ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 130

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

16.5
16.5.1
16.5.2
16.5.3
130
Register Descriptions
Atmel ATtiny24/44/84 [Preliminary]
USIBR – USI Data Buffer
USIDR – USI Data Register
USISR – USI Status Register
The USI uses no buffering for the serial register, i.e., when accessing the data register
(USIDR) the serial register is accessed directly. If a serial clock occurs during 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 on the USICS1..0 bit settings. The shift operation
can be controlled by an external clock edge, by a timer/counter 0 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 (transpar-
ent) 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.
The latch ensures that data input is sampled and data output is changed on opposite clock
edges.
Note that the corresponding Data Direction Register to the pin must be set to one for enabling
data output from the Shift Register.
The Status Register contains Interrupt Flags, line Status Flags and the counter value.
• Bit 7 – USISIF: Start Condition Interrupt Flag
When two-wire mode is selected, the USISIF flag is set (one) when a start condition is
detected. When output disable mode or three-wire mode is selected and (USICSx = 0b11 and
USICLK = 0) or (USICS = 0b10 and USICLK = 0), any edge on the SCK pin sets the flag.
Bit
0x10 (0x30)
Read/Write
Initial Value
Bit
0x0F (0x2F)
Read/Write
Initial Value
Bit
0x0E (0x2E)
Read/Write
Initial Value
USISIF
R/W
MSB
MSB
R/W
7
0
R
7
0
7
0
USIOIF
R/W
6
0
R/W
R
6
0
6
0
USIPF
R/W
5
0
R/W
R
0
0
5
5
USIDC
4
R
0
R/W
R
4
0
4
0
USICNT3
R/W
3
0
R/W
R
3
0
3
0
USICNT2
R/W
2
0
R/W
R
2
0
2
0
USICNT1
R/W
R/W
1
0
R
1
0
1
0
USICNT0
LSB
LSB
R/W
R/W
R
0
0
0
0
0
0
7701E–AVR–02/11
USIBR
USIDR
USISR