atmega3250v-8auatmega325v-8ai ATMEL Corporation, atmega3250v-8auatmega325v-8ai Datasheet - Page 191
atmega3250v-8auatmega325v-8ai
Manufacturer Part Number
atmega3250v-8auatmega325v-8ai
Description
Atmega3250 8-bit Microcontroller With In-system Programmable Flash
Manufacturer
ATMEL Corporation
Datasheet
1.ATMEGA3250V-8AUATMEGA325V-8AI.pdf
(362 pages)
- Current page: 191 of 362
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21.3
21.3.1
21.3.2
21.3.3
21.3.4
21.3.5
21.4
21.4.1
2570L–AVR–08/07
Alternative USI Usage
Register Descriptions
Half-duplex Asynchronous Data Transfer
4-bit Counter
12-bit Timer/Counter
Edge Triggered External Interrupt
Software Interrupt
USIDR – USI Data Register
trol Unit will hold the SCL low until the slave is ready to receive more data. This may reduce the
actual data rate in two-wire mode.
When the USI unit is not used for serial communication, it can be set up to do alternative tasks
due to its flexible design.
By utilizing the Shift Register in Three-wire mode, it is possible to implement a more compact
and higher performance UART than by software only.
The 4-bit counter can be used as a stand-alone counter with overflow interrupt. Note that if the
counter is clocked externally, both clock edges will generate an increment.
Combining the USI 4-bit counter and Timer/Counter0 allows them to be used as a 12-bit
counter.
By setting the counter to maximum value (F) it can function as an additional external interrupt.
The Overflow Flag and Interrupt Enable bit are then used for the external interrupt. This feature
is selected by the USICS1 bit.
The counter overflow interrupt can be used as a software interrupt triggered by a clock strobe.
The USI uses no buffering of the Serial Register, i.e., when accessing the Data Register
(USIDR) the Serial Register is accessed directly. If a serial clock occurs at the same cycle the
register is written, the register will contain the value written and no shift is performed. A (left) shift
operation is performed depending of the USICS1..0 bits setting. The shift operation can be con-
trolled by an external clock edge, by a Timer/Counter0 Compare Match, or directly by software
using the USICLK strobe bit. Note that even when no wire mode is selected (USIWM1..0 = 0)
both the external data input (DI/SDA) and the external clock input (USCK/SCL) can still be used
by the Shift Register.
The output pin in use, DO or SDA depending on the wire mode, is connected via the output latch
to the most significant bit (bit 7) of the Data Register. The output latch is open (transparent) dur-
ing the first half of a serial clock cycle when an external clock source is selected (USICS1 = 1),
and constantly open when an internal clock source is used (USICS1 = 0). The output will be
changed immediately when a new MSB written as long as the latch is open. The latch ensures
that data input is sampled and data output is changed on opposite clock edges.
Bit
(0xBA)
Read/Write
Initial Value
MSB
R/W
7
0
R/W
6
0
R/W
5
0
R/W
4
0
ATmega325/3250/645/6450
R/W
3
0
R/W
2
0
R/W
1
0
LSB
R/W
0
0
USIDR
191
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