ATtiny25 datasheet(96/196 Pages) ATMEL

96 / 196 page

7598H–AVR–07/09

ATtiny25/45/85

16. Universal Serial Interface – USI

The Universal Serial Interface, or USI, provides the basic hardware resources needed for serial

communication. Combined with a minimum of control software, the USI allows significantly

higher transfer rates and uses less code space than solutions based on software only. Interrupts

are included to minimize the processor load. The main features of the USI are:

• Two-wire Synchronous Data Transfer (Master or Slave, f

SCLmax = fCK/16)

• Three-wire Synchronous Data Transfer (Master or Slave f

SCKmax = fCK/4)

• Data Received Interrupt

• Wakeup from Idle Mode

• In Two-wire Mode: Wake-up from All Sleep Modes, Including Power-down Mode

• Two-wire Start Condition Detector with Interrupt Capability

16.1

Overview

A simplified block diagram of the USI is shown on Figure 16-1. For the actual placement of I/O

pins, refer to “Pinout ATtiny25/45/85” on page 2. CPU accessible I/O Registers, including I/O

bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are listed

in the “USI Register Descriptions” on page 103.

Figure 16-1. Universal Serial Interface, Block Diagram

The 8-bit Shift Register is directly accessible via the data bus and contains the incoming and

outgoing data. The register has no buffering so the data must be read as quickly as possible to

ensure that no data is lost. The most significant bit is connected to one of two output pins

depending of the wire mode configuration. A transparent latch is inserted between the Serial

edge of the data input sampling. The serial input is always sampled from the Data Input (DI) pin

independent of the configuration.

The 4-bit counter can be both read and written via the data bus, and can generate an overflow

interrupt. Both the Serial Register and the counter are clocked simultaneously by the same clock

source.

Two-wire Clock

Control Unit

(Output only)

DI/SDA

(Input/Open Drain)

USCK/SCL

(Input/Open Drain)

4-bit Counter

USIDR

USISR

USICR

CLOCK

HOLD

TIM0 COMP

[1]

USIDB

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