185
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
Bit 5 – USIPF: Stop Condition Flag
When Two-wire mode is selected, the USIPF Flag is set (one) when a stop condition is detected. The flag is
cleared by writing a one to this bit. Note that this is not an Interrupt Flag. This signal is useful when implementing
Two-wire bus master arbitration.
Bit 4 – USIDC: Data Output Collision
This bit is logical one when bit 7 in the Shift Register differs from the physical pin value. The flag is only valid when
Two-wire mode is used. This signal is useful when implementing Two-wire bus master arbitration.
Bits 3...0 – USICNT3:0: Counter Value
These bits reflect the current 4-bit counter value. The 4-bit counter value can directly be read or written by the
CPU.
The 4-bit counter increments by one for each clock generated either by the external clock edge detector, by a
Timer/Counter0 Compare Match, or by software using USICLK or USITC strobe bits. The clock source depends of
the setting of the USICS1:0 bits. For external clock operation a special feature is added that allows the clock to be
generated by writing to the USITC strobe bit. This feature is enabled by write a one to the USICLK bit while setting
an external clock source (USICS1 = 1).
Note that even when no wire mode is selected (USIWM1:0 = 0) the external clock input (USCK/SCL) are can still
be used by the counter.
21.5.3
USICR – USI Control Register
The Control Register includes interrupt enable control, wire mode setting, Clock Select setting, and clock strobe.
Bit 7 – USISIE: Start Condition Interrupt Enable
Setting this bit to one enables the Start Condition detector interrupt. If there is a pending interrupt when the USISIE
and the Global Interrupt Enable Flag is set to one, this will immediately be executed. Refer to the USISIF bit
description on page
184 for further details.
Bit 6 – USIOIE: Counter Overflow Interrupt Enable
Setting this bit to one enables the Counter Overflow interrupt. If there is a pending interrupt when the USIOIE and
the Global Interrupt Enable Flag is set to one, this will immediately be executed. Refer to the USIOIF bit description
on page
184 for further details.
Bit 5:4 – USIWM1:0: Wire Mode
These bits set the type of wire mode to be used. Basically only the function of the outputs are affected by these
bits. Data and clock inputs are not affected by the mode selected and will always have the same function. The
counter and Shift Register can therefore be clocked externally, and data input sampled, even when outputs are dis-
Bit
7
6
5
4
3
2
1
0
(0xB8)
USISIE
USIOIE
USIWM1
USIWM0
USICS1
USICS0
USICLK
USITC
USICR
Read/Write
R/W
R/WW
W
Initial Value
0