
eUSCI_B Operation – I
2
C Mode
In I
2
C slave mode, no internal clock source is required because the clock is provided by the external
master. It is possible to operate the eUSCI_B in I
2
C slave mode while the device is in LPM4 and all
internal clock sources are disabled. The receive or transmit interrupts can wake up the CPU from any low-
power mode.
20.3.11 eUSCI_B Interrupts in I
2
C Mode
The eUSCI_B has only one interrupt vector that is shared for transmission, reception, and the state
change.
Each interrupt flag has its own interrupt enable bit. When an interrupt is enabled and the GIE bit is set, the
interrupt flag generates an interrupt request. DMA transfers are controlled by the UCTXIFGx and
UCRXIFGx flags on devices with a DMA controller. It is possible to react on each slave address with an
individual DMA channel.
All interrupt flags are not cleared automatically, but they need to be cleared together by user interactions
(for example, reading the UCRXBUF clears UCRXIFGx). If the user wants to use an interrupt flag he
needs to ensure that the flag has the correct state before the corresponding interrupt is enabled.
20.3.11.1 I
2
C Transmit Interrupt Operation
The UCTXIFG0 interrupt flag is set whenever the transmitter is able to accept a new byte. When operating
as a slave with multiple slave addresses, the UCTXIFGx flags are set corresponding to which address
was received before. If, for example, the slave address specified in register UCBxI2COA3 did match the
address seen on the bus, the UCTXIFG3 indicates that the UCBxTXBUF is ready to accept a new byte.
When operating in master mode with automatic STOP generation (UCASTPx = 10), the UCTXIFG0 is set
as many times as defined in UCBxTBCNT.
An interrupt request is generated if UCTXIEx and GIE are also set. UCTXIFGx is automatically reset if a
write to UCBxTXBUF occurs or if the UCALIFG is cleared. UCTXIFGx is set when:
•
Master mode: UCTXSTT was set by the user
•
Slave mode: own address was received(UCETXINT = 0) or START was received (UCETXINT = 1)
UCTXIEx is reset after a PUC or when UCSWRST = 1.
20.3.11.2 Early I
2
C Transmit Interrupt
Setting the UCETXINT causes UCTXIFG0 to be sent out automatically when a START condition is sent
and the eUSCI_B is configured as slave. In this case, it is not allowed to enable the other slave addresses
UCBxI2COA1-UCBxI2COA3. This allows the software more time to handle the UCTXIFG0 compared to
the normal situation, when UCTXIFG0 is sent out after the slave address match was detected. Situations
where the UCTXIFG0 was set and afterward no slave address match occurred need to be handled in
software. The use of the byte counter is recommended to handle this.
20.3.11.3 I
2
C Receive Interrupt Operation
The UCRXIFG0 interrupt flag is set when a character is received and loaded into UCBxRXBUF. When
operating as a slave with multiple slave addresses, the UCRXIFGx flag is set corresponding to which
address was received before.
An interrupt request is generated if UCRXIEx and GIE are also set. UCRXIFGx and UCRXIEx are reset
after a PUC signal or when UCSWRST = 1. UCRXIFGx is automatically reset when UCxRXBUF is read.
20.3.11.4 I
2
C State Change Interrupt Operation
describes the I
2
C state change interrupt flags.
550
SLAU272C – May 2011 – Revised November 2013
Enhanced Universal Serial Communication Interface (eUSCI) – I
2
C Mode
Copyright © 2011–2013, Texas Instruments Incorporated