ATMEGA169 ATMEL [ATMEL Corporation], ATMEGA169 Datasheet - Page 179

• Manufacturer Part Number: ATMEGA169
• Description: 8-bit Microcontroller with 16K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

USI – Universal Serial
Interface
Overview
2514P–AVR–07/06
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:
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A simplified block diagram of the USI is shown on Figure 76. For the actual placement of
I/O pins, refer to “Pinout ATmega169” on page 2. CPU accessible I/O Registers, includ-
ing 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 185.
Figure 76. 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 Register Output and output pin, which delays the change of data out-
put 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.
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. This allows the counter to count the number of bits received
or transmitted and generate an interrupt when the transfer is complete. Note that when
an external clock source is selected the counter counts both clock edges. In this case
the counter counts the number of edges, and not the number of bits. The clock can be
selected from three different sources: The USCK pin, Timer/Counter0 Compare Match
or from software.
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
USIDR
USICR
USISR
2
4-bit Counter
3
2
1
0
3
2
1
0
D Q
LE
[1]
TIM0 COMP
0
1
Two-wire Clock
Control Unit
ATmega169/V
CLOCK
HOLD
DO
DI/SDA
USCK/SCL
(Output only)
(Input/Open Drain)
(Input/Open Drain)
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