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102

8048C–AVR–02/12

ATtiny43U

14. USI – Universal Serial Interface

14.1

Features

Two-wire Synchronous Data Transfer (Master or Slave)

Three-wire Synchronous Data Transfer (Master or Slave)

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

14.2

Overview

The Universal Serial Interface (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.

A simplified block diagram of the USI is shown in Figure 14-1 on page 102. For actual placement

of I/O pins, refer to “Pinout of ATtiny43U” 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 “Register Descriptions” on page 109.

Figure 14-1. Universal Serial Interface, Block Diagram

The 8-bit USI Data Register (USIDR) contains the incoming and outgoing data. It is directly

accessible via the data bus but a copy of the contents is also placed in the USI Buffer Register

(USIBR) where it can be retrieved later. If reading the USI Data Register directly, the register

must be read as quickly as possible to ensure that no data is lost.

The most significant bit of the USI Data Register is connected to one of two output pins (depend-

ing on the mode configuration, see Table 14-1 on page 110). There is a transparent latch

between the output of the USI Data Register and the output pin, which delays the change of data

DATA

BUS

USIPF

USITC

USICLK

USICS0

USICS1

USIOIF

USIOIE

USIDC

USISIF

USIWM0

USIWM1

USISIE

Bit7

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

Bit0

[1]

USIDB

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