ATtiny44 datasheet(117/236 Pages) ATMEL

117 / 236 page

117

8006H–AVR–10/09

ATtiny24/44/84

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 For actual placement of I/O pins

refer to “Pinout ATtiny24/44/84” on page 2. Device-specific I/O Register and bit locations are

listed in the “Register Descriptions” on page 124.

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 “USICR – USI Control Register” on page 126). There is a

transparent latch between the output of the USI Data Register and the output pin, which delays

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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