wire mode. In two-wire slave mode the Two-

wire Clock Control Unit will hold the SCL low until the slave is ready to receive more

data. This may reduce the actual data rate in two-wire mode.

Alternative USI Usage

When the USI unit is not used for serial communication, it can be set up to do alternative

tasks due to its flexible design.

Half-duplex Asynchronous

Data Transfer

By utilizing the Shift Register in Three-wire mode, it is possible to implement a more

compact and higher performance UART than by software only.

4-bit Counter

The 4-bit counter can be used as a stand-alone counter with overflow interrupt. Note

that if the counter is clocked externally, both clock edges will generate an increment.

12-bit Timer/Counter

Combining the USI 4-bit counter and Timer/Counter0 allows them to be used as a 12-bit

counter.

Edge Triggered External

Interrupt

By setting the counter to maximum value (F) it can function as an additional external

interrupt. The Overflow Flag and Interrupt Enable bit are then used for the external inter-

rupt. This feature is selected by the USICS1 bit.

Software Interrupt

The counter overflow interrupt can be used as a software interrupt triggered by a clock

strobe.

ATMEGA649 Datasheet(PDF) 194 Page - ATMEL Corporation