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116
8183F–AVR–06/12
ATtiny24A/44A/84A
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
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-
latch between the output of the USI Data Register and the output pin, which delays the change
of data output to the opposite clock edge of the data input sampling. The serial input is always
sampled from the Data Input (DI) pin independent of the configuration.
DATA
BUS
USIPF
USITC
USICLK
USICS0
USICS1
USIOIF
USIOIE
USIDC
USISIF
USIWM0
USIWM1
USISIE
Bit7
Two-wire Clock
Control Unit
DO
(Output only)
DI/SDA
(Input/Open Drain)
USCK/SCL
(Input/Open Drain)
4-bit Counter
USIDR
USISR
DQ
LE
USICR
CLOCK
HOLD
TIM0 COMP
Bit0
[1]
3
0
1
2
3
0
1
2
0
1
2
USIBR