attiny25v ATMEL Corporation, attiny25v Datasheet - Page 118

• Manufacturer Part Number: attiny25v
• Description: Microcontroller With 2/4/8k Bytes In-system Programmable Flash
• Manufacturer: ATMEL Corporation

15.4.4
15.4.5
15.5
15.5.1
15.5.2
118
Register Descriptions
ATtiny25/45/85
Edge Triggered External Interrupt
Software Interrupt
USIDR – USI Data Register
USIBR – USI Buffer Register
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 Data Register can be accessed directly but a copy of the data can also be found in the
USI Buffer Register.
Depending on the USICS1:0 bits of the USI Control Register a (left) shift operation may be per-
formed. The shift operation can be synchronised to an external clock edge, to a Timer/Counter0
Compare Match, or directly to software via the USICLK bit. 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.
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 USI Data Register.
The output pin (DO or SDA, depending on the wire mode) is connected via the output latch to
the most significant bit (bit 7) of the USI Data Register. The output latch ensures that data input
is sampled and data output is changed on opposite clock edges. The latch is open (transparent)
during 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 is written as long as the latch is open.
Note that the Data Direction Register bit corresponding to the output pin must be set to one in
order to enable data output from the USI Data Register.
Instead of reading data from the USI Data Register the USI Buffer Register can be used. This
makes controlling the USI less time critical and gives the CPU more time to handle other pro-
gram tasks. USI flags as set similarly as when reading the USIDR register.
The content of the USI Data Register is loaded to the USI Buffer Register when the transfer has
been completed.
Bit
0x0F
Read/Write
Initial Value
Bit
0x10
Read/Write
Initial Value
7
MSB
R/W
0
7
MSB
R
0
6
R/W
0
6
R
0
5
R/W
0
5
R
0
4
R/W
0
4
R
0
3
R/W
0
3
R
0
2
0
2
R
0
R/W
1
R/W
0
1
R
0
0
LSB
R/W
0
0
LSB
R
0
2586K–AVR–01/08
USIDR
USIBR